Singularities developed for the British Isles

Climatologists have always been alive to the fact that similar weather patterns/types occur at certain times of the year with varying degrees of regularity - an annual 'singularity'. For a while, before dynamical methods of long-range forecasting were used, singularities were very popular, though controversial.

1. CEP Brooks
2. HH Lamb
3. Alexander Buchan
4. Barry and Perry (after Lamb as above)
5. Warmest days/periods
6. Coldest days/periods

IMPORTANT: Remember that many of the below were developed mainly using data for the latter half of the 19th, and first half of the 20th centuries. The climate has undoubtedly changed, whatever the reason! Use these with much caution.

[ The term 'singularity' was apparently coined (according to HH Lamb) by A. Schmauss in Berlin in 1938. In an article entitled "Synoptische Singularitäten", Schmauss demonstrated that the curves (in graphical terms) of meteorological elements (such as temperature), show singular points in the mathematical sense (dy/dx=0), and apparently the name came quickly into use for this reason.]

1. C.E.P. Brooks 

(ref: 1946: "Annual recurrences of weather; 'singularities'."; Weather, London, 1. pp. 107-130)

Period studied: 1889 - 1940
Principal groupings:
(1) October to early February, stormy periods with minor anticyclonic interludes.
(2) February to May, cold waves associated with northeasterly winds.
(3) The summer period of alternating cool fresh northwesterly and warm, sultry southwesterly winds.
(4) September and early October, spells of anticyclonic conditions and late "summers".

 Guide  Type  Overall period  Notional 'peak'  Frequency in 52 yr.
 Early January  "Stormy"  Jan 5th - Jan 17th  Jan 8th  45/52=87%
 Mid January  "Anticyclonic"  Jan 18th - Jan 24th  Jan 20th/21st  45/52=87%
 Late January  "Stormy"  Jan 24th - Feb 1st  Jan 31st  44/52=85%
 Early February  "Anticyclonic"  Feb 8th - Feb 16th  Feb 13th  29/52=56%
 Late February  "Cold spell"  Feb 21st - Feb 25th  Feb 22nd  22/52=42%
 Late February & early March  "Stormy"  Feb 26th - Mar 9th  Mar 1st  46/52=88%
 Mid-March  "Anticyclonic"  Mar 12th - Mar 19th  Mar 13th/14th  27/52=52%
 Late March  "Stormy"  Mar 24th - Mar 31st  Mar 28th  35/52=67%
 Mid April  "Stormy"  Apr 10th - Apr 15th  Apr 14th  37/52=71%
 Late April  "Unsettled"  Apr 23rd - Apr 26th  Apr 25th  27/52=52%
 June  "Summer monsoon"  Jun 1st - Jun 21st  n/a  40/52=77%
 July  "Warm period"  Jul 10th - Jul 24th  n/a  n/a
 Late August  "Stormy"  Aug 20th - Aug 30th  Aug 28th  35/52=67%
 Early September  "Anticyclonic"  Sep 1st - Sep 17th  Sep 10th  43/52=83%
 Mid September  "Stormy"  Sep 17th - Sep 24th  Sep 20th  31/52=60%
 Early October  "Stormy"  Oct 5th - Oct 12th  Oct 8th/9th  35/52=67%
 Mid October  "Anticyclonic"  Oct 16th - Oct 20th  Oct 19th  35/52=67%
 Late October & early November  "Stormy"  Oct 24th - Nov 13th  Oct 29th, Nov 9th, Nov 12th  52/52=100%
 Mid November  "Anticyclonic"  Nov 15th - Nov 21st  Nov 18th, 20th  34/52=65%
 Late November & early December  "Stormy"  Nov 24th - Dec 14th  Nov 25th, Dec 9th  51/52=98%
 Pre-Christmas  "Anticyclonic"  Dec 18th - Dec 24th  Dec 19th - 21st  29/52=56%
 Post-Christmas  "Stormy"  Dec 25th - Jan 1st  Dec 28th  43/52=83%

2. Singularities affecting the British Isles (after HH Lamb)

Hubert Lamb identified five 'natural' seasons, which don't perfectly correspond with calendar months. They are defined thus:-

 High summer:  18th June to 9th September
 Autumn:  10th September to 19th November
 Early winter:  20th November to 19th January
 Late winter:  20th January to 29th March
 Spring:  30th March to 17th June

According to Lamb, these are the important characteristics of each of the five 'seasons' as defined above.
HIGH SUMMER [18th June - 9th September]:
High frequency (using lengthy datasets) of similar weather types ... i.e. dry/warm or wet/cool ... in the period analysed by Lamb (late 19th & first-half 20th centuries), years with cyclonic/wet sequences are twice as likely as persistently anticyclonic (A) types ... in other words, in 'high summer', you are more likely to experience an 'unsettled' summer than a quiet, fine one; a common experience! ... depressions tend to be shallow, moving less rapidly than in winter (weak driving polar front jetstream [PFJ]) ... can remain slow-moving for many days ... the fine (A type) summers tend to result from 'offshoots' of the Azores high moving east across southern Britain / northern France ... frontal systems being weak and only of passing concern ... mainly affecting (in their most active phase) the north-west of these islands ... there is a tendency for high cells to move east (progression) and to end up over Germany or Denmark, introducing a warm or very warm S (southerly, Tc) type across central and SE Britain ... such periods are ended by troughs approaching from the Atlantic ... perhaps with a warm-plume advection type ahead ... thunderstorms & heavy rain etc. ... however, occasionally the PFJ is displaced much further south & somewhat stronger than 'summer-time' average (e.g. 2007) ... frequent depressions of an active nature ... plenty of rain ... temperatures near or below average.

AUTUMN [10th September - 19th November]:
First week (10th September onwards) driest of year on average, especially central & eastern areas ... but some years show tendency to a mid-September cyclonic / wet spell ... localised heavy (thundery) rainfall resulting from slow-moving areas of low pressure ... early / mid October can be 'two-faced': often 'unsettled' with rain/showers, but occasionally can give rise to anticyclonic / warm conditions, with southerly [S] weather types lifting temperatures to 'near-record' levels ... between 23rd October & 11th November, strong signal for wet/stormy weather with a sharp reduction in A-type weather; sharp reduction this type last week of October c.f. first week.

EARLY WINTER [20th November - 19th January]:
Lengthy spells of any weather type less likely than 'high summer' or 'autumn' ... any extended types tend to be westerly / zonal /mild in nature ... unusual for type established early in this period to persist to end (or into 'Late winter') ... tendency to a post-Christmas 'stormy' period [recent notable examples 1997, 1998 & 1999] ... in former times, if these were to the south, perhaps associated with significant snow [ but not so much in recent years ] ... circulation type around 'New Year' some guide to type later in winter ... significant correlation [Lamb] between cold weather late December / early January & cold winters overall.

LATE WINTER [20th January - 29th March]:
In 50% cases, 'lengthy spells' evident, but no preference to one type or the other (i.e. as between zonal-mild-windy & blocked/cold) ... coldest winters when persistent blocking highs Scandinavia/Iceland regions (Pc/Am types) ... cold, northerly types tend not to last much more than 5-7 days (and in recent years, not particularly cold anyway) ... mildest winters from high zonality [high NAOI] (Tm or rPm types) ... strong correlation between wet winters & mild winters ... dry/anticyclonic mid-March conditions often extend (persistence of type) for 2 weeks or more ... with right conditions, droughts can be prevalent & potentially severe, depending upon precipitation totals earlier.

SPRING [30th March - 17th June]:
Least likely to have extended 'runs' of similar weather ... changeability from day-to-day marked ... N & E types fairly frequent (high pressure anywhere from NW to NE), especially second-half April ... 'late' snowfalls not unusual (even in these days) due northerly outbreaks / polar lows or troughs ... May often brings quiet/dry with increasing chance of extended periods A-type.
[ Based on "The Climate of the British Isles", Longman, 1976 & Lamb's original work.]

  5th - 11th January
 Westerly type very frequent, esp on 8th. Anticyclonic type very infrequent. The mild/oceanic air masses do not penetrate into central Europe as often here as with earlier cyclonic singularities of 26th Nov, 28th Dec, and (later) 1st Feb.


  20th - 23rd January
 Dry/frosty in Europe. Anticyclonic, Southerly & Easterly types frequent in Britain.


  27th January - 3rd February
  Lows pass into North & Central Europe from Atlantic. The first lows of the series commonly approach England from the South-West (N.B. liability for freezing rain with advance of mild air after some days of frost). Anticyclonic type rare.


  8th - 13th February
 Year's highest frequency of winter sports conditions in Alps & Southern Germany (a second peak follows in central & Eastern Europe 19th to 24th Feb.). Record frosts in cold winters. Fogs common inland in Britain when high forms in maritime air: Anticyclonic, Southerly & Easterly types common in Britain.


  26th February - 9th March
  Northerly outbreak from Norwegian Sea and cyclonic weather over N. Sea and surrounding lands. Cyclonic type maximum 26th February - 2nd March; Northerly type maximum 28th February - 3rd March.


  12th - 22nd March
 Very quiet weather - large diurnal range of temperatures. Anticyclonic, Northerly & Easterly types common.


  28th March - 1st April
  First of a series of northerly outbreaks from Norwegian Sea, with Cyclonic conditions in West, Central Europe and the Mediterranean. These outbreaks commonly occur about 4 times in April & early May, punctuated by warm, quiet anticyclonic intervals.


  12th - 19th April
 The most regular of the successive cold stormy spells in April, (see above). Northerly type very common about 17th - 19th April. Atlantic highs liable to affect West & South-West Britain.


  29th April - 16th May
  Northerly type very common, but often anticyclonic in west. Easterly type common; and Westerly type less common than at any time of the year.


  21st - 31st May
  Anticyclonic & Southerly types very common. Fine, dry period.


  1st - 4th June & 12th - 14th June
  Atlantic lows move across UK with Cyclonic type very frequent 1st to 4th June. Succesive lows travel rather further north and Westerly types become more common. The 2nd monsoonal wave reaches Germany around 12th - 14th June and the 3rd wave 18th - 22nd June (see below) brings westerlies in across the British Isles as the commonest type once more. These events are punctuated by recoveries of the anticyclonic tendency associated with advances of the Azores High. Thunder is very common over continent 3rd - 5th June as the cool oceanic air moves in over the hot land.


  5th - 11th June
 Anticyclonic intervals to June monsoon (see above). Anticyclonic type very common around 7th.


  18th - 22nd June (and following fortnight)
 The most regular monsoonal invasion of the continent by cool, oceanic air from the west and NW leading to thunder and cyclonic activity. The S & SW of Britain often remaining anticyclonic: But over most of Britain, Westerly type is very common, especially around 20th. This 3rd wave of the monsoon reaches Germany about 24th - 26th June. Later waves are usually less pronounced and occur with varying regularity in early, mid & late July and mid-August.


  23rd - 30th July & following week
  Stagnant lows common. Cyclonic type very frequent, especially around 4th - 8th August. Westerly & North-Westerly types also commonly occur. The mean temperature curve reaches its seasonal peak in most of the British Isles in the week 30th July - 6th August. (See also below: re "when do the warmest days occur?") Following the last singularity, late-summer anticyclones give Anticyclonic as common type around 15th August before the first storms of Autumn.


  16th - 30th August
  Depressions passing in high latitudes frequently produce cold N'ly outbreaks in the Norwegian Sea. In the British Isles, the weather is commonly Westerly or Cyclonic types, though Anticyclonic type may persist in the south.


  5th - 30th September
  Peak dates for Anticyclonic type in Britain are 7th - 10th, 16th - 21st and 30th Sep. The highs pass across the British Isles into Europe and Siberia. More cyclonic and Southerly type weather affects Britain after each 'High' system has passed away east into the continent. Cyclonic type is quite common around 24th Sep, and in cyclonic autumns, when the anticyclones keep further south, the period around 24th Sep is particularly liable to gales with vigorous lows passing over or close to the country. There is an increasing cyclonic trend during October in most years, but the frequency of anticyclones remains high early in the month, 1st to 10th Oct.


  24th October - 13th November
  Stormy Cyclonic type frequent, especially in 2 peak periods 26th - 29th October and 9th - 12th Nov. In the interval there is a tendency to fair, mild Southerly type weather with high pressure over the continent. Anticyclonic type is very uncommon in Britain 26th - 28th Oct. Westerly types become increasingly frequent througout the period, as successive lows pass further north, reaching a peak on 8th - 12th Nov. Other common types in this period are: - Easterly type around 1st/2nd Nov; Northerly type around 25th/26th Oct. This last marks the first and most prominent autumn N'ly outbreak - which is followed in some years by rising pressure in Scandinavia with continental anticyclones common around 30th Oct - 6th Nov.


  15th - 24th November
  Brief period, especially about 17th to 19th Nov. when Anticyclonic type is common, the anticyclones forming over Britain & W. Europe in maritime air. Westerly & Cyclonic types are uncommon at this time.


  25th November - 10th December
  Cyclonic period associated with progressive intensification of Atlantic westerlies and mobile lows, rather than with stagnant cyclonic situations over Europe. Waves of mild air spread east across Britain until blocked and lifted by the monsoonal development of stagnant cold air in the heart of the Eurasian continent. Cyclonic type is very common 25th - 29th November & 6th - 12th Dec. Westerly & North-Westerly types common throughout the period.


  19th - 23rd December
 Quiet, frosty weather on the European lowlands. Southerly type very common in Britain with Anticyclonic and Easterly types also quite frequent. Gales still frequent in Scotland. A preliminary, well-marked anticyclonic spell about 12th Dec commonly affects only East & Northern Europe.


  25th - 31st December
  Cyclonic & Westerly types common in Britain, carried by a second surge into central Europe.



(NB: strictly based on records across south-east Scotland, and specifically Edinburgh.)

Nine periods where put forward by Alexander Buchan in 1867 on the basis of 50 years of observations (though some texts quote only 1857-1866), constituting fairly reliable periods of unseasonal cold (6 cases) or warmth (3 cases).
Note that Buchan himself did not claim these as 'singularities' and it is widely accepted that they have little real predictive merit: they are included here for the sake of historical completeness. (**=shows some correspondence with Lamb etc.)
7th - 14th February: COLD**
11th - 14 April: COLD
9th - 14th May: COLD**
29th June - 4th July: COLD/COOL
12th - 15th July: WARM
6th - 11th August: COLD/COOL
12th - 15th August: WARM**
6th - 13th November: COLD
3rd - 14th December: WARM/MILD**

4. BARRY & PERRY (based on / updates by Lamb, as above & later)

( extracted from "The Climate of the British Isles", ed: Chandler & Gregory, chapter author A.H. Perry )
Where the 'type frequency' is=> 60%, then the characteristic is in bold type.

 Period:  Circulation type (Lamb):  Characteristics:  Type frequency (%)
(& significance level):
 20 - 23 January  A,S, & E together  Generally dry and sunny in central & southern England.  50  1890-1950 (about 10 years)
 Year's lowest frequency of C type (10-12%) 24-26 January. (5% level - probably significant)
 12 - 23 March  A,N & E together  Notable rainfall (precipitation?) minimum in central & southern England.  70  1890-1950 (about 10 years)
 12-14 March peak of AC.  35 (1% level - significant)
 12 - 18 May  N type  Annual maximum about these dates;
14-20 May is sunniest week of the year in Ireland.
 30  1873-1961
 21 May -
10 June
 A type  Annual maximum frequency, 40% or more on some days during most of this period; driest weeks of year in Scotland, Ireland: more year-to-year variations in southern half of England.  (5% level - probably significant)  1873-1961
 18 - 22 June  W, NW & A together  Generally dry and sunny in southern England: cloudy & wet in Scotland & Ireland.  70  1890-1950 (about 10 years)
W type frequency 52% on 20 June  (1% level - significant)
 31 July -
4 August
 C type  Sharp peak (replaced by twin maxima around 20 July & mid-August).  35%+ (5% level - probably significant)  1873-1961 var.
 17 August -
2 September
 W & NW together  Wet in most areas.  70  1890-1950 (about 10 years)
 C type  Peaks 19 and 28 August.  30 (5% level - probably significant)
 6 - 19 September  A,N & NW together  Dry, especially east and central England.  80  1873-1961
 C type frequency, >20% between 6-12 September.  (5% level - probably significant)
 5 - 7 October  A type  Slight check to seasonal cooling.  40 (5% level - probably significant)  1890-1950 (about 10 years)
 24 - 31 October  C, E & N types  Great decline to year's minimum frequency of A type (<10%) about 28th - 31st October.  (1% level - significant)
(5% level - probably significant)
 17 - 20 November  A type  Dry, foggy period in central and southern England.  30 (1% level - significant)  1873-1961
 3 - 11 December  W & NW together  Wet and stormy in most areas with 3 - 9 December generally wettest week of year on average.  70  1873-1961
 17 - 21 December  A type  Generally dry, foggy weather.  25  1873-1961



Although the longest day (maximum theoretical incoming solar radiation) occurs around 21st / 22nd June (the Summer Solstice), on average, the warmest days turn up about 4 or 5 weeks after this time, often occurring in early August.

> Using the mean 5-day temperature series for 1961-90 for three inland stations, the highest mean day maxima for Glasgow, Manchester and Heathrow fall in the period 23rd to 27th July.
In fact, with only 0.1degC or so difference, the period 13th July to 7th August is the 'high risk' period for the higher mean maximum temperatures.

This period also covers the warmest nights, which on average occur from 23rd July to 7th August.

The 'lag' beyond the date of the summer solstice is due to a combination of three effects:

1. The thermal inertia of the surfaces over which the air passes - particularly adjacent sea areas: the effect is not unlike the action of a 'night-storage' heater, offsetting the slightly weakening incoming solar radiation (slightly lower altitude of the sun / greater path-length through the atmosphere) and shorter day-lengths.
2. Warmer air can 'hold' a greater amount of water vapour (higher dew-points). In response to the lag (above), the warmest days will tend 'normally' to come after mid-June, and therefore the dew-points tend to be higher late June through to early September. The main effect of this is that night-time minima tend to be higher (water vapour acts as a 'blanket' absorbing / re-radiating outgoing earth-based radiation), so the 'day' starts off with higher temperatures, and given sufficient insolation, the day maxima will respond accordingly.
3. As 'High Summer' (July/August) is often associated with a weakened Polar Jet, cyclonic developments will be weaker, will find more difficulty in penetrating from the Atlantic, and the incidence of blocking is higher. Given the right circumstances, and the fact (from effects above), the land (and seas) will be warmer, this means that advection of warmed air from elsewhere, plus local stagnation of air-mass is much more likely.

>Using the series after JC Webb ('Weather' & 'Journal of Meteorology' - as updated) which lists the highest temperatures known by day (across the UK as a whole), the absolute high temperature of 38.1degC occurred on 10th August (2003) at Kew/Royal Botanic Gardens. [ Many stations across the southeast of England broke the old record (37.1, Cheltenham, 3rd August [1990]) on this day. In particular, 38.5degC was recorded at Faversham, Kent, but there are some doubts surrounding this figure.]
For Scotland only, the highest known/accepted temperature (32.9degC at Greycrook in the Scottish Borders) occurred on the 9th August (2003).

> Using the CET daily series, the period with the highest mean temperature runs from the 2nd week of July to the 2nd week of August. (roughly 10th July to 15th August). Inspecting the graph showing the progress of the mean daily CET record (1961-90), the peak occurs in late July, though with secondary, but slightly lower peaks, 2nd week of July & 2nd week of August.

> For several 'primary' synoptic-reporting stations around the UK, the values and dates of highest maxima are as below (but not updated due to station closure etc.):
GLASGOW: 4th August, 1975 31.2degC (Abbotsinch .. now closed)
MANCHESTER: 2nd August, 1990 33.7degC (Ringway .. now closed)
BIRMINGHAM: 3rd August, 1990 34.9degC (Elmdon .. now closed)
LONDON: 10th August, 2003 37.9degC (Heathrow)
CARDIFF: 3rd August, 1990 33.5degC (Rhoose .. now closed)

> The average maxima around the 'peak' period as noted above are circa 1.0degC higher than those around the 'longest' day.


> For the whole of the UK, using the series after JC Webb (JMet), the lowest absolute values in that series are:
(minus) 27.2degC, which occurred on the 30th December, 1995 at Altnaharra (Highland) and on 10th January, 1982 at Braemar (Grampian).

> > Using the mean daily Central England Temperature (CET) series [ 1961-1990 ], the curve dips to a first winter 'minimum' around the end of December or within the first few days of January, i.e. the 'turn' of the year. There is then a 'false' recovery in mean temperature, before the series minimum is reached in mid-February. In both cases of course, these minima occur after the point of shortest daylength - in the case of the lowest (mean) temperature, a good 7 weeks after this date. The main reason for this marked 'lag' is again a function of the fact that sea temperatures in particular do not reach their lowest values around our islands until well into February or even early March. There are other factors though, such as a greater tendency to northerly or easterly types as the winter progresses and the increasing likelihood of anticyclonic 'blocks' allowing night radiation to become fully effective.