LIVE VIDEO: 9NEWS Live at Noon    Watch

Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z


Pacific Fronts - A variety of cold fronts and storm systems move onto the west coast during the winter months. Coming on shore with plenty of moisture, the mountains of California, Oregon and Washington squeeze out much of that water. Fortunately for our ski areas, many systems retain enough moisture to dump heavy snow on the mountains of Colorado. By the time these storms get past our mountains, they are relatively dry. As the saying goes..."Pacific front, mountains bear the brunt. Southeast low, Denver gets snow."

PDS Watch - A tornado watch with enhanced wording (Particularly Dangerous Situation).

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

Penetrating Top - Same as overshooting top.

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 typically are 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. 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 considered 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 east 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.

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

Precipitation - All forms of water particles, liquid or frozen that reach the ground.

Pressure Gradient - The difference in air pressure between two points or places (or example between two weather systems, ie high pressure and low pressure). The tighter the pressure gradient, or the greater the pressure difference between weather systems, the higher the wind speed between them as well. Likewise, the less the pressure gradient, the lighter the wind speeds.

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 Dopper radar.

Pseudo-Cold Front - A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the meso-cyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded meso-cyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. 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 meso-cyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the meso-cyclone. See pseudo-cold front and beaver tail.

Psychrometer - An instrument used to measure the water vapor content of the air. It measures the dry-bulb and wet-bulb temperatures and with the air of tables, relative humidity and ew-point can be found.

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 overshooting top, 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. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society