Paleosol: Old, buried soil that may furnish some evidence of the nature of past climates because climate is the most important factor in soil formation.

Pannus (pan): Ragged low cloud often found low down in wet and windy weather; A layer of shredded accessory clouds below the main cloud. Occurs with nimbostratus, cumulus, and cumulonimbus.

Parametrization: This is carried out by developing semi-empirical equations which relate the large-scale effects calculated at grid points to features such as showers which are too small to be calculated explicitly.

Parhelic Circle: The parhelic circle can be an impressive white arc spanning 360 degrees in azimuth when complete, running through the sun, the two 22-deg. parhelia, the two 120-deg parhelia and the anthelion. It is parallel to the horizon, and at the same elevation as the sun. The parhelic circle is caused by sunlight reflecting from the vertical faces of horizontally aligned, plate-like ice-crystals. Sometimes, the arc can also be colored, in the anthelion area, due to a more complex light path actually through the plate crystals.

Parhelion (also known as Sun Dog): Parhelia is the plural form of the word. See Mock Sun for definition. There are 9, 18, 22, 23, 120 degree parhelia...

Parry Arcs: There are several types of Parry arcs. The most common are the upper "sunvex" and "suncave" Parry arcs, referring to their shape - convex or concave towards the sun. The photos below show the upper suncave Parry arc. It is a variation of the standard upper tangent arc, and is also caused by horizontally oriented pencil-shaped ice crystals, but these do not have a freedom of rotation along their long horizontal axis - one of the side crystal faces must be parallel to the horizon. The upper suncave Parry arc shows as a second upper tangent arc, just over the normal tangent arc. (The upper sunvex Parry arc would appear between the suncave arc and the upper tangent arc as a small V shape). Lower Parry arcs are also possib[l]e.

Partial Pressure: The pressure exerted by one gas in a mixture. This is the pressure remaining if all other gases are removed but the volume is unchanged.

Particulate Matter: Solid particles or liquid droplets that are small enough to remain suspended in the air. Also called aerosols.

Pascal (Pa): A unit of pressure. One hectopascal (1hPa) is 100 pascals and equals one millibar. A number of meteorological services have replaced millibar by hPa on charts and diagrams.

Period: The time taken to complete an orbit or oscillation and return to the original value. It is the reciprocal of frequency.

Periodic Orbit: This exactly repeats its past behavior after a fixed interval.

Perlucidus (pe): A sheet of cloud with gaps; An extensive patch, sheet, or layer of clouds with well-defined but sometimes small spaces between its elements. The Sun, Moon, sky, or overlying clouds can be seen through the spaces. Applies to altocumulus and stratocumulus.

Permafrost: The permanent freezing of the subsoil in tundra regions.

Persistence Forecast: A forecast that assumes that the weather occurring upstream will persist and move on and will affect the areas in its path in much the same way. Persistence forecasts do not account for changes that might occur in the weather system.

Phase Line: A vertical cross-section of lee wave flow often shows that the position of wave crests and troughs varies with height. Lines drawn through the crests and troughs at every level are phase lines.

Phase Space: An imaginary space with as many dimensions as the number of variables needed to specify a dynamical system. A simultaneous value of these variables specifies a coordinate in phase space.

pH Scale: A zero to fourteen scale that is used for expressing the exact degree of acidity or alkalinity of a solution. A pH of 7 signifies a neutral solution. Values below 7 signify an acid solution, and values above 7 signify an alkaline solution. e.g. pH of 2 is more acidic than a pH of 5. A pH of 12 is more basic than a pH of 9.

Photochemical Reaction: A chemical reaction in the atmosphere that is triggered by sunlight, often yielding a secondary pollutant.

Photochemical Smog: See Smog.

Photodissociation: The cleavage of a molecule into two or more (smaller) atomic or molecular fragments through the absorption of radiant energy (most often from the Sun).

Photon: A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light (300Mm/sec).

Photosynthesis: The production of sugars and starches by plants using air, water, sunlight, and chlorophyll. In the process, atmospheric carbon dioxide is changed to organic matter and oxygen is released.

Pileus (pil) Cloud: A small lenticular cap of cloud often formed when the rising top of a cumulus lifts moist air above it. The cumulus may subsequently grow through the pileus; A small horizontal accessory cloud, in the form of a hood, that occurs above or on top of a cumuliform cloud. Occurs with cumulus and cumulonimbus.

Pilot Balloon: Originally a small hydrogen-filled balloon released to show the direction a full-sized balloon would drift. Pilot balloons can be tracked by theodolite to find the wind velocity aloft. To see a picture and description of a theodolite, visit our Instrumentation page.

Planck's Radiation Law: All the properties of the radiation emitted by a blackbody (spectrum, intensity, total energy, and so forth) are determined once the temperature of the blackbody is known (Planck's constant, Boltzmann's constant, and the speed of light also enter into the calculation but are well known).

Plate Tectonics Theory: A theory that states the outer portion (uppermost ~100km) of Earth is made up of several individual pieces, called plates, which move in relation to one another upon a partially molten zone below. As plates move, so do continents which literally ride on top of them. This explains some climatic changes in the geologic past. Plate tectonics is Geomorphology's paradigm and guides understanding of the field and all Earth sciences in general.

Pleistocene Epoch (or Ice Age): The most recent period of extensive continental glaciation that saw large portions of North America and Eurasia covered with ice. It began about 2,000,000ybp (years before present) and ended about 10,000ybp.

Polar (P) Air Mass: Air which has spent a relatively long time at high latitudes and has low temperatures and little moisture content. There are subdivisions such as Polar Arctic which travels fairly directly towards lower latitudes, Polar Maritime which follows a track over the sea, Polar Continental which travels over large land masses, and Returning Polar Maritime which makes a long sweep over the sea and swings back to arrive from a southerly direction.

Polar Climates: Climates in which the mean temperature of the warmest month is below 10°C; climates that are too cold to support the growth of trees.

Polar Easterlies: In the global pattern of prevailing winds, winds that blow from the polar high toward the subpolar low. These winds, however, should not be thought of as persistent winds, such as the trade winds.

Polar Front: A front dividing tropical and polar air masses.

Polar Front Theory: A theory developed by J. Bjerknes and other Scandinavian meteorologists in which the polar front, separating polar and tropical air masses, gives rise to cyclonic disturbances that intensify and move along the front and pass through a succession of stages.

Polar High: Anticyclones that are assumed to occupy the inner polar regions and are believed to be thermally induced, at least in part.

Polar Ice Cap Climate: A climate characterized by extreme cold, as every month has an average temperature below freezing.

Polar Jet Stream: A midlatitude jet stream that migrates between the latitudes of 30° and 70°. Outbreaks of thunderstorms and tornadoes follow its seasonal migration. It is usually located at altitudes between 9km and 12km (29,500 and 39,000 feet).

Polar Low: A small depression, usually without fronts, which forms in polar air at high latitudes, chiefly during the winter. They often bring heavy snowfall.

Polar Orbiting Satellite: See Polar Satellite.

Polar Satellite: Satellites that orbit the poles at rather low altitudes of a few hundred kilometers and require only 100 minutes per orbit! These have much higher resolution than other satellites above lower latitudes which have to orbit at altitudes of 35,000km or more.

Polar Tundra Climate: A climate characterized by extremely cold winters and cool summers, as the average temperature of the warmest month climbs above freezing but remains below 10°C (50°F).

Polar Vortex: A cyclonic circulation around the polar regions in both hemispheres. In winter it becomes strong enough to produce a westerly Polar Night Jet Stream at high levels. In summer high-level heating changes the flow to an easterly.

Pollutants: Any gaseous, chemical, or organic matter that contaminates the atmosphere, soil or water.

Pollutant Standards Index (PSI): An index of air quality that provides daily air pollution concentrations. Intervals on the scale relate to potential health effects. (Following Index adapted from p. 303 of Ahren's "Essentials of Meteorology".)

PSI Value	Description	General Health Effects	PSI Episode Level	Cautionary Statements
0-50	Good	none
51-100	Moderate	None
101-199	Unhealthful	Mild aggravation of symptoms in susceptible persons, with irritation symptoms in the healthy population		Persons with existing heart or respiratory ailments should reduce physical exertion and outdoor activity.
200-299	Very unhealthful	Significant aggravation of symptoms and decreased exercise tolerance in persons with heart or lung disease, with widespread symptoms in the healthy population	Stage 1 Health Advisory Alert	Elderly and persons with existing heart or lung disease should stay indoors and reduce physical activity.
300-399	Hazardous	Premature onset of certain diseases in addition to significant aggravation of symptoms and decreased exercise tolerance in healthy persons.	Stage 2 Health Advisory Warming	Elderly and persons with existing diseases should stay indoors and avoid physical exertion. General population should avoid outdoor activity.
400-500	Extremely Hazardous	Premature death of ill and elderly. Healthy people will experience adverse symptoms that affect their normal activity.	Stage 3 Emergency	All persons should remain indoors, keeping windows and doors closed. All persons should minimize physical exertion and avoid traffic.

Positive-Feedback Mechanism: As used in climatic change, any effect that acts to reinforce the initial change. A sort of domino effect.

Potential Energy: The energy that exists by virtue of a body's position with respect to gravitation.

Potential Instability: See Stability.

Potential Temperature: The temperature air would have if it were moved along a dry adiabatic until the pressure reached 1,000 mbar. For example, if air at 10°C descended 5,000 feet to the 1,000 mbar level, the temperature would rise to 25°C.

Precipitation: Generating precipitation that falls from a cloud and reaches the ground. This occurs with altostratus, nimbostratus, stratocumulus, stratus, cumulus, and cumulonimbus.

Precession (of Earth's axis of rotation): The slow migration of Earth's axis that traces out a cone over a period of ~26,000 years.

Precipitation: A term which includes drizzle, rain, snow and hail, often used when it is uncertain which will reach the ground: e.g. precipitation may fall as rain on low ground but snow on the mountains. It is any water (solid or liquid) that reaches Earth's surface from above.

Precipitation Fog: See Frontal Fog.

Pressure: The force, exerted equally in all directions, by a fluid or gas upon a unit area. For meteorological purposes, it is usually expressed in millibars (mbar) or their equivalent hectopascals (hPa). 1 mbar = 1hPa = 0.02953 inches of mercury = 100 newtons per square meter. MSL (mean sea level) pressure in the standard atmosphere is 1,013.25 mbar. The following are quantitative equivalents of normal, sea-level atmospheric pressure:
1013.25 mb
1013.25 hPA
29.921 Inches of mercury
760.00 mm of mercury
14.696 lbs per square inch
1033.2 kg per square meter
Note that hPA (hectopascals) are equal to mb (millibars).

Pressure Altitude: The altitude shown by an altimeter with its subscale set to 1,013.25 mbar. It differs from the true altitude if the MSL (mean sea level) pressure changes or the temperatures aloft differ from those in the ICAO (International Civil Aviation Authority) standard atmosphere.

Pressure Gradient: Usually refers to the horizontal force which acts on the air; it is a horizontal vector perpendicular to the isobars (generally at mean sea level).

Pressure Gradient Force (PGF): The force due to differences in pressure within the atmosphere that causes air to move and, hence, the wind to blow. It is directly proportional to the pressure gradient.

Pressure Tendency: The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction.

Prevailing Westerlies: The dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.

Prevailing Wind: A wind that consistently blows from one direction more than from any other.

Primary Air Pollutant: A pollutant emitted directly from an identifiable source.

Probability Forecast: A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions.

Prognostic: Another word for forecast, often abbreviated to Prog.

Prognostic Chart (prog.): A chart showing expected or forecasted conditions, such as pressure patterns, frontal positions, contour height patterns, and so on.

Psychrometer: A pair of matched thermometers mounted together, consisting of a dry bulb measuring ambient air temperature and a wet bulb (covered with muslin kept moist by a supply of distilled water). From the difference between the two, one may calculate humidity and dew point. Special tables or a humidity slide rule are used.

Purple Red: The purple red is a purple to red sky coloration sometimes visible about 15 to 30 minutes after sunset. It is caused by fine aerosols in the atmosphere, deposited there by volcanic eruptions. During the great Krakatoa eruption around 1880, many years followed with intense sunsets and extensive purple red skies. Sunlight, deeply reddened by atmospheric absorption and Rayleigh scattering, shines on the high stratum of aerosols on its under-surface. Together with the deep blue clear sky this shows like a purple veil.

Pyrocumulus: Cumulus formed above a large fire on the ground.