Weather Glossary

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P

PALMER DROUGHT INDEX

A long-term meteorological drought severity index produced by the NOAA/USDA (Department of Agriculture) Joint Agricultural Weather Facility. The index depicts prolonged times, as in months or years, of abnormal dryness or wetness. It responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration.

PALOUSER

A strong, dangerous, katabatic wind that descends from the mountains into the Palouse River valley in northern Idaho and eastern Washington. May be called a Cow-Killer.

PARCEL

A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.

PARHELION

The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns. 

PARTIAL OBSCURATION

Denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. It often reduces horizontal visibility but not the vertical. It is reported as "X" in an observation and on the METAR.

Related term: obscuration

PARTLY CLOUDY

The state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. The National Weather Service does not have an amount of sky cover for this condition.

Related terms: clear, few, scattered, broken, and overcast

PASCAL

The unit of pressure produced when one newton acts on about one square meter.

PASCAL'S LAW

When an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. It means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. Formulated by Blaise Pascal (1623-1662), a French mathematician, theologian, and physicist.

PEAK GUST

The highest instantaneous wind speed observed or recorded.

PERIGEE

The point nearest the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of apogee.

PERIHELION

The point of the earth's orbit that is nearest to the sun. Although the position is part of a 21,000 year cycle, currently it occurs around January, when the earth is about 3 million miles closer to the sun than at aphelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of aphelion.

PHOTOMETER

Any of a number of atmospheric phenomena which appear as luminous patterns in the sky. They do not directly cause adverse weather. They include halos, coronas. rainbows, and fogbows.

PHOTOSPHERE

The intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000°F, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.

PILOT BALLOON

A small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. Also known as a pibal.

PILOT REPORT

A report of in-flight weather by an aircraft pilot or crew member. Often referred to as a PIREP.

PLAN POSITION INDICATOR

Also known as a PPI Scope, it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.

PLOW/PLOUGH WIND

The spreading downdraft and strong straight-line winds preceding a thunderstorm. So named in the American Midwest because of its ability to flatten tall grasses as it passes.

Related term: first gust

POLES/POLAR

The poles are the geographic point at 90 degrees latitude North and South on the earth's surface. They are equal distance from the equator. The polar region is considered to be that area between 60° and 90° latitude, both North and South.

POLAR AIR MASS

An air mass that forms over a high latitude region. Continental polar air (cP) is formed over cold surface regions and is typically very stable with low moisture. Maritime polar air (mP), produced over warmer waters, is less stable with high moisture.

POLAR FRONT

A semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. An integral part of an early meteorological theory known as the Polar Front Theory.

POLAR JET

Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.

POLAR-ORBITING SATELLITE

A satellite whose orbit passes over both of the earth's between poles.

Related term: geostationary satellite

POLLUTANT

Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.

POSITIVE VORTICITY ADVECTION

The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.

related term: negative vorticity advection

POUNDS PER SQUARE INCH (PSI)

A unit for measuring pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one square inch.

PRECIPITATION

Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period. 

PRE-FRONTAL SQUALL LINE

A line of thunderstorms that precedes an advancing cold front.

Related term: squall line

PRE-FRONTAL TROUGH

An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.

Related term: trough

PRESSURE

The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.

Related terms: atmospheric pressure and barometric pressure

PRESSURE ALTIMETER

An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.

PRESSURE ALTITUDE

The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.

PRESSURE CHANGE

The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.

PRESSURE CHARACTERISTIC

The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.

PRESSURE GRADIENT

The amount of pressure change that occurs over a fixed distance at a fixed altitude.

PRESSURE JUMP

A sudden increase in the observed atmospheric pressure or station pressure.

PRESSURE TENDENCY

The pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation.

PREVAILING WIND

A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

PREVAILING VISIBILITY

It is considered representative of visibility conditions at the observation station. It is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous.

PROFILER

A type of Doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere.

PROGNOSTIC CHART

A chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. Also known as a prog.

PSYCHROMETER

An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

PULSE

A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic radiation emitted by the radar.

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