Miscellaneous Articles

A selection of articles on various topics, written by Martin Rowley.

Blue Moon: The Facts

When the subject of 'what is a blue moon' turns up in a scientific newsgroup, it is obvious that the 'two full moons in one calendar month' definition is becoming widely known, and is often given, in my view erroneously, as the ONLY correct answer to the question (and we now [1999] know that this is in any case wrong and was never the definition intended).

This note attempts to clarify the situation... note that it includes extracts from other's work which I duly acknowledge and give the www links for reference:

Extract from the alt.english.usage FAQ: "blue moon". (notes by Philip Hiscock)

"The phrase "blue moon" has been around a long time, well over 400 years, but during that time its meaning has shifted around a lot. I have counted six different meanings which have been carried by the term, and at least four of them are still current today". (items 1 to 5 deleted)

"6.Finally, in the 1980s, a sixth meaning was popularized (chiefly by the game Trivial Pursuit): the second full moon in a month. The earliest reference cited for this is The Maine Farmers' Almanac for 1937. Rumour has it that when there were two full moons in a calendar month, calendars would put the first in red, the second in blue".



However, from the March, 1999 issue of 'Sky and Telescope', it appears that even this was wrong! Apparently, the use of the term 'blue moon' relates to its seasonal occurrence, not its monthly occurrence. According to the almanac, a "Blue Moon" was said to occur when a season (defined by using the solstices and equinoxes) has four full Moons, rather than the usual three. It is then the third of these 4 full moons that is 'blue'. In other words, it would have been the last full moon in that season, but for the fact that, in some astronomical years (not every year) we need to pack 13 moon cycles in a 12 month year (winter solstice to winter solstice). The (now known as totally erroneous) definition that the 2nd full moon in a calendar month was 'blue' is thought to derive from a 1980's popular radio programme, which in turn based its answers on mis-interpretation of original articles in 'Sky and Telescope'. 

So, to collate our understanding of 'blue moon' usage in terms of various newsgroups:

1. To those who know the scientific definition, it means, what it has always meant, something that occurs very rarely or in extreme cases, never... i.e. in the 'pigs might fly' category. The event is not predictable, and you might go a lifetime without experiencing the event.

2. To those who grew up with the 'two full moons in a calendar month' definition (many parts of US and some areas in Canada), and those who play 'Trivial Pursuit', it will mean that an event happens now-and-then, but with longish gaps in between; is predictable, most/all people will experience such events in their lifetime, at irregular intervals. What happens in the future, now that we know it was really meant as the third full moon in a season with four... I suspect that we are stuck with the former!

British/Irish Radarnet

British/Irish Radar Map

 Beacon Hill (Druim a' Starraig)
[Isle of Lewis, Hebrides]
 Lat = 58 DEG 13 MIN N.
 Long = 6 DEG 11 MIN W.
 Hill of Dudwick (NE Scotland)
 Lat = 57 DEG 26 MIN N.
 Long = 2 DEG 02 MIN W.
 Corse Hill (Strathclyde)
 Lat = 55 DEG 41 MIN N.
 Long = 4 DEG 14 MIN W.
 Castor Bay (Armagh, NI)
 Lat = 54 DEG 30 MIN N.
 Long = 6 DEG 20 MIN W.
 Lat = 53 DEG 27 MIN N.
 Long = 6 DEG 14 MIN W.
 Shannon Airport(Co. Limerick, RoI)
 Lat = 52 DEG 42 MIN N.
 Long = 8 DEG 56 MIN W.
 Hameldon Hill (Lancashire)
 Lat = 53 DEG 45 MIN N.
 Long = 2 DEG 17 MIN W.
 Ingham (Lincolnshire)
 Lat = 53 DEG 20 MIN N.
 Long = 0 DEG 33 MIN W.
 Clee Hill (Shropshire)
 Lat = 52 DEG 24 MIN N.
 Long = 2 DEG 36 MIN W.
 Crug-y-Gorllwyn (Ceredigion)
 Lat = 51 DEG 59 MIN N.
 Long = 4 DEG 27 MIN W.
 Chenies (Hertfordshire)
 Lat = 51 DEG 41 MIN N.
 Long = 0 DEG 32 MIN W.
 Cobbacombe (Devon)
 Lat = 50 DEG 58 MIN N.
 Long = 3 DEG 27 MIN W.
 Dean Hill (Wilts/Hants border)
 Lat = 51 DEG 02 MIN N.
 Long = 1 DEG 39 MIN W.
 (ex- Wardon Hill)
 Predannack (Lizard, Cornwall)
 Lat = 50 DEG 00 MIN N.
 Long = 5 DEG 13 MIN W.
 Jersey (La Moye, CI)
 Lat = 49 DEG 11 MIN N.
 Long = 2 DEG 13 MIN W.
 Thurnham (nr. Maidstone / Kent)
 Lat = 51 DEG 17 MIN N.
 Long = 0 DEG 35 MIN E.
 East Anglia(?)


International Standard Atmosphere (ISA) and Pressure Settings

Based on the International Standard Atmosphere for dry air (ICAO 1964), which is defined as under:-

1. At mean sea level (msl), the pressure = 1013.25 hPa and temperature = 15.0 degC
2. From msl to 11 km, a decrease in temperature (or lapse rate) of 6.5 degC/km
3. From 11 km to 20 km, the temperature is held to be isothermal (not changing) at a value of - 56.5 degC
4. From 20 km to 32 km, an increase in temperature of about 1 degC/km

hPa (mbar) ......... height (metres) ......... height (feet) ......... temperature (degC)
»...... 10 ................... 31 055 ................ 101 885 ................. -45.4
»...... 20 ................... 26 481 .................. 86 881 ................. -50.0
»...... 30 ................... 23 849 .................. 78 244 ................. -52.7
»...... 40 ................... 22 000 .................. 72 177 ................. -54.5
»...... 50 ................... 20 576 .................. 67 507 ................. -55.9
»...... 70 ................... 18 442 .................. 60 504 ................. -56.5
».... 100 ................... 16 180 .................. 53 083 ................. -56.5
».... 150 ................... 13 608 .................. 44 647 ................. -56.5
».... 200 ................... 11 784 .................. 38 662 ................. -56.5
»... (226/ISA TROP.. 11 000 .................. 36 091 ................. -56.5)
».... 250 ................... 10 363 .................. 33 999 ................. -52.3
».... 300 ..................... 9 164 .................. 30 065 ................. -44.5
».... 400 ..................... 7 185 .................. 23 574 ................. -31.7
».... 500 ..................... 5 574 .................. 18 289 ................. -21.2
».... 600 ..................... 4 206 .................. 13 801 ................. -12.3
».... 700 ..................... 3 012 .................... 9 882 ................... -4.6
».... 800 ..................... 1 949 .................... 6 394 ..................... 2.3
».... 850 ..................... 1 457 .................... 4 781 ..................... 5.5
».... 900 ........................ 988 .................... 3 243 ..................... 8.6
».... 950 ........................ 540 .................... 1 773 ................... 11.5
».. 1000 ........................ 111 ....................... 364 ................... 14.3
». (1013.25/ISA MSL ....... 0 ........................... 0 ................... 15.0)
».. 1050 ...................... - 302 .................... - 989 ................... 17.0

For practical aviation purposes, i.e. Flight Weather charts, the following relationships are used:


100 ............................................ F530
150 ............................................ F450
200 ............................................ F390
250 ............................................ F340
300 ............................................ F300
400 ............................................ F240
500 ............................................ F180
600 ............................................ F140
700 ............................................ F100
850 ............................................ F050

(below F050, heights usually expressed in altitude, above mean sea level, rather than pressure altitude - SEE DEFINITIONS BELOW.)


These definitions are intended to give a quick reference to the various pressure settings in use. Users should refer to standard textbooks (e.g. Handbook of Aviation Meteorology, The Met.Office/HMSO) for more details, and to current air safety/air traffic control regulations for in-flight use.

National Severe Weather Warning Service

An attempt to explain what all the various warning categories are

National Severe Weather Warning Service (NSWWS) (UK service only)

Within the United Kingdom, the UK Met Office is responsible for the issue of various classes of warnings under this service. These are funded by contributions from central & regional governments, and are issued when the weather is expected to be of sufficient severity to cause significant dislocation / disruption to transport and / or loss of life (or in the case of the Heatwave warnings, extreme stress to vulnerable groups of people). (The criterion used to be 'widespread' dislocation: in practice, this was always difficult to define, and in recent years, any dislocation, no matter how small the area, has been targetted; similarly, population density is now not a critical determinant when warnings are issued - so you will see warnings for sparsely populated areas that in years past we would never have considered issuing).

Warnings - public (UK service only):

Under the NSWWS the UK Met Office issues various levels of warnings of weather conditions sufficient to cause significant disruption to communication (road, rail etc.), and / or loss of life (or stress due to high temperatures). There are strict criteria observed which are 'weather related' (i.e. amount of rain in a given time), but some latitude may be employed when considering the "end-effect". As an example, 24hr of light but steady snowfall in the Hebrides is unpleasant & unwelcome, but would not generate anything other than a low-level local advisory: 2 or 3 hr of similar snowfall in the English Home Counties on a Monday morning would cause chaos!

The various types of warnings cover the following weather conditions:
Severe Gales, Storms, Heavy Snow, Very heavy snow, Blizzards, Severe blizzards, Heavy Rain, Dense Fog, Glazed Frost/Widespread Icy roads & Heatwave.

The two classes of warnings are: Early and Flash.

"EARLY" warning (UK service only)

These warnings (for all except 'Heatwave' in the list above**) are issued by the Met Office (HQ only) ahead of an expected event, ideally 3 or 4 days before, but sometimes with less than 24hr lead-time. They are only issued when there is a 60% or more chance of the expected disruption occurring: this means that given 10 similar synoptic outcomes (usually determined by inspection of NWP ensemble output - see entries elsewhere), on at least 6 of them, populated locations in the UK should experience conditions sufficient to cause significant disruption to communication, or there is a high risk of loss of life. It should be carefully noted that this means that on 4 or fewer occasions, the expected disruption may NOT occur: this is not a failure, but is the standard agreed between the Met Office & the users of the system (i.e. local authorities, emergency services, public utilities etc.) [ For more on the use of probabilities in this scheme, click HERE.]

( ** Heatwave warnings have a different text-based format - the initial warning is put up on the Met Office home page, with a link to a 'Heat Health-watch' page, which divides the country up into regions, with a % prob. risk of exceedence of the critical temperature and explanatory text. )

An Early warning is usually issued (when required) around 0900 - 1000 each day, but it can be issued at any time when the situation warrants. It is updated each morning, (with revised regional risk assessment if necessary). An early warning will be updated until either it expires or until it is superceded by Flash messages, or, it may be cancelled ahead of time if the overall (UK-wide, NOT the individual regional value) probability falls too low for it to be valid.

These warnings prompt the issue of a 'Weather Watch' by the BBC Weather Centre: see below.

"FLASH" warning (UK service only)

These are issued in the hours leading up to the expected event, when confidence is very high that the conditions will cause disruption etc. They are issued by HQ MetO at Exeter, after consultation with the regional Met Offices. Exceptionally, "Emergency Flash" warnings are issued by HQ when 'extreme' conditions are expected (e.g. exceptionally high winds or heavy, prolonged blizzard conditions).

[ The "FLASH" tag belongs to the days of the precedence categories used to relay signals over closed military/diplomatic communications networks: The Meteorological Office was much more closely tied to RAF & MOD signals protocols than it is now, and would often use the same (or parallel-bearer) links to relay meteorological warnings. ]
These warnings will generate an appropriate alert on the Met Office web site & also prompt issue of a 'Weather Warning' by the BBC Weather Centre: see below.
[ NB: as of September 2005, 'Advance' & 'Motoring' warnings are no longer issued.]


BBC Weather Centre warnings:

These are based upon the warnings issued by the Met Office, but unfortunately, different terminology is used. It is probably best to visit the BBC Weather Centre site to get the full picture, but in essence:

 Met Office category  BBC Weather Centre category
 Early Warning  Weather Watch
 Flash Warning  Warning of Severe Weather

For more on warnings issued by the UK Met Office, visit their site: navigate via ' Products and Services', then 'Public Sector', or follow the appropriate links from the warnings listing on the home page - much more of interest on the categories etc.
For more on warnings issued by the BBC Weather Centre, visit their site: navigate via 'UK & Warnings' links.
For some general points surrounding Warnings to the general population (not just weather related), visit: the "UK Resilience" site, which also has information on other warnings for public safety.

Early warnings and probabilities

Although it doesn't always come across very well in broadcast presentations, when an 'Early Warning' is issued for a particular meteorological event, forecasters are issuing what is known as a 'probabilistic' forecast not a 'deterministic' one. It is important to understand the distinction between the two.

When a forecast says something like ... " strong winds and heavy rain will sweep north from SW England, across Wales reaching SW Scotland this evening ", then that is a 'yes/no' (or deterministic) forecast - that is, the expected evolution of the weather with time.

When a NSWWS 'Early Warning' is issued, the probability of those conditions occurring somewhere within the UK must be 60% or higher. That is, on the balance of probabilities, the event is more likely than not to occur (> 1 in 2) .. BUT it is not a certainty.

Increasingly, the level of risk will be set using objective techniques based upon NWP ensemble output - some very sophisticated routines are being developed which will take such output and, based on past cases and the knowledge of current meteorological & infrastructure conditions, produce a probability - which of course can be modified manually if required.

So, what does a probability figure mean? Well, if a figure of 60% probability is issued, it means that given 10 situations with the same set of precursor ( & forecast ) indicators, on 6 of those, 'something' extreme (meteorologically) will occur which will lead to a significant impact on the community - with consequences for the emergency services, local authorities, public utilities etc. However, this also means that on 4 occasions, nothing 'extreme' (in terms of public impact) is likely. The primary customers understand this - unfortunately the general public is not too well educated about the latter point. Even in the dizzy heights of 90% probability (not often used for events more than 12 hours ahead), then there is still a slim chance that nothing untoward will occur.

Within the individual warning, the probability for any particular area may be even lower. It may be that the 'threat' to Wales, SW England and SW Scotland is 60%, but for adjacent areas, such as NW England, the West Midlands etc., perhaps 40%. In this latter case, it is even more likely that nothing untoward will occur (i.e. 60% probability of a 'non-event').

This gives rise to the charge ... "well, they can't get it wrong, can they!" Well, believe me if forecasters continually issued warnings and nothing happened time after time, then the 'powers-that-be' would soon get involved. Each warning is carefully monitored after issue to find out how well (or otherwise) both the forecasters & the end-users coped. Central (UK) government also gets involved directly and indirectly and would view over-use of these warnings with concern. In fact, it turns out that forecasters, being human beings, actually try to get each one exactly right - which is not the idea of the scheme!

So, why use probabilities then? If you are a county planner, or a team leader with the emergency services, or responsible for having 'stand-by' resources available to support the public utilities (electricity supply, rail transport, airport management etc.), then the idea of attaching a % level of risk is supposed to help in the provision of stand-by/back-up services: it allows planners to avoid over-preparing by bringing expensive resources 'on-stream' for a low-threat event (say 20% or one-in-five possibility), but have plenty of back-up available for a 80% (or roughly four-in-five probability) event. Even in this last occasion though, remember that the wind may blow, the snow may fall, but there is still that slim chance that little 'disruption' will occur.

For the general public, the probabilities tend to get 'smeared-out' into very broad "low", "moderate" or "high" risks of disruption on radio and television presentations. But, even when a "high" risk is indicated, it must not be assumed that trouble will ensue. The information is presented to enable YOU the user to decide whether to carry on with your planned journey, building project etc., or to delay perhaps by 24 hours, setting this against the cost in monetary and other terms of either the delay, or carrying-on regardless and 'taking a chance'. This is what 'probabilistic' forecasting is all about - you, the user have extra information to assess the likely outcome of the event, and the costs/risks associated.

On the road in severe wintry weather

Winter weather in the United Kingdom is somewhat less demanding than, for example, that in North America, but even when temperatures are only a few degrees below zero (°C), and given strong winds and/or sufficient snow, some of our upland areas can be hazardous for travellers; and, of course, more people travel further afield now throughout Europe and North America during winter/early spring, e.g. on business or on skiing holidays.

The notes below may be of some use.


Keep a survival kit in your car... this should include:....
(*=items that might usefully be on the vehicle at all times.)



(Notes re: hypothermia:...Death from cold (aka as hypothermia), will occur when the overall body temperature fall below 28 °C. The ‘inner core’ of the body must remain at around 37.5°C in order to function properly; A couple of degrees colder, and mild hypothermia occurs, and this can be detected as shivering, which is the way the human body will have of trying to create heat; Below about 33°C, the situation becomes serious, and is likely to lead to unconsciousness and unless the body temperature is brought back to near-normal levels quickly, the lack of activity will soon lead to death.)



In the United Kingdom, the Met.Office is responsible for the National Severe Weather Warning Service, and by this mechanism, relayed via the normal tv and radio weather forecasts, will attempt to give at least 2 days warning of potential severe weather. Notwithstanding this service, all forecasts should be monitored closely if you intend to travel in areas that might be subject to severe winter weather. Remember though that because of time constraints imposed by broadcasters, details of extreme weather in remote areas cannot be given the due time it deserves. This is particularly true of Scotland, where forecasts provided on a (UK) national basis often concentrate on the central belt, and the populated east and north-eastern regions of the country. Forecasts provided by regional weather centres, via recorded telephone or local/regional television and radio presentations are better.

Ozone in the atmosphere

It might be as well to emphasise that "Ozone" [ O3 ] is not new! Although it has become part of the 'green' vocabulary, and is rightly a topic of concern, Ozone has, as far as we (humans) are concerned, always been a constituent part of the earth's atmosphere, albeit with variations in its distribution in time and space.

What should be of concern to us is that the concentrations at low altitudes (on / close to the surface) under certain atmospheric and human-influenced conditions can increase to uncomfortable, or even hazardous levels.

However, in the upper atmosphere (roughly between 12 and 50 km altitude, with a peak concentration in the 15 to 25 km altitude band), where Ozone is of benefit to all animal and plant life, total amount, and the regional distribution have varied alarmingly over short periods, over and above the natural variation that is a part of normal atmospheric processes: at these high altitudes, a sustained depletion of natural Ozone is a potential long-term catastrophe for life on this planet. Without a healthy, self-sustaining stratospheric Ozone layer, life as we know it on this planet would not be possible.

The rest of this note looks at each problem in turn:

a. Ozone at low levels:

There is always a small background level of naturally occurring Ozone, but that level can be significantly enhanced, because the unburnt residues involved in the combustion of hydrocarbon fuels, (particularly those concentrated in vehicle exhaust emissions), contain the ingredients to produce Ozone, when acted upon by strong sunlight, in combination with Nitrogen Oxides, also produced by transport exhaust, and in certain power station emissions.

Under light wind / low level stability conditions (see the uk.sci.weather FAQ entry "Stable and unstable air masses"), the Ozone ingredients (NOx; unburnt hydrocarbons), along with a mixture of other unsavoury gases, such as sulphur-dioxide, nitrogen-dioxide etc., accumulate, causing breathing problems, particularly in those at risk from asthma attacks, and other respiratory problems.

Ozone is a particularly sneaky problem, as its peak of concentration often occurs after several hours of sunshine, when people are opening windows to benefit from the supposedly fine weather - in fact, concentrations of ground level Ozone tend to be significantly higher in the summer months, due to the strength of the sunshine - an essential ingredient for the production of low-level Ozone.

Further, the problem is not necessarily concentrated in the inner cities. Because many major road / motorway interchange complexes are situated in semi-rural areas, under conditions of near-stationary traffic, a rapid build-up of engine exhaust pollution can occur, which if the low-level atmospheric conditions are correct, will not be dispersed. Also, the build-up of the primary pollutants that produce Ozone often drift out of town, so rural / suburban residents in areas downwind of major conurbations are often more at risk from high Ozone pollution level events, due to concentrations that have built up in town earlier in the day.

There is also a paradox in that some emissions from vehicles actually combine with the emerging Ozone radicals and offset the production of the gas – so statistically, rural areas often experience high levels of Ozone pollution because of lower levels of traffic! (Of course, in light wind/hot/high stability conditions, other pollutants will cause problems in the cities, so this difference is a bit spurious – also, large cities act as ‘heat islands’ local / mesoscale inflow of air will drag Ozone pollution back into the towns.)

Ozone also attacks the cell membranes of plants, leading to stress. Significant 'die-back' of otherwise healthy-looking foliage, particularly noticeable in the canopy of mature trees, and is often the first sign of repeated high-Ozone attacks.

Ozone is a "greenhouse gas": that is, it contributes to the normally beneficial extra-warming of the planet. However, any increase over and above the 'natural' level would add to any perceived problem with enhanced global warming. However, the effect, compared to other gases of concern, e.g. CO2, methane and water vapour, is smaller, though still significant.




Carbon Monoxide (CO)

Motor engine exhausts; some industrial processes.

Sulphur dioxide (SO2)

Power generators using coal or oil etc., that contain sulphurous elements.

Particulate matter

Vehicle exhausts; many industrial processes; incineration; power generation etc.

Lead (Pb)

Vehicle exhausts.

Nitrogen dioxides (NO2)
[ formed from Nitric oxide (NOx) after emission from vehicle exhausts - those burning fossil fuels]

Vehicle exhausts. power generators. Fertiliser plants.

Photochemical oxidants (including Ozone O3)

Reaction of NOx and unburnt hydrocarbons (from vehicle exhausts) to sunlight.

Carbon dioxide (CO2)

all sources that involve combustion.


Solvents (vapour), fuel combustion vapours etc.


For more details on air quality monitoring (and background to the problem), see: http://www.airquality.co.uk/
or: http://www.defra.gov.uk/environment/airquality/index.htm
and BBC CEEFAX (p417) and ITV TELETEXT (p156) services carry current data, as do some newspapers.

b. Ozone at high levels:

In the stratosphere, Ozone is an important constituent gas of the atmosphere for two key reasons:

1. The radiation received from the sun, our primary source of energy, irradiates the atmosphere across a broad band of wavelengths, but it is the ultra-violet part of the spectrum that concerns us here, accounting for about 7% of the total radiation received at the earth's upper atmosphere. [The other important wavelengths are: visible (41%) & infra-red (52%)].

The following simple graphic shows how ultra-violet radiation 'fits in' with the rest of the electromagnetic spectrum. (nm=nanometres) .

Gamma rays X-rays Ultra-violet Visible Infra-red Microwaves Radio
     200 to 400 nm  ~400 to 760nm  > 760nm    


Ultra-violet radiation has a complex relationship with high-altitude Ozone .... one particular uv wavelength band contributes to the formation of Ozone, and other wavelengths are involved in the dis-association of the same gas. In the process, sensible heat is released which contributes significantly to the heat budget at these Ozone-rich altitudes, with a peak of absorption (& therefore stratospheric warming) at 50km: if the Ozone levels were to be significantly depleted on a permanent basis, the stratosphere, and the troposphere below it (where all the 'active' weather is), would have a significantly different vertical temperature profile, with consequences for the whole atmospheric system.

The generally accepted ultra-violet ‘bands’ used in the high-level Ozone debate, and their possible / probable effects are:-

 UV-A:  315/320 - 400 nm  Not (significantly) absorbed by the stratospheric Ozone layer.  10-15% of ‘burning’: possible connection with the formation of malignant melanomas. These wavelengths are responsible for primary skin 'tanning' & skin ageing.
 UV-B:  280 – 315/320 nm  Absorbed by Ozone in the stratosphere. Ozone absorbs UV radiation without being reduced; the overall result being to convert UV radiation to heat. This is why the temperature of the stratosphere increases with increasing altitude. Any significant reduction in Ozone (caused by us) will have an impact upon the thermal character of the Stratosphere.  85 – 90 % ‘burning’; involved with both malignant & benign cancerous growths. Also linked to eye cataracts. Effects on growth of plants and marine life – but variable; generally though, any increase in UV-B thought to be harmful.
 UV-C:  200 - 280 nm  highly absorbed (by Oxygen molecules): involved with formation of Ozone (wavelengths < 240 nm in particular).  not (thought to be) significant, mainly because of its efficient absorption at high-altitudes: large-scale weakening of the Ozone layer though may revise this opinion.


2. The processes outlined above involve the 'absorption' of uv radiation preferentially in a wavelength band known to be important in the formation of human cancers: UV-B (280-320 nm). In studies in both this country & in North America, UV-B was found to be responsible for some 85 to 90% of ‘burning’: that is, increasing the risk of formation of cancerous growths on the skin. UV-B is also involved in a widely accepted link with eye cataracts. In very rough terms, the shorter the wavelength, the higher the impact upon living tissues at near-ground level, whether human or plant life. When the Ozone concentration is dramatically reduced (the so-called 'Ozone-hole' - actually an Ozone weakness), then more of this harmful radiation reaches the earth's surface, and without adequate protection, would eventually lead to an increase in several types of skin cancer, damage to human (and other) immune systems, and an increase in the incidence of eye defects.

UV radiation has always reached the surface of course, and there are naturally occurring variations in Ozone, which we can do nothing about. These occur on time-scales of hours (connected with upper tropospheric ‘weather’ disturbances), to seasons (due to varying solar radiation levels as the earth orbits the sun with its tilted axis.)

However, certain classes of man-made chemicals, used in such processes as refrigeration, foam packaging manufacture and (formerly), in aerosol sprays, find their way to the stratosphere, residing there for long periods, and accelerating the destruction of Ozone. Stratospheric circulation systems concentrate these Ozone minima to the polar regions. Protocols have now been put in place to phase out the problem substances, but it will be many years, perhaps decades, before the 'normal' background levels of Ozone are restored.

It is important to understand that at present (early 21st century), the occurrence of 'Ozone holes' is a high latitude problem, observed around the return of the spring sunshine to high-altitudes. The effect was first described in the mid-1980's (though probably a developing problem since the mid-1970's), and at that time, and even currently, is primarily associated with the Antarctic polar vortex; however, recently a similar, though not as yet dramatic, depletion has been noted in the Arctic sector.

Elongation of the polar vortices 'swing' a marked depletion of total column Ozone (i.e. Ozone found taking a sample from bottom to 'top' of the atmosphere) across major population centres, for example across the southern tip of South America & southern parts of Australia and New Zealand. However, within our (mid-latitude) bands, although we cannot be complacent, an 'Ozone-hole' as such does not occur: there has been though, a broad-scale steady loss of total column Ozone, which obviously will not help the situation.

Changes in life-style since the mid-1960's are a major contributory factor to the increased incidence of skin cancers etc: increased disposable income has encouraged more and more people to travel to the sunspots of the world.

Within the UK, the Health Education Authority indicated in its 1996 pamphlet that the increase in skin cancer reporting is probably due more to the habit of 'soaking up the sun' rather than altered levels of uva and uvb, although it doesn't rule out the latter as a cause. Certainly, over a period of 25 or more years, more people have travelled abroad for holidays in the summer months at lower latitudes, where the strength of the sun is notably stronger. Such widespread travel abroad would have been unthinkable to my parents' generation, and although uv radiation level increase has been detected, it is more likely that our self-inflicted increased exposure to strong sunshine has a lot to do with the problems of skin cancer.

for more information on the stratosphere & high-altitude Ozone see:
and BBC CEEFAX (p418) provide forecasts which attempt to convey the ‘risk’ associated with exposure to the sun.

Q - code: meteorological section

In the early part of the 20th century, communication between aviators and the ground was by means of wireless telegraphy (W/T), using the Morse Code (once radio sets had become robust enough to fit in aircraft of course). The pilot would have the transmitter key near at hand (often strapped to his/her upper leg / thigh), and would 'tap' out a message in 'morse' to ground staff, who would reply in similar fashion, the message being heard by the pilot on a headset. Later, larger aircraft (both civil and military) had radio officers on board, but the practice was still the same - W/T exchange of coded messages.

To minimise the time taken to exchange important information, and to standardise the exchange, the 'Q-code' was devised, some portions of which are still used, having been carried over into the 'R/T' (radio-telephony) era that soon replaced the cumbersome morse-code method.

The list below is not exhaustive, but gives a 'flavour' of the system as it applies to meteorology.
In these days of email, SMS/text etc., perhaps the Q-code might make a come-back!

The 'Q' part of the code indicates that a 'query' is being performed, thus the sequence " QAN LGW " heard on the ground would be interpreted as ... "what is the latest wind report for Gatwick airport?", and the reply would be along the lines ... " QAN GEO LGW 280/16KT QNT 28KT "

QAM (what is / here is the ) ... latest surface weather report
QAN (what is / here is the ) ... surface wind (Geo after Q signal to indicate degT)
QAO (what is / here is the ) ... upper wind (to be followed by height)
QBA (what is / here is the ) ... visibility
QBB (what is the / here is the ... ) amount and base of significant cloud above station level
QBC (what is the / here is the ... ) weather report from aircraft in plain language or POMAR code
QBI Instrument Flight Rules compulsory (weather unfit for visual landing)
QBJ (what are the / here are the ... ) cloud tops above msl
QBT (what is the / here is the ... ) runway visual range
QDF (what is the / here is the ... ) difference in actual height of pressure level to height of same pressure level in ICAO standard atmosphere (the 'D' - factor).
QFA (what is the / here is the ... ) route forecast (location from / to)
QFB Request for fresh met.reports
QFC Cloud base above msl (used for forecasts and aircraft reports)
QFE Barometric pressure reduced to airfield level
QFF Barometric pressure reduced to MSL
QFT Icing
QFY Present meteorological landing conditions
QFZ Landing forecast
QGO Airfield unfit for any landings (usually due to weather)
QMW Freezing Level
QMX Temperature at upper levels
QNH Altimeter setting in tenths of millibar ( I believe this used to be QUH prior to 1950)
QNT Gusts
QNY Present met. conditions...plain language
QUB Information in the following order: visibility, height of cloud, direction and speed of surface wind. (however, my earlier Met. for Aviators says this is QVB ... it may have changed in 1950 when the codes were updated.)
  (and not strictly meteorological, but used to 'end' message exchange)
QSL Please acknowledge / I acknowledge


Rossby Waves

If the spinning earth's crust was completely uniform in height and character (i.e. no mountains, seas etc.), but a surface made of the same substance, it is likely that the differential heating between poles and the equator would still produce a west-to-east airflow in the lower atmosphere [ i.e. the troposphere and lower stratosphere ], ( assuming of course in this unlikely situation we had an atmosphere), but without the very variable wave-like motion that can be seen on both daily and time-average maps at any particular level in the atmosphere. (N.B. there would be detectable wave-trains due to the inertial forces on the atmospheric mass on a rotating sphere.)

The major (or 'long') waves (also known as Planetary Waves) we see on weather maps for the upper air are produced largely because the atmosphere in motion encounters barriers to its progress, and is forced to ascend (by the changing surface level), then allowed to descend (under gravitational influence), and the resultant "squashing" and "stretching" respectively of the air columns involved lead to alterations to the rates of "spin" of the air flow (vorticity). These variations in the rate of spin must be balanced on a rotating earth for the system to remain stable -- assuming there are no other forces at work: see later. 

The principle is known as CAV, the Conservation of Absolute Vorticity, and was investigated by Carl-Gustav Rossby and others in the late 1930's. When considering the northern hemisphere, air that is forced to ascend tends to turn to the right, and as it descends again, it tends to turn to the left, inducing a ridge/trough pattern to the broadscale westerlies. These long waves are often known as Rossby waves, after the person who did so much to investigate their character. 

Major mountain chains provide obvious sources of such deflection, and the Rockies and the Andes, which lie astride the westerly flow in each hemisphere, provide good examples. These long-waves are key elements in the atmospheric circulation, and can be traced well into the stratosphere. At any one time, there are between 3 and 7 such waves, the number in any particular latitude band dependent upon a fine balance between the speed of the airflow through the trough/ridge system and the wavelength. Rossby, following his investigation of these long-waves, derived the relationship: The Rossby Equation

From this equation, we see:

  1. At any particular latitude (phi), and for a given wavelength(L), the governing factor which forces change is U, which is the mean west-to-east airflow through the system. This is in turn governed by short-term developments in the synoptic pattern; however, the development/vigour of such "synoptic-scale" features (i.e. frontal or baroclinic systems), is directly influenced by the position and strength of the aforementioned long-wave pattern. In general, a strengthening zonal flow drives the major long-waves apart; a weakening zonal flow allows a 'closing-up' of the wave pattern. There is therefore a delicate feedback loop at work ... ideal for super-computers running sophisticated NWP packages to get their teeth into.
  2. For a particular latitude band, there is a combination of U and L which will give C=0, i.e. the wave-train is stationary. These conditions often apply downstream of the so-called 'anchor troughs', which form due to the principles of CAV outlined above. From this, we can predict how many long-waves might be found in any latitude band for a particular zonal component of wind, and the wavelength of same...the table below summarises the various variables and results:
  3. The wave speed decreases with the square of the wave-length, i.e. very long waves are slow-moving, and this is commonly observed in the real atmosphere. The average west-to-east movement of a major long wave is around 2 to 5 degrees of longitude per 24 hr.
Table for Rossby long-waves
latitude zonal wind speed        
>>>> knots 10 20 30 40 50
60 deg.(N/S) 76 deg long. 108 deg long. 132 deg long. 152 deg long. 170 deg long.
50 deg.(N/S) 52 deg long. 74 deg long. 90 deg long. 104 deg long. 116 deg long.
40 deg.(N/S) 40 deg long. 57 deg long. 69 deg long. 80 deg long. 90 deg long.
approx. number of waves (at 50 degN/S) 7 5 4 3 to 4 3


All the foregoing assumes that other convergent/divergent effects due for example to cyclonic development, are negligible; this is unrealistic for the real atmosphere except perhaps at the level of non-divergence (LND) around 600 mbar, and operational forecasting models attempt to take such factors into account. The same principles will apply though.

In Summary

The circulation in the "upper air" (say 700 mbar [or hPa]) upwards, consists of a large circumpolar vortex system -- on this broad flow are superimposed a system of variable, but usually slow-moving long waves, sometimes stationary, or very occasionally retrogressive (i.e. they move east-to-west). Down the scale again, there are a series of short-waves running quickly (circa 15 degrees of longitude per 24 hr: +/- 5 degrees) through the flow, driving, and being in turn modified, by the familiar synoptic scale disturbances in the lower troposphere (i.e. frontal systems), which in turn feed-back energy (in all forms) up the scale to influence the broadscale pattern. 

It is no wonder that L.F. Richardson, postulated in 1922, that computing power of some weight would be needed to properly solve the equations governing atmospheric motion, and much endeavour in operational and research meteorology has been devoted to this end ... and the story goes on!

Thermodynamic Diagrams

No, this is not an in-depth discourse on the mathematics behind such diagrams .. I don't claim to be that clever! I am though going to try and outline the characteristics of the three main diagrams used in weather services around the world, and indicate some sites which deal with how they can be used.

This note is sub-divided as follows:

  1. General Introduction:
  2. A little History:
  3. Description of the various 'lines' on the charts:
  4. Comparison of the three different diagrams under consideration:
  5. Sites with more information, including worked examples:
  6. How to get hold of current data:



There are three diagrams that I am going to discuss in this note:

Thermodynamic (otherwise called adiabatic or aerological) diagrams of various types are in use, and the earliest dates from the late 19th century. They are all, however, based on the same principles, and differences are mainly in appearance. Each chart contains five sets of lines: isobars, isotherms, dry adiabats, pseudo-adiabats & saturation moisture lines.

Why do we not use a simple bit of graph paper? Well, although it would be perfectly fine to use such and indeed atmospheric stability etc., could be judged by the shapes plotted on the graph, by using the more rigorous diagrams presented here, calculations based on the basic laws of thermodynamics and humidity can be accomplished very quickly, without the use of calculators etc. The diagrams are such that equal area represents equal energy on any point on the diagram: this simplifies calculation of energy and height variables if required.

It is however, important not to get bogged down in the background, mathematics etc., involved. For basic calculation of such as condensation level, temperature of free convection etc., then it will be enough to remember what the various sets of lines mean, and more importantly, how to recognise them. I have included a little history for those that like these things.


Tephigram_basic The Tephigram takes its name from the rectangular Cartesian coordinates used: temperature and entropy. Entropy is now usually denoted by capital letter S, but in earlier texts the Greek letter 'phi' was used, hence Te-phi-gram. The diagram was developed by Sir William Napier Shaw, a British meteorologist about 1922 or 1923, and was officially adopted by the International Commission for the Exploration of the Upper Air in 1925. Although this diagram is one of the earliest, it is not the first; that honour appears to belong to the Emagram (see Skew-T/Log(P)), used by H Hertz in 1884. Shaw was Director of the UK Meteorological Office from 1905 to 1920, and thus this diagram was introduced readily into that organisation and offices/countries that have been, or are allied to this formation. The tephigram strictly should be arranged so that temperature lies along the x axis, and theta (dry-bulb potential temperature) lies along the y axis. In earlier texts, you will see this arrangement. However, since about the late 1940s, the diagram has always been used in its 'rotated-right' format, whereby isobars decrease upwards to the top of the chart; rather more logical than the original! (For UK-based meteorologists, this is the diagram that they are most used to, and as such in the uk.sci.weather newsgroup for example, often all thermodynamic diagrams tend to get called (erroneously) tephigrams.)


Stuve_basic The Stüve diagram was developed circa 1927 by G. Stüve and quickly gained widespread acceptance in the United States, and has a simplicity in that it uses straight lines for the three primary variables, pressure, temperature and potential temperature. In doing so though it sacrifices the equal-area requirements (from the original Clapeyron diagram) that are satisfied in the other two diagrams. For practical purposes though, this is not important.


SkewTLogP_basic The SkewT/Log(-P) diagram is also in widespread use in North America, and in many services with which the United States (various) weather services have had connections. This is in fact a variation on the original Emagram, which was first devised in 1884 by H. Hertz. In the emagram, the dry adiabats make an angle of about 45degrees with the isobars and isopleths of saturation mixing ratio are almost straight and vertical. In 1947, N. Herlofson proposed a modification to the emagram which allows straight, horizontal isobars, and provides for a large angle between isotherms and dry adiabats, similar to that in the tephigram. This chart has much in common with the tephigram, and superficially at least provides a similar-looking trace when a sounding is plotted on it. Hence the two charts are often confused.

So, in summary, the chronology of the various diagrams (including a couple not discussed above) is (as far as I can ascertain):
(I would be grateful if anyone who has more information on the history behind the developments of these diagrams would let me know so I can add a little to the above.)

 Chart  Date  Who by
 Emagram  1884  H. Hertz
 Tephigram  1922 or 1923  N. Shaw
 Stüve  1927  G. Stüve
 Aerogram  1935  A. Refsdal
 Pastagram  1945  J.C. Bellamy
 SkewT/Log(-P)  1947  H. Herlofson


There are five sets of fixed lines (isopleths) printed / displayed on most thermodynamic charts. They are:

 Line  Usual symbol  What they are
 Isotherms  T  Lines of equal temperature
 Isobars  P  Lines of equal atmospheric pressure
 Dry Adiabats  theta  Lines of constant potential temperature
 Saturated (or Wet) Adiabats  theta e  Lines of constant equivalent potential temperature
 Saturated Humidity Mixing Ratio  w s  Lines of constant saturation mixing ratio with respect to water

In addition, you may find others, such as the MINTRA line on UK Tephigrams; used in the forecasting of condensation trails, and the zero degC, MS20degC and MS40degC isopleths highlighted to pick out significant temperature values.


On another page, I have put a detailed list of how to compare the various types of charts. If you know what you are looking at, then ignore this section.


Sites that explain how to ‘do’ things on these diagrams are rather thin on the ground. However, the following might help:
(CAUTION: web sites of a specialist nature are notorious for appearing and disappearing without warning! Don’t be too surprised to get a ‘404’ error sometimes. You can often find a useful site by using a decent search-engine)

Jack Harrison, a highly knowledgeable pilot based in the UK, has prepared a tutorial which beginners will find most useful.

Unisys site.- explanation of Skew-T diagrams, indices etc.


and of course, you will want to put all this knowledge to the test, so I have put a few links to sites with current data below:-

University of Wyoming (European data)
Barcelona University
University of Cologne (choose 'European Radiosoundings')

The three thermodynamic diagrams in common use in operational meteorology

The three plots below represent the main thermodynamic diagrams in use around the world.

image of the three diagrams in use




they are intended to demonstrate the differences in appearance between the three versions, rather than be a strictly accurate representation of that particular diagram. For this reason, I have deliberately left off the labels!

This table will attempt to explain the differences between each diagram, and also explain a little about each printed 'line' on the charts, with the units usually used for the variable displayed.

Line on diagram 

 units used (usually)




 ISOTHERMS (lines of constant 'real' temperature)


 straight & parallel; angled 45deg/slope to right. Equal spacing (linear); angled circa 90deg to Dry Adiabats up to 300hPa.

 straight, parallel & vertical. Equal spacing (i.e. linear).

 straight & parallel; angled 45deg/slope to right. Equal spacing (linear); angled exactly 90deg to Dry Adiabats whole diagram.

 ISOBARS (lines of constant atmospheric pressure)

 hPa (or millibars in old money)

 straight, parallel & horizontal; increased spacing per unit pressure change with altitude.

 straight, parallel & horizontal; increased spacing per unit pressure change with altitude.

 very slightly curved upwards (i.e. convex towards top of diagram). [ not really noticeable for routine use.]; quasi-horizontal; increased spacing per unit pressure change with altitude.

 DRY ADIABATS (lines of constant potential temperature for a dry air sample [ i.e. an unsaturated air parcel path. ] )

 degC (but strictly, and sometimes found, degrees Kelvin are used)

 curved: approx. 45deg to left near 1000 hPa, decreasing to within 10deg of vertical near 100 hPa.

 straight - sharply angled to left - gently convergent to left. (meet at a theoretical point where P=0; T(K)=0)

 straight & parallel; angled 45deg/slope to left; Equal spacing (linear); angled exactly 90deg to Isotherms whole diagram.

 SATURATED ADIABATS (lines of equivalent potential temperature for a saturated [or 'wet'] air parcel path.)

 degC (but strictly, and sometimes found, degrees Kelvin are used)

 curved - but not constant; on right-hand side of diagram, curve starts right and bears left above 400 hPa; on left-hand side, curve starts left and quickly become parallel with Dry Adiabats.

 slightly curved to left with height - curve minimal left-hand side of diagram; a gently increasing left turn on right hand side.

 The only notably curved lines on this diagram: On the right-hand side, starts slightly right before curving left; on left-hand end, curve all to left. On most diagrams, not shown above about -50degC.

 SATURATED HUMIDITY MIXING RATIO (lines of constant saturation mixing ratio with respect to a plane water surface.)

 g/kg ( i.e. ratio of mass of water vapour in given volume to the mass of the dry air in that sample. )

 quasi-straight*; angled to right, at less than 45deg to the vertical; gently convergent to a point well above the top of the diagram. (* for practical work can be regarded as straight & parallel)

 quasi-straight*; angled to left, at less than 20deg to the vertical; gently convergent to a point well above the top of the diagram. (* for practical work can be regarded as straight & parallel)

 quasi-straight*; angled to right at less than 45deg to vertical, i.e. less slope than isotherm. (* for practical work can be regarded as straight & parallel)

… and diagrammatically, I have attempted to highlight the various lines here…. the diagrams are not true representations of each style or necessarily to scale, but are close enough to pick out the major differences listed above:





image of the three diagrams comparing isotherm plots





images of the three diagrams showing the isobars





images of the three diagrams comparing the dry adiabats





images of the three diagrams showing saturated adiabats





images of three diagrams showing the HMR lines

Some definitions/abbreviations:- (not already given elsewhere by hyperlink)


 degrees Celsius

 Dry Adiabats

 describes the 'parcel path' on a thermodynamic diagram when that parcel is unsaturated: i.e. 'dry'. Taken to be 3degC per 1000 ft, or 9.8 degC per km.


 the ratio of the amount of heat absorbed by an object in undergoing a reversible thermodynamic process to the absolute temperature of the object (dQ/T) is defined to be the increase in entropy.


 grams (of water vapour) per kilogram (of dry air)


 hecto Pascal … same as millibars.

 Potential temperature

 is defined as the temperature an air parcel would have, if it were moved vertically (upwards >> decreasing pressure >> expansion; downwards >> increasing pressure >> compression ), from its existing pressure and temperature to a standard level (usually defined to be 1000 hPa). Provided the parcel remains 'dry' or more strictly un-saturated, then the rate of cooling (upward motion), or warming (downward motion) occurs at the Dry Adiabatic Lapse Rate (DALR), which is 9.8 degC per km ( or 3 degC per 1000 feet ). This value is constant. Once a parcel becomes saturated (i.e., the initially un-saturated parcel is lifted until it cools to its dew-point temperature), then the subsequent release of latent heat of vaporization offsets the cooling rate, and the Saturated Adiabatic Lapse Rate (SALR) is consequently less than the DALR. (see Saturated Adiabats)

 Saturated Adiabats

 describes the 'parcel path' on a thermodynamic diagram when that parcel is saturated: Taken to be, very roughly, 1.5degC per 1000 ft, or 5 degC per km IN THE LOWEST FEW HUNDREDS OF MILLIBARS OF THE TROPOSPHERE. Towards a temperature of minus 50degC, tends to the Dry Adiabatic Lapse Rate, as air at lower temperatures holds less and less moisture, hence less offset from the latent heat release.

Advantages and disadvantages

Very much in the eye of the beholder here. The tephigram is regarded as near-perfect for strict thermodynamic calculations, and its large angle between isotherms and dry adiabats renders it the most effective as assessing degrees of stability. However, the skewT also has this property, and tephigrams aren't exactly plentiful on the net. The skewT though has curved dry adiabats, though for short vertical calculation perhaps not a major problem. Indeed, for a parcel undergoing saturation, you are going to have to cross over to the saturated adiabats anyway, which on all the diagrams are curved. The Stüve is clean and simple in that it has the most straight lines, and is perhaps intuitive in that isotherms are vertical against a pressure plot. The disadvantage is that the angle between isotherm and dry adiabat is not as great as the other two versions. Again, constant use would offset this minor difficulty. For those used to the tephigram, the skewT is most like what you are used to.

UK Bank Holidays

Some history ....

Some variations ....

Other background notes ....

A check list of current Bank/Public Holidays

 Holiday  England & Wales  Northern Ireland  Scotland
 New Year's Day (1st January*) BPH BPH BH
 New Year (additional)
(2nd January*)
 St. Patrick
(17th March*)
 Good Friday
 Easter Monday
(1st Monday in May)
(Last Monday in May)
 Battle of the Boyne
(12th July*)
(1st Monday in August)
(last Monday in August)
 Christmas Day
(25th December*)
 Boxing Day
(26th December*)



* - or the following Monday (or Tuesday) if the day falls on a weekend
BPH - Bank & Public Holiday
BH - Bank Holiday
HBC - Holiday by convention ('common law')

Upper Air Meteorology

Some Preliminaries

The mid-latitude westerlies blow in response to the global-scale difference in temperature between colder/high latitude regions and warmer, lower latitude zones. The earth does not receive its heat uniformly over the globe: much more heat is available at equatorial latitudes, than at the poles. Circulations must be in place to distribute the excess on/near the equator to other regions north and south of the tropics: the meandering westerlies, often concentrated into ribbons of fast-moving 'jets' of strong winds play an important part in maintaining the overall global heat balance.

The patterns are not uniform of course, as anyone looking at upper air analyses/forecasts will readily appreciate. Variations occur in response to seasonal changes in hemispheric temperature differences. In winter, with a generally strong north-south temperature gradient, the resultant thermally driven winds are much stronger than in summer, when temperature differences are somewhat less pronounced, due to the summer hemisphere receiving more insolation. The situation is complicated at times in that disturbances running through the long wave pattern in the upper flow can in themselves lead to advection of warm air polewards, and cold air equator-wards, offsetting the temperature differences which produced the broad-scale westerlies in the first place! There are also variations when the conservation of absolute vorticity theorems are applied, which govern the shape/propagation of waves in the upper westerlies. These complex 'feedback loops' are a common feature in meteorology.

The long waves - also known as Rossby waves: What produces these?

A pure west to east flow, perhaps implied by the simple model of warm/low latitudes and cold/high latitudes outlined above, is difficult to sustain in the real atmosphere, with orography, and differential thermal capacities of land versus sea playing a major part in causing perturbations in the broad-scale westerly flow. On a mean (e.g. monthly) analysis, and often on individual days, long wave trough/ridge systems can be found.

On average, there are between 3 and 7 such waves in each hemisphere, and considering just for the moment the Northern Hemisphere, there are two that are regarded as semi-permanent features: one roughly downstream of the North American Rockies, and another downwind of the Himalya range. The position of these features, plus their semi-permanence, suggest that the massive mountain chains involved are the primary reason for their existence. The mechanism is almost certainly a need to conserve absolute vorticity in a column of the atmosphere that is forced to climb abruptly in this fashion.

The number of long waves is largely influenced by the magnitude of the north/south temperature gradient - when the thermal gradient is weak, typically in summer, then the westerlies are able to meander rather more than in winter and early spring, when the flow is stronger. The number and position of the major (Rossby) long waves is very important in the study of hemispheric weather patterns, and even slight changes in amplitude/longitudinal position can have major repercussions thousands of kilometres downwind. 

For more information on the long/Rossby waves, see here.

How and why does the pressure change in the upper air?

Imagine the atmosphere consisting of an infinite number of slices in the vertical. As heat is pumped into each slice, by whatever method (advection from somewhere else; heating from below etc.), that slice expands (gains energy), and as well as expansion sideways, and downwards (against a net expansion upwards by the layer below), there is a general expansion upwards. The definition of atmospheric pressure is the force per unit area due to the weight of the atmosphere above any defined point. Thus, for any level in the atmosphere, if heat is supplied to a column of these infinite number of slices, the whole column expands effectively upwards, and because more of the atmosphere is now above any one point, the pressure at that point increases.(There must be a net expansion upwards, because the earth's surface forms an effective block to net downward expansion.) The opposite occurs for cooling of a column.

Why do meteorological charts show contour heights etc., and not isobars?

In meteorology, we don't take a fixed height, and measure the pressure of that height (though in the early days of upper air meteorology, that was in fact done), but instead define a pressure level ( 850 hPa, 700 hPa, 500 hPa etc.), and find the height (amsl) at which that pressure value is found. There are several reasons why this is so: It was easier when balloon sounding using pressure sensors was the only method of upper air measurement to note the height at which pressure levels were found; and in computation of upper dynamics (e.g. wind vectors), by using this scheme you can eliminate density from the equations ... an obvious variable. There are also advantages when relating the upper air patterns to the readings on a pressure-altimeter.).

On a constant pressure level map then (say 500 hPa), you will have heights plotted as, for example 5340, 5432, 5286, 5567 metres etc., and then contours are drawn (just like isobars, which in fact they are), and so we get to the often smoothly flowing patterns with ridges/troughs etc., familiar to those used to MSLP charts of isobaric plots.


                                Example of 500 hPa product

What is the significance of high versus low contours?

From the first paragraph, in cold air the 'column' of the infinite number of slices collapses downwards, and in warm air, the 'column' expands upwards: So, in summary: a cold troposphere is associated with low contour values. a warm troposphere is associated with high contour values. And further, as a very rough/first approximation: higher contour values are associated with warmer weather; lower contour values with colder weather, or Polar/low contours vs. Sub-Tropical/high contours. (This doesn't always hold true right at the bottom of the atmosphere of course: it is not unusual for cold, low level air to undercut warmer air aloft with high contour values, and vice-versa.)

Why do we get zones of very strong winds aloft/jetstreams?

The gradient of temperature from cold/low contour areas to warm/high contour areas tends not to be gentle in mid-latitudes, but often abrupt for a variety of reasons (continental/ocean thermal contrasts; convergence of airstreams etc.), and temperature contrasts are enhanced from time to time - drawn on our maps as fronts. It is important to remember, that although we show fronts as surface features, they extend right through the Troposphere, as a sharp contrast in temperatures in the horizontal, and this gives rise to the strong winds aloft, as I shall try to explain below.

The effect of these temperature contrasts is cumulative. Therefore at lower levels, say around 850 hPa, the temperature differences across a frontal surface, which then lead to pressure differences (see opening paragraphs), give winds say around 30 knots. However, by the time we get to 400 or 300 hPa, the warmer column of air will have a greater cumulative expansion that the colder column of air, and the consequent pressure difference will be greater, hence the resultant wind will be stronger: typically at these levels 70-110 knots, and just below the Tropopause (top of the troposphere/"weather zone"), maybe up to 150 knots or more.

Only where you get these marked thermal contrasts do you get the potential for strong winds aloft, and so, jetstreams are associated with active frontal systems in the mid-latitudes. We call the jet associated with the Polar Front, the discontinuity between ex-Polar air and ex-SubTropical air, the Polar Front Jet (PFJ)

Below is a simplified diagram of the relationship of the PFJ to a classical/mid-latitude frontal system in the Northern Hemisphere. Note that the 'arrangement' also applies in the Southern Hemisphere, but it would be inverted.

Why does the jet vary in strength/direction etc?

The PFJ lengthens/shortens; weakens/strengthens etc., in response to complex changes in the structure of the related air mass boundaries. Jet strengths are usually strongest across the Atlantic during the autumn (as the first signs of polar outbreaks collide with still warm sub-Tropical air masses to the south), and early spring (as the northern air is still cold after the hemispheric winter, but the tropics/sub-tropics start to warm in response to the increasing insolation further south).

Many irregularities occur along the jet length: There are feedback mechanisms at play, whereby a developing cyclonic system can both temporarily strengthen, and eventually decay the vigour of a jet. In general terms again:

Shorter wave-length trough/ridge systems are also found on a day-to-day basis running mostly through the very long wave pattern: the number and speed of translation governed by synoptic-scale balances in development/baroclinic instabilities: 

What is the significance of the position of the jet stream?

The position of the PFJ governs the overall weather type at all seasons of the year. If it sweeps from SW to NE between Iceland and Scotland, then the BI/NW maritime Europe is generally mild/windy with occasional warm anticyclonic periods; If it comes due west to east around 55N, then a stormy run of weather, with frequent, but well distributed rainfall; if it comes 50 N or below, then generally a cold or cool type, with storms running along the Channel or over northern France etc., with wintry/or cold weather to the north. Further, the wave patterns in the upper westerlies, in very large measure, govern weather events on a variety of time scales ranging from a day to weeks, or perhaps even months.

Why does the jet stream change?

The position/orientation of the jetstream, and its strength/continuity is governed by such things as the distribution of Pacific SST anomalies; Pacific/North American temperature constrasts; injection of energy from active tropical disturbances etc. (These are deeper matters which are inappropriate for this article.) Why the PFJ in some years runs well to the north (mild winters/potentially very warm summers) or well to the south (cold winters/unsettled summers) or becomes blocked is still a subject of debate. My own opinion is that a primary trigger must be Pacific SST anomalies allied to the strength/depth of the Polar vortex, which in turn is coupled to the Stratospheric flow and the switch from summer to winter circulation type. If you talk to someone else, you will get other answers.

Sometimes, upper air patterns change only slowly - whats going on?

At mid to high latitudes in the upper part of the troposphere (above roughly 5 km ), the mean wind flow exhibits a broadly west-to-east motion - this applies in both hemispheres, and is a consequence of the global-scale heating differential between the cold/polar regions and the warm/tropical latitudes, coupled to the deflection of air on a spinning sphere: the earth.

On many occasions, particularly in mid-latitude/temperate zone regions, the flow is directed more or less directly from west to east, crossing few latitude zones within the same longitude range: this is a 'highly zonal' type - any short-wave disturbances embedded in the flow will be carried quickly along and the weather is ever-changing as a succession of frontal systems, interspersed with transient ridge conditions cross any one point.

diagram of a zonal jet arrangement
diagram of a zonal jet arrangement


However, on both average (e.g. monthly) pressure maps and on individual days, long-wave trough/ridge patterns can be found - some having large amplitude, i.e. the airflow meanders a long way north and south around the loops of the pattern, crossing many parallels of latitude in a relatively limited longitudinal range: a 'meridional' type; Usually, some west-to-east progression of the looped pattern can be seen over a 24hr period, and the associated surface weather type changes, albeit more slowly than the zonal type described earlier.

diagram of a meridional pattern


However, if the 'loops' in the pattern become locked in one geographical area, then depending where you are in relation to the upper flow, the associated surface patterns are often little changed from one day to another, and in extreme cases, from one week to another - the pattern is said to be 'blocked'. Some examples of blocked patterns are shown below:

Omega and diffluent blocks

In, and just to the east of a slow-moving trough in the upper flow, the surface weather will tend to be of a low pressure/convective/showery type, and perhaps cool for the time of year (but not necessarily); In, and just to the east of a static ridge in the flow, the surface pressure will tend to be high, with settled conditions lasting until the block is destroyed. This latter case is responsible (over land) for prolonged dry/hot weather in summer, but cold/sometimes grey conditions in winter, and considerable pollution build-up can occur at all seasons due to the stagnation of the lower level air and high air-mass stability encountered.

What's all the jargon associated with upper air features?

The flow in the upper Troposphere, roughly above 500 hPa / 18000 ft / 5.5km is much smoother and behaves like a vast river, not of water, but of air. As with rivers, the flow can take up a variety of patterns; from swift/straight to slow/meander and variations in-between. When it is a swift and straight flow from a largely westerly point to a largely easterly point, this is known as ZONAL flow (because the flow crosses many time/longitude zones in a short time), and if the flow is directly from west to east, this is a HIGHLY ZONAL flow. Disturbances/short wave upper troughs embedded in this flow (and the attendant surface disturbances below 500 hPa), move quickly from west to east. When the flow buckles into large amplitude meanders, the flow is said to be MERIDIONAL (because the net movement is north/south along meridians of longitude over time), and blocked situations are said to be HIGHLY MERIDIONAL: this sort of situation gives us either persistent wet weather (we are locked into the trough portion of the loop), or persistent dry weather (we are locked into the ridge portion of the loop). Large amplitude troughs/ridges forming part of the upper flow can move steadily eastwards (progressive motion), stay sensibly stationary (blocked), or move backwards against the general trend (retrogress), though the latter case is usually accompanied by disruption of a trough/spawning of a high cell, where the general flow gives birth to a closed low/high which then lives a life away from the general upper flow, or alternatively, the flow is diverted around the low/high (bifurcation).

How are the upper features related to surface weather patterns?

Broadly speaking, msl low pressure is found in and forward of a marked upper trough, and msl high pressure is found in and forward of a marked upper ridge. These troughs and ridges need not be large amplitude affairs that sweep over hundreds of kilometres of the globe: ~~ A minor RIDGE in the upper flow over a synoptically 'small' area such as southern Britain, or northwest Germany, can act to dampen any tendency to convection: the ridge will be an area of descent, which will produce subsidence warming/drying of the air, and act against vigorous shower activity. ~~ A minor TROUGH, (or short-wave length trough, 'shortwave' for short!) perhaps only just detectable on a chart, can be the cause of much heartbreak for forecasters! The trough may be a slight hesitation in the upper flow, but combined with adequate moisture, and perhaps enhanced by land heating, can lead to notably more intensive shower activity, and operational forecasters are always on the look-out for such irritations at such levels as 500 hPa or 300 hPa. Operational NWP models don't always capture the vigour of such features correctly - perhaps not sharpening them sufficiently, or slightly mis-locating them, and human-adjustments, coupled with analysis of satellite imagery (particularly water vapour imagery), can be very beneficial. The shape of the trough/ridge is also important:

Troughs can be:

Ridges can be:

Diagram of various trough/ridge types

Water Vapour Imagery

(This note wouldn't pass muster as an answer to a question in a degree-atmospheric physics paper, so don't use it a such! It's simply an attempt to put into words for the intelligent layman the basic concepts of some powerful principles used in current operational and research meteorology)

The advent of high-quality water-vapour imagery (WVI), received from geostationary satellites, offer meteorologists a "new" way of visualising developments in the atmosphere as they occur ('real time').

At its most basic, viewing areas of moist (usually presented as white, or very light-grey shades) as distinct from dry (dark/black areas), is obviously useful. [ However, it is important to remember that the sensors are detecting radiance from a broad (altitude) band, with diffuse boundaries between 400 and 200hPa (circa 7 to 11km)]. Now, not only can tangible clouds of water droplets or ice crystals be observed and followed, but also the potentially cloudy areas can be observed - at least in the upper troposphere.

Once frequent images were obtained (at least hourly), then these moist and dry regions could be followed. One use is to monitor the advection (movement) of dry (dark) areas over the top of humid and potentially unstable lower tropospheric air-masses (detected by other traditional synoptic methods). Such situations can lead to "explosive" convective activity.

In the field of aviation meteorology, the sharp discontinuity (light vs. dark/black) between dry & moist regions are associated with jet-stream shear; not only useful to track the position of the jet of course, but also areas of potential clear air turbulence (CAT).

However, allied to theories associated with vorticity-forced cyclonic development, looped WVI really comes into its own.

In adiabatic, frictionless flow, potential vorticity (PV) can be defined as the product of two variables:
the [ absolute vorticity ] of the air (on an isentropic* surface) and ...
its [ static stability ];
or more loosely, a measure of the tendency of air to 'twist' as it flows downstream and the degree of 'damping' to vertical motion. The resultant variable is strictly called 'Isentropic Potential Vorticity' or IPV.
(* isentropic: a thermodynamic process not involving change of entropy - crudely equivalent to adiabatic in meaning. )

Wherever these two quantities have a high & positive value, then PV will be high.

So, how does all this fit in with the use of WVI?

Stratospheric air in association with a powerful jet (and less dramatically with short-wave troughs), have high stability (see the definition relating to the stratosphere), and high, positive absolute vorticity - provided the air is being sampled on the cold side of the strongest flow aloft, or in the area in or just ahead of the axis of an upper trough.

Stratospheric air is also dry. WVI detects such dry regions very efficiently. If we 'see' a dark/dry slot in the imagery, then we can infer that stratospheric air has descended to lower levels, dominating the 7 to 11km column that the satellite is monitoring; these must be areas of high (stratospheric) PV - and "PV anomalies" (discrete dark slots associated with marked cyclonic development) can be used to monitor the vigour, location, phasing etc., of developmental regions.

Why should such discrete areas of dry, stratospheric air be so important? One way of trying to understand what it going on is to understand that if stratospheric air has descended, (revealed as the dark slot on WVI), then there must be motions causing this descent, and where air is going sharply down, there must be air going up! The greater the 'vigour' of the air going down (sharply darkening dry slot over a relatively small area), the greater must be the effect of the air going up - leading to thick cloud cover, rapidly falling pressure, and associated developments involved in cyclonic development.

A more rigorous view, at least in terms of IPV development theory, is to regard well-formed upper-level IPV anomalies as the initiator of cyclonic circulations in the column below it. [(This is analogous to the effect you get when you stir the surface of an initially still bath of water - a small circulatory motion at the surface of the bath will eventually translate to motion deeper within the water: thus it is with the atmosphere (only another fluid after all)].
If this 'stirring' at high levels (the PV anomaly) is coincident with a low-level marked baroclinic zone (i.e. a classical frontal boundary), then, other factors being allowed for, the induced circulation will lead to poleward warm-air advection (and thus falling pressure), equator-ward cold-air advection, and atmospheric 'development' (formation or enhancement of low pressure areas) will occur. (On operational charts, the warm front moves north, the cold front sweeps south, (reverse directions if viewing in the Southern Hemisphere) and the wave depression deepens.)

If the PV forcing is marked and is co-incident with sharp baroclinicity (large gradient of temperature with horizontal distance), then 'explosive deepening' of a depression will occur.

Modern NWP diagnostics packages can output 'pseudo' WVI - this is overlaid on 'real' WVI and any mismatch in model analyses readily seen and allowed for. Forecasters have become used to using water vapour imagery to diagnose and monitor development in mid-latitudes - a powerful tool indeed.
If you want to know a little more about water vapour imagery, see here.
And a classic example of WVI associated with a major NE Atlantic autumn storm is shown here.

WVI - Some additional notes


 **  Spacecraft sensors integrate radiant heat energy through the column of the atmosphere within the field of view (FOV), NOT from a fixed level.
 **  WV radiances are detected using Channel 10, which is tuned to 6.7 microns, and allows a resolution of around 5 km (Meteosat & GOES) at the sub-satellite point (SSP)
 **  Radiation "seen" is biased towards nearest WV layer in the FOV, or the underlying (relatively warm) surface, if no cloud or very dry air is present.
 **  Maximum response: 80% of radiation from 620 to 240 mbar, with notional maxima of response for "standard atmosphere" at around 400 mbar. (However, this 'fixed' level is mis-leading: it is best to remember that the radiation is integrated through a layer having diffuse upper & lower bounds.)
 BLACK  : "warm" (low altitude / near-surface radiation source; small amounts of water vapour (in the sub-satellite column) in the FOV)
 DARK GREY  : "cool" (slightly colder than low altitude - typical of AC / thin, low AS clouds, therefore typical of low or mid - troposphere around or just below the level of non-divergence (LND)
 LIGHT GREY  : "cold" ( typical of thicker AS levels, or tops of NS, or thinner (but low) CI/CS OR areas of high humidity [ but no clouds evident ]-- around and just above the LND, and at the theoretical maxima of radiation that the sensor is responsive to.
 NEAR WHITE  : "very cold" ( typical of thick / high CI/CS)
 BRIGHT WHITE  : typical of CB clusters which poke out above the general moist / cloudy levels. (Useful for detection of MCS)
 ** Importance of WV imagery lie not just as an instantaneous image, but with loops over time.
 Blacker-with-time: (implies) >  warming / lowering WV content.
   Either (or combination of):
   (a): Descending air or
   (b): Advection of drier air or
   (c): Clouds being replaced by non-cloudy air
 Whiter-with-time: (implies) >  cooling / increasing WV content.
   Either (or combination of):
   (a): Ascending air or
   (b): Advection of moister air or
   (c): Non-cloudy zones turning cloudy

( : but slight changes between moist / non-cloudy and moist / cloudy zones cannot be inferred from WVI, especially at CI levels.)

 **  WVI patterns take up the character of the flow they are in. In the FOV, a WV signature, especially in developmental situations, will almost certainly not be at one level. It is therefore a "tracer" of horizontal and vertical atmospheric motions. Even at jet-stream altitudes, where we tend to assign a 'fixed' level to the jet core, the jet often wanders up and down through several hundreds of metres, and where marked cross-contour flow is involved, then at the entrances and exits to such jets, changes in altitude of at least 1500 m are not unusual.
 **  There are often abrupt changes between "black" and "light-grey" areas: i.e. as between dry / descending and moist / ascending zones. [ US & Canadian Met. sources point to these regions as most likely ones for Clear Air Turbulence (CAT) - indeed they quote a successful detection rate of some 80 % for CAT in these areas. The sharper the boundary, the more likely is CAT to be found, though no inference (as yet detected) can be made as to the severity of the CAT. ]
 **  Very useful in detecting vorticity patterns - which, with attendant moisture indicators, can give useful clues to development.

   **  Beginning of dry slot ("dry intrusion") cyclonically curved into the vortex centre -- associated centre is "closing / cutting off"
   **  Broadening / blacker dry slot -- indicates development is still in place -- maximum cyclogenesis about to begin. (Intrusion of high tropospheric / lower stratospheric air)
   **  If dry slot is ill-defined / not-warming (darkening) with time -- development of parent system possibly arrested early.
   **  System is weakening when dry / descending air totally encircles the vortex.
[ Sometimes, the dry slot encircles the vortex several times.]
   **  Well defined "hook" on dry imagery: defines classic PVA areas -- indicating strong development.
   **  Short-wave troughs (SWT): active troughs, generating "+SHRA/+RA/TS" rather than just an enhancement of the general shower regime - often marked by a noticeable dry-line (or abrupt change from 'moist' (white) to 'dry' (dark grey/black); found either along or just to the rear of the lower tropospheric trough axis (i.e. ~700hPa).



 standard analysis at 06Z  This is the conventional surface analysis with the familiar fronts, lows etc. After this point, the depression deepened smartly (but NOT rapidly), to a value of 988 hPa (or mbar) in the Oslo area of Norway some 36 hrs later. 'A' indicates the area influenced by the warm-air conveyor ahead of the driving upper trough - showing how warm, humid air is thrown well ahead of the developing centre to produce the 'white' area seen on the WV image: 'B' indicates the cold low-level air cutting in behind the development.
 image of developing dry slot here  A: 'Dry' slot tucking in behind developing depression: darkening of the dry intrusion indicated that development (falling surface pressure) was to be expected.
 B: High water vapour content detected in the 'conveyor' streams associated with the baroclinic development - leading to thick cloud & rain/snow etc.
 C: The Polar Front Jet lay just to the north of the black / white discontinuity along this clear-cut edge ... however note comments below, as the PFJ can only be placed like this in certain circumstances.

   **  Beware placing jetstreams along WVI boundaries (see introductory note). Water vapour patterns trace the level within which the WV maxima lies, which varies with both space [ all three dimensions ] and time -- e.g. the conveyors associated with extra-tropical depressions.
   **  Some general rules:
> a strong / non-buckling jet can be located by the sharp, well-defined poleward cloud edge of an associated cirrus shield (but see comments above re: placement of jet).
> the stronger the jet -- the better the definition (the greater the ageostrophic forces).
> the location of the dark / descending zone relative to the jet depends on the curvature of the flow:
>> CYCLONIC: darkest zone equatorwards of jet core
>> ANTICYCLONIC: dark zone is poleward of jet core.
(NB: however, I have not found this to be a very true statement, and some reservations are held about this.)
> jet streaks can be traced at the head of a dark zone which is known (from independent analysis) to be within the jet region.
   **  For release of Convective instability -- decrease of ThetaW with height -- need dry, mid-level air over-running moist low level air. Use (a): WVI loops to trace mid-level moisture and (b): NWP ThetaW fields to trace low-level moist plumes.
   **  Preferred location for maxima of such development is along the leading edge of a dark zone.
   **  Use loop to infer movement of edges of moist zones -- i.e. does movement agree with appropriate NWP frames at the same time -- if not, adjust NWP derived output as required.

 Some other notes that might be useful .....
 **  Use change of "whiteness" to ascertain ascent / development or descent / decay.
 **  Broad troughs often have a sharp WV (dry/moist) boundary immediately to the rear of the axis - i.e. along the line that marks the abrupt reversal of the sign of vertical motion / change of sign of relative vorticity.
 **  Sub-tropical jets (STJ): usually show a marked edge which enables the translation / shape change to be monitored, thus confirming, or amending NWP ideas.
 **  NWP relative humidity fields can be cross-checked with WVI with high degree of correlation. Any deviations due to the model 'atmosphere' deviating from the "real" atmosphere can be seen and allowed for.
 **  Sharp WVI discontinuity along jets caused by sensing of lower stratospheric air (low WV content) and dry / descending polar maritime air.


WVI example

This led to high rainfall totals and 'damaging' winds in the NE Atlantic/NW European region in late autumn 2002 (The "Prestige" Storm).


Water vapour example & 300 hPa



 A  Dry slot of rapidly descending stratospheric air to the immediate rearward of the synoptic feature. Strongest gusts occurred as the leading edge of this feature encountered landfall over northern Portugal/NW Spain.
 B  Area of broad-scale ascent associated with the warm conveyor of the developing low.
 Other notes:  This magnificent image in the 'water-vapour' channel from the ESA/EuMetSat 'Meteosat' platform neatly captures the major development that occurred through the 13th November 2002, which in the next few hours was to produce high-winds and heavy rain over NW Iberia, with gusts reported to at least 60 knots. One ship in Sea Area 'FitzRoy' reported a mean wind of 65 knots as the leading edge of the 'dry slot' swung east. (Incidentally, this storm was responsible for the severe damage to the Tanker "Prestige", which subsequently broke up on the 19th, with consequent loss of its contents - a major pollution episode ensued.)

During the evening/night to come, the storm swept NE to bring heavy rainfall (with flooding) to many parts of NW Europe (including Britain), along with high winds. Squally winds were reported for the west, and later the northern Departments of France, and in the period 14/0300 to 14/0900 UTC, gusts of at least 60 knots were reported just inland of the central and eastern English Channel, with 'on-coast' gusts to at least 70 knots (possibly more in isolated spots).


 1  Note the implied accumulation of air just below the jet-core as the high-speed air on the NW'ly jet has to slow abruptly as it encounter the weaker & markedly cyclonically curved gradient in the trough axis: the air has to spread out and descend, encouraging the downward penetration of high-PV/stratospheric air. This action brings air having a high velocity to low altitudes, mixing with the already 'perky' low-atmosphere flow.
 2  If the air is going down sharply (1), then it must be going up somewhere, and this region, just forward of the highly diffluent trough and associated PVA-max region, is just one such, where heavy rainfall was reported.
 3  The equatorward 'tail' of the cirrus outflow shield, curling back around the dry intrusion, where in former times a classical occlusion might have been drawn, but is really an upper-air feature, and not necessarily frontal either. Various workers have found that the strongest gusts (possibly tornadic-enhanced) occur near/just in front of the 'tail' of this feature in the dark slot.