Weather Glossary - P

Pascal

(Pa) The unit of pressure (force per unit area) Kg/m². Average air pressure is 1013.25 hPa (101325 Pa).

Pendant echo

Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Penetrating top

See overshooting top.

Percentile

The term for denoting thresholds or boundary values in frequency distributions. Thus the 5th percentile is that value which marks off the lowest 5 per cent of the observations from the rest, the 50th percentile is the same as the median, and the 95th percentile exceeds all but 5 per cent of the values. When percentiles are estimated by ranking the items of a finite sample, the percentile generally falls between two of the observed values, and the midway value is often taken. (From Bureau of Meteorology glossary)

Pileus

Latin - cap
A cloud in the form of a cap above or attached to the upper part of a cumuliform cloud, indicating strong updrafts within the cloud.

Polar Jet

Marked by concentrations of isotherms and strong vertical wind shear this jet is the boundary between polar air and subtropical air. Its position migrates north in the Southern Hemisphere winter and south in the summer.

Popcorn convection

[Slang] Showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms are typically of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Positive area

The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. See sounding. Positive area is a measure of the energy available for convection; see CAPE.

Positive CG

A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)

Positive tilt trough

An upper level system which is tilted to the west with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Potential temperature

The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.

PPI

Plan Position Indicator. A radar scanning method which uses a single elevation of the radar antenna to detect and range rainfall surrounding the site. Multiple PPI scans at various elevations are often combined to create a CAPPI (Constant Altitude Plan Position Indicator) display of the radar data. This is the most common type of radar display used in Australia.

PPINE

Plan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range.

Pre frontal trough

An elongated area of low pressure preceding a cold front. Is usually associated with a shift in wind direction and a slight temperature drop.

Precipitable water

The vertical integral of the water content in a column of the atmosphere, and is measured in millimetres. It is a measure of the amount of water that can be "wrung out" of the atmosphere. High values (above 40mm) indicate the potential for heavy rainfall.

Precipitation

Any or all of the forms of water particles, whether liquid (e.g. rain, drizzle) or solid (e.g. hail, snow), that fall from a cloud or group of clouds and reach the ground.

Duration of precipitation

• Brief: Short duration.
• Intermittent: Precipitation which ceases at times.
• Occasional: Precipitation which while not frequent, is recurrent.
• Frequent: Showers occurring regularly and often.
• Continuous: Precipitation which does not cease, or ceases only briefly.
• Periods of rain: Rain is expected to fall most of the time, but there will be breaks.

• Few: Indicating timing not an area.
• Isolated: Showers which are well separated in space during a given period.
• Local: Restricted to reatively small areas.
• Patchy: Occurring irregularly over an area.
• Scattered: Irregularly distributed over an area. Showers which while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.
• Widespread: Occurring extensively throughout an area.

Pressure

The force per unit area. In meteorology, pressure refers to the weight of air in a column directly above a point.
The standard atmospheric pressure at mean sea level is 1013.25 hPa, though surface pressures of 870 hPa (Typhoon Tip, October 1979) and 1084 hPa (Agata, Siberia, December 1968) have been recorded.

Pressure gradient

The pressure change over a fixed distance at a fixed altitude. The larger the pressure gradient the stronger the winds.

Prevailing wind

A wind that blows from one direction more frequently than any other during a given period.

Probabilities, or Probabilistic Forecasts

An attempt to convey the uncertainty in a forecast by expressing its likelihood of occurrence as a percentage. High probabilities do not guarantee an outcome - they merely indicate that that outcome is highly likely.

Profiler

An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.

Prognostic chart

A chart displaying a forecast of meteorological elements

Pseudo-cold front

A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone centre, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gustfront. See also pseudo-warm front.

Pseudo-warm front

A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone centre, usually toward the east or southeast, and normally is either nearly stationary or moves northward or north-eastwards ahead of the mesocyclone. See pseudo-cold front and beaver tail.

Pulse storm

A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See air mass thunderstorm, cyclic storm.

PVA

Positive Vorticity Advection. Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air (Northern Hemisphere). PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.

Pyrocumulus

Clouds which form on top of a rising column of smoke over a fire. These can produce precipitation and/or lightning in rare cases.
Pyrocumulus