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P

Pa

Pascal - allocated in honour of Blaise Pascal, to a unit of one N/m2, the basic unit of pressure in the SI system.

Parametrisation

Some atmospheric processes are below the grid-scale/wavelength of operational meteorological computer models and cannot be handled explicitly by such schemes - for example individual showers, which are not only important for local weather, but have a feedback effect within the atmosphere that needs to be included in the NWP routines to maintain a realistic model of the real atmosphere. Larger scale model parameters (e.g. wind vector, temperature, humidity) are used to diagnose and represent the effects of such sub-gridscale processes: this is know as parametrisation. [ See HERE ]

Partial drought

A period of at least 29 consecutive days, whose mean daily precipitation does not exceed 0.01 inches / 0.2 mm. (See also drought; absolute drought; dry spell).

PGF

(abbr) Pressure gradient force

Phenology

Is the study of times of naturally occurring events, such as the first blossom appearance in a long established species, or the departure of migratory birds. From 1875 to 1948, a register of such events was maintained by the Royal Met. Society, but after a period when the science was in the doldrums, the Woodland Trust and Centre for Ecology & Hydrology combined in the late 1990's to 'kick-start' the observing network, recognising that such data can complement studies into long-term climate change. For more detail, see:- http://www.phenology.org.uk/

PL

(abbr) Ice Pellets (was PE); used in aviation weather reports.

PMC

(abbr) Polar mesospheric clouds (or Noctilucent clouds).

PMSL

(abbr) Pressure at mean sea level: often seen in connection with NWP model products.

PO

(abbr) Well developed sand/dust swirls; as used in aviation weather reports.

Polar front

A boundary that separates polar air masses from tropical air masses.

Polar low

(or Polar depression or Polar meso-cyclone) See "What is a polar low?".

Polar mesocyclones

A term now used to encompass the whole 'family' of disturbances resulting from arctic air flowing equatorward over progressively warmer seas; the term 'Polar Low' (q.v/above) is now often used only for 'extreme' systems where gale or near gale-force winds are observed.

Polar mesospheric clouds

(PMC) See entry under Noctilucent clouds.

"Poor Man's" Ensemble Technique

A true NWP ensemble (q.v.) is the product of multiple iterations of a single atmospheric model on a single centre's computer: the individual members of the ensemble run are obtained by perturbing the initial conditions very slightly to simulate the uncertainty that is always present at analysis time. However, long before these techniques were perfected, operational forecasters would (and still do) absorb the differing output from various international centres (e.g. EC, NCEP, DWD etc.) and / or different 'runs' from the same centre - treating all the various outputs as 'members' of what has been dubbed a 'Poor Man's Ensemble'. As with true ensembles, the more model runs that agree at a certain lead time, the higher is the confidence in that particular solution.

Potential Instability

(also known as Convective Instability) Said to exist when forced lifting (e.g. ascent over mountains or broad- scale/dynamic ascent) causes a layer, initially (just) stable to such forced ascent to become unstable. Decreasing humidity aloft is required within the layer, and heavy rain/thunder can be the result. Theta-W or Theta-E(q.v.) difference charts are often used to find such areas of potential instability: the usual levels used are at 850hPa and 500hPa. The value at 850hPa is subtracted from that found for 500hPa, and negative values so found indicate potential instability. Only slightly negative differences can lead to some significant convective activity .. all other factors being favourable of course. [ Such layers can also be inferred using a thermodynamic diagram (or tabular listing of Theta E or Theta W), noting where values decrease with increasing altitude within the low-to-middle troposphere (roughly up to 400hPa). ]

Potential Vorticity

The ratio of the absolute vorticity (q.v.) of an atmospheric column to the (defined) pressure difference across the column. This quantity is used to label air in much the same way as we use other conservative properties. As a column of air moves along, it 'shrinks' vertically (due to mass divergence) in just the right amount to decrease its absolute vorticity; as it expands vertically (due to mass convergence), its absolute vorticity increases. Therefore, Potential Vorticity tends to remain constant following the motion of the flow, for adiabatic motion.

Precipitation

Anything 'precipitated' by clouds (rain, snow, hail, drizzle etc.) is covered by this noun. Often abbreviated to 'ppn' or 'pptn'. (for definitions of various types of precipitation, see:- "Beaufort Letters")

Pressure Gradient

The difference in atmospheric pressure over a defined (usually horizontal) distance. (See "Why does the wind blow?")

Pressure Gradient Force

(abbr. = PGF)The force exerted on the air due to a pressure gradient, causing a tendency for movement (i.e. 'wind') from areas of high pressure to areas of low pressure.(See "Why does the wind blow?")

Prevailing visibility

Used in METAR (& TAF) coding - see Visibility [ reports ].

Prevailing wind

The most frequent wind direction for any particular location in a given period, e.g. a day, month, year or climatological period.

PRFG

Partial fog (i.e. fog "banks"; substantial portion of airfield covered by fog - but not completely; visibility < 1000m.)

PROB

Probability (as used in aviation forecasts, e.g. TAFs; in the latter, under current [2002] rules, only PROB30 or PROB40 are allowed, e.g. 'moderate' probability of an event occurring.

Probability forecasting

Given that there is always a measure of uncertainty in forecasting the weather, the likelihood of an event happening can be expressed as a probability: thus a 70% chance of rain, 20% chance of thunderstorms etc. Often useful in finely balanced situations i.e. rain vs. snow; severe storms vs. no storm etc. (see also Deterministic forecasts).

Progression

When large scale features in the upper air, such as a 500 or 300 hPa trough/vortex drift west-to-east this is said to be a 'normal' progression of the pattern. (See also retrogression).

PSC

(abbr: Polar Stratospheric Clouds) During the polar 'night' (i.e. the period in the middle of the winter when insolation does not penetrate to ultra-high latitudes), the stratosphere cools significantly leading to 'closed-loop' circulations (both vertical and horizontal) which virtually isolate these polar stratospheric regions - the "Polar night vortex" is found, within which temperatures can be found well below (minus)75degC. In these extremely cold conditions, clouds are observed to form in the stratosphere, which appear to be composed of a combination of nitric acid and water. Stratospheric clouds can also form from ordinary water ice (i.e. as in the troposphere) but these are much less common at these high altitudes as the stratosphere is normally very dry and water-ice clouds only form at the lowest temperatures. The presence of PSC's and the part they play in the chemical interactions at these levels have been a subject of much debate in recent years. (See main FAQ here for the Stratospheric Night Jet and here for Stratosphere & various web-sites dealing with upper atmosphere ozone depletion.)

Pulse storms

(a term often used in North America) Random air-mass thunderstorms forming in an environment of little or no vertical wind shear, which appear as individual returns (without any obvious organisation) on radar/high-resolution satellite imagery systems. They usually last 20 to 30 minutes, perhaps up to 60 minutes, and give rise to small hail, sometimes heavy rain and perhaps weak tornadoes. They can be regarded as a more intense version of the single-cell convective type discussed in the main FAQ here, i.e. higher CAPE values are involved than for an 'ordinary' shower.

PVA region

An area where marked advection (movement) of positive, or cyclonic vorticity (q.v.) is occurring - hence Positive Vorticity Advection; often associated with a small upper trough running through the broadscale upper pattern. Cyclonic development will occur - other factors being favourable.