Weather Words - P

-- P --

Pacific air: A type of air mass whose mild, moist characteristics are originally developed over the Pacific Ocean, but whose characteristics become mild and dry after passage across the Rocky Mountains and other western U.S. highlands. In the winter, Pacific air brings mild, dry weather to Chicago.

Pacific warm pool: That part of the tropical Pacific Ocean located north and east of New Guinea and usually about the size of the United States, whose surface water temperature never goes below 86 degrees F.

Pack ice: Great spreads of broken ice that move freely with winds and water currents; commonly found in the open waters of the Great Lakes. Shore ice is firmly attached to the shore or anchored on the sea bed.

Palatable water: Water, at a desirable temperature, that is free from objectionable tastes, odors, colors, turbidity and chemicals that might constitute a health hazard.

Paleoclimate: A prehistoric climate; that is, the climate of a time period in the geologic past.

Paleoclimatology: The study of prehistoric climates and the causes of their variations; the study of climates in the geologic past.

Paleotempestologist: A meteorologist who studies the occurrence of hurricanes and other catastrophic storms in the geologic past.

Paleotempestology: The study of hurricanes and other catastrophic storms before the inception of human records. Environmental information (such as sediment cores) is used rather than human historical accounts.

Palm Sunday Tornadoes: An especially vicious outbreak of tornadoes on Sunday, April 11, 1965, ripped paths of devastation through Iowa, Wisconsin, Illinois, Michigan, Indiana and Ohio. 255 people were killed and 3,331 were injured. 37 tornadoes were rated F2 or greater; 18 violent F4 tornadoes and two maximum-intensity F5 tornadoes occurred. In far northwest suburban Crystal Lake, there were 6 deaths and 75 injuries.

Palmer Drought Severity Index: A comprehensive moisture index based on long-term (weeks, months or longer) departures of temperature and precipitation from normal and upon soil moisture supply and demand. Chicago's current Palmer Index: near normal.

Pancake ice: Small pieces of newly-formed sea ice, usually less than six feet across and almost always circular in shape, with slightly raised edges caused by collisions between individual pieces.

Panhandle hook: A low pressure system that forms in the Oklahoma Panhandle area, moves southeast to about eastern Texas, then turns and accelerates northeast to the lower Great Lakes region. These storms, usually laden with moisture, produce Chicago's heaviest snows.

Parhelion: A bright spot of light at opposite sides of a circular halo around the sun and at the same elevation as the sun; a sun dog. The moon's counterpart is a paraselene.

Particulates: Solid or liquid particles in the air, usually small enough and light enough to be suspended in the air. They can be emitted into the air (like smoke) or formed in the air (like cloud droplets).

Partly cloudy sky: The state of the sky when clouds are conspicuously present but do not dull the sky or aspect of the day at any moment.

Partly/mostly cloudy: The state of the sky when clouds are conspicuously present but do not completely cover the sky. Partly cloudy: more open sky than clouds; mostly cloudy: more clouds than open sky.

Partly/mostly sunny: The state of the daytime sky when clouds are present but the aspect of the sky is still bright. Mostly sunny: The prevailing condition is sunny, but some clouds are present. Partly sunny: Some sun, but clouds block the sun more than half the time.

Pea-soup fog: Not a meteorological term, but in popular usage any particularly dense fog. The National Weather Service defines dense fog as fog which reduces horizontal visibility to 1/4 mile or less.

Percent of possible sunshine: The ratio, expressed as a percentage, of the amount of time bright sunshine occurs on a given day to the total time the sun is above the horizon on that day.

Percolation: The gravity-driven movement of water downward through the soil; also, the downward motion of water through snow.

Pereletok: A sub-surface frozen layer of ground that might persist for a few years, but is not permanently frozen and is not considered to be permafrost. The term is Russian, meaning "survives over the summer."

Perihelion: The point on the orbit of the Earth (or any other body in orbit about the sun) which is nearest to the sun; the opposite of aphelion. At perihelion, usually in early January, the Earth is 91.3 million miles from the sun, or about 3.1 million miles closer to the sun than at aphelion (the point when the Earth is farthest from the sun).

Permafrost: A layer of soil or bedrock beneath the surface in which the temperature is permanently below freezing and water in the soil is permanently frozen. Permafrost exists where the annual average temperature is below about 23 degrees F.

Perry: A sudden, heavy fall of rain and (in England) it is sometimes known as "half a gale". Other spellings are: parrey, parry, pirrie, pirry.

Perseid meteor shower: Peak viewing time for this annual meteor shower occurs just before sunrise, about August 12, as Earth passes through a debris trail left by the Swift-Tuttle comet.

Persistence Forecast: A weather forecast that the future weather will be the same as the current conditions; that is, that tomorrow's weather will be the same as today's.

Petrichor: The distinctive smell that accompanies the first rain after dry weather. Volatiles evaporate from plants during dry weather, are absorbed by the ground, then released into the air when it rains.

Phengophobia: The irrational or morbid fear of sunshine or daylight.

Phlogiston: A hypothetical substance believed in the 17th and 18th centuries to be a constituent of all flammable materials, released as flame during burning. The belief lost credence with the 1774 discovery of oxygen and its role in combustion.

Phobia: A morbid fear or dread. In meteorology, keraunophobia is the irrational fear of lightning and brontophobia is the irrational fear of thunder.

Phoenix' heaviest snow: The greatest snowfall officially reported at Phoenix, Ariz., is 1.0 inch on Jan. 20, 1933. That all-time snow record was tied exactly four years later (Jan. 20, 1937) when another 1.0 inch of snow was recorded.

Photochemical smog: Smog that forms in the presence of sunlight. The word "smog" was coined in 1905 by Harold Des Veaux, a London physician, to describe natural fog contaminated by smoke.

Photoperiod: The length of time that light, either natural or artificial, shines on an organism; the duration of daily exposure to light and dark periods.

Photosphere: The intensely bright portion of the sun visible to the unaided eye; the lowest layer of the three main layers of the sun's atmosphere, and the layer from which the sun's visible light emanates.

Phototoxic reaction: (1) A sunburn-like skin reaction to ultraviolet in sunlight when using certain oral medications or when a topical agent (like lemon) containing sensitizing chemicals is on the skin. (2) A sunburn-like skin reaction to ultraviolet light when using certain oral medications (such as doxycycline) or when a topical agent (like lime, lemon, parsley, etc.) containing photosensitizing chemicals is on the skin prior to sun exposure.

Pileus: An accessory cloud of limited horizontal extent that appears in the form of a scarf, hood or cap occurring either attached to or above the top of rapidly growing cumulus clouds.

Pineapple express: A portion of the subtropical jet stream, originating near the Hawaiian Islands, that occasionally impinges on the Pacific Coast of North America, bringing mild and very rainy weather.

Plainfield tornado: On Aug. 28, 1990, a devastating tornado, the worst in the U.S. that year, killed 29 people and injured 350, with a $165 million path of destruction across Kendall and Will counties in northeast Illinois.

Pluvial: Pertaining to rain, especially to an abundant amount; more generally, pertaining to heavy precipitation. Also, pertaining to an interval of geologic time marked by a wet climate.

Pluviometer: The general name for an instrument designed to measure the depth of rain that has fallen; also called rain gauge, ombrometer, precipitation gauge.

Pluviophobia: The irrational or morbid fear of rain.

Pogonip: Ice fog (fog consisting of tiny suspended ice crystals). Pogonip is an American Indian word (in the Ohlone language) applied mainly to ice fogs in mountain valleys of the western United States.

Polar climate: (also called arctic climate, snow climate) The climate of geographical polar regions, but especially the climate of polar regions which are too cold to support the growth of trees.

Polar day: In polar regions, the portion of the year when the sun is continuously in the sky. Its length varies from 20 hours at the Arctic/Antarctic circles (66 degrees 33 minutes N/S) to 186 days at the North and South poles.

Polar front: The frontal zone which develops in suitable conditions between air originating in polar regions and air from low latitudes. It is a major front in terms of air-mass contrast and susceptibility to development of low pressure systems.

Polar night: In polar regions, the portion of the year when the sun does not rise above the horizon. Its length varies from 20 hours at the Arctic/Antarctic Circles (latitude 66 degrees 33 minutes N/S) to 179 days at the poles.

Polar outbreak: The rapid movement of a mass of air originating above the Arctic Circle southward to the Gulf of Mexico, or farther. Polar outbreaks bring Chicago its lowest winter temperatures; also called arctic outbreak.

Polluted: Descriptive of a degraded state of the environment resulting from the release of substances or energy directly or indirectly harmful to living things or damaging to property.

Pollution: Substances or energy that are released by humans into the environment and that have (or are likely to have) deleterious effects.

Ponding: The accumulation of standing (as opposed to flowing) water in flood-prone areas. In relatively flat terrain such as is prevalent across metropolitan Chicago, flash flooding often takes the form of ponding of water in low-lying residential areas and on roadways.

Poor man's thermometer: Crickets. Count the number of chirps produced by a cricket in 15 seconds and add 37. The sum will be very close to the prevailing Fahrenheit temperature at the cricket's location.

Popcorn convection: Showers and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to daytime heating.

Port Martin, Antarctica: The world's windiest place. It has an average wind speed of 40 m.p.h. (64 km/hour). It experiences gale force winds (sustained 39 m.p.h. or stronger) on more than 100 days per year.

Potamophobia: The morbid or irrational fear of rivers.

Potential hurricane disaster: Explosive population growth in hurricane-vulnerable areas of the U.S. Gulf and Atlantic coasts greatly increases the potential for a hurricane disaster. For example, the population of metropolitan Miami now exceeds the 1930 population of the entire coast from Texas to Virginia.

Powder snow: A skiing term for a cover of dry snow that has not been compacted in any way.

Precipitable water: The total depth of water that would result if all the water vapor in a column of air extending from the Earth's surface upward to the "top" of the atmosphere were condensed out.

Precipitation: All of the kinds of water particles, whether in liquid or frozen form, that fall from the atmosphere and reach the ground. Dew, frost and fog are not considered to be precipitation.

Precipitation intensity terminology: In order of increasing intensity: very light, light, moderate, heavy. very light precipitation will not cover or completely wet a surface, regardless of duration.

Precipitation probability terminology: The Chicago National Weather Service uses four categories of terminology to describe the probability of precipitation in its forecasts: no mention when the probability is 20 percent or less; "chance" corresponding 30, 40, 50 percent; "likely" for 60, 70 percent; unqualified mention for 80, 90, 100 percent.

Pressure: A type of stress characterized by uniformity in all directions (equally up, down, and sideways) and it is expressed in units of force per unit area. Air pressure is the pressure exerted by the atmosphere.

Pressure gradient: The rate of change of air pressure per unit of horizontal distance. A large pressure gradient causes high winds, a small pressure gradient causes light winds.

Prevailing westerlies: (1) Two atmospheric belts paralleling the equator at 35-55 degrees N and S latitude, in which weather systems move from the west. Most of the United States lies within the northern hemisphere zone. (2) The dominant west-to-east motion of the atmosphere centered over the middle latitudes of both hemispheres and paralleling the equator at 35-55 degrees N and S latitude. Most of the United States lies within the northern hemisphere zone of prevailing westerlies.

Priestley, Joseph: (1733-1804) English physical scientist and one of the discoverers of oxygen. On August 1, 1774, he isolated oxygen (calling it dephlogisticated air) by heating red mercuric oxide.

Probability: The chance that a prescribed event will occur, represented by a number in the range from zero to one. The probability of an impossible event is zero and that of an inevitable event is one.

Probability of precipitation: The likelihood, expressed as a percentage, that precipitation will occur at any given spot within the forecast area during a given period of time.

Progressive weather: Weather patterns in a constant state of change, or weather patterns that are moving (as contrasted to stagnant or only very slowly moving weather patterns).

Psithurism: The sound of wind in trees or rustling leaves.

Psychrometer: An instrument, consisting of wet and dry thermometers, that measures humidity. Humidity is calculated from the temperature difference between the dry and evaporation-cooled wet thermometers.

Psychrophobia: The irrational or morbid fear of cold temperatures.

Ptolemy, Claudius: (about 87-150 A.D.) The Greco-Egyptian astronomer who first systematized the popular theory that the universe was revolving around a stationary Earth at its center. The doctrine was so compelling that it withstood the tests of observation and was accepted into the 16th century.

Puddle: A small, shallow pool of water up to several feet across but only a few inches deep; also, an accumulation of meltwater on top of a layer of ice.

Pulse thunderstorm: A thunderstorm that develops and intensifies rapidly, briefly produces severe weather (a few minutes), then abruptly weakens.

Punxsutawney Phil: The Pennsylvania groundhog who predicts (but only with 39 percent accuracy) upcoming late-winter weather depending on sky conditions on Groundhog Day (Feb. 2): milder if cloudy (no shadows to be seen); six more weeks of winter if sunny (and therefore shadowed).

Purl: To flow or ripple with a murmuring sound; the sound made by rippling water. In radar meteorology, the looping flight path made by a weather observation aircraft during a storm investigation.

Pyroclastic flow: A hot (often 800 degrees F or higher), ground-hugging but airborne avalanche of volcanic gas, ash, pumice and rock fragments that surges down the side of a volcano during an eruption.

Pyrocumulus clouds: Cumulus clouds that form at the top of warm updrafts fueled by the heat of large fires. They are often observed above forest fires.