P

Paraselene

Photometeor of the halo family, analogous to the parhelion, but in this case less brilliant, being the Moon the luminary.

Parcel

Imaginary volume of fluid, to which may be assigned various thermodynamic and kinematic quantities.

Parcel mixing

The result of irregular fluctuations in fluid motions on all scales, from the molecular to large eddies.

Parhelia / Parhelion / Paranthelion, Sun dog

Photometeor of the halo family, consisting of white luminous spots, with a diameter slightly bigger than that of the Sun.

Partial drought

Relative drought of at least 29 consecutive days, during which the average daily rainfall does not exceed 0.01 inches (0.25 mm).

Particulate

Term that designates solid and liquid macro and microscopic particles in the air.

Particulate matter

Very tiny particles of solid matter or liquid droplets, small enough to remain in the air during long periods of time.

Perihelion

Point on the terrestrial orbit that is closest to the sun. Nowadays, this occurs about January 1st , but the date has irregular variations from year to year.

Permafrost

Layer of soil or bedrock beneath the surface, where temperature has been consistently below freezing during thousands of years. It occurs where summer heating does not reach the base of the layer of frozen ground, and annual mean temperature is less than –5° C.

Persistence method

Forecasting method based on the assumption that the present weather will not present changes throughout the forecast period. It is based on the persistence of weather conditions.

Photochemical smog

Air contaminated with the presence of air pollutants, such as ozone, nitrogen oxides and hydrocarbons, accompanied or not by natural fog, which diminishes visibility.

Photosphere

Visible portion of the sun, consisting of an intensely bright and thick gaseous layer, from where light is emitted.

Pilot balloon

Small ascending balloon destined to collect data for the computation of the measurement (speed and direction) of upper-air winds.

Plasma

Ionized gas composed of protons and electrons, possibly neutrons and molecules, of almost equal charge density. When coined, the term designated “that portion of an arc-type discharge in which the densities of ions and electrons are high but substantially equal”.

Pleistocene

Period from roughly 2 million to 10 thousand years ago, characterized by the periodical expansion of continental glaciers over sub-polar regions in both hemispheres.

Pleistocene Ice Age

Particular period within the Pleistocene during which extensive ice sheets covered many parts of the Earth´s surface.

Plotting symbols

Conventional ideograms on a synoptic chart that represent various meteorological elements observed.

Polar cell

Direct and weak meridional circulation in the troposphere, approximately between the pole and 60° latitude, characterized by a sinking motion near the poles and an equatorward flow near the surface, an ascending motion in the sub-polar latitude, around latitude 60°, and a poleward flow aloft, completing the cell.

Polar Easterlies

Low level easterly winds, located poleward of the sub-polar low pressure belt, characterized by being irregular, diffuse and rather shallow. In the Northern Hemisphere, they have a significant presence only north of the Aleutian low and the Icelandic low.

Polar orbiter

Satellite whose orbital plane passes through both poles, North and South poles.

Polar stratospheric clouds (PSC) / Nacreous clouds

Clouds that resemble cirrus or altocumulus lenticularis, but with a very marked irisation, particularly when the sun is several degrees below the horizon, when they show especially bright colors. In the Southern Hemisphere, they have been observed only over the Antarctica, due to the prevalent low temperatures in the circumpolar vortex; in the Northern Hemisphere, they form over Scotland and Scandinavia and eventually in Alaska. Among its components, there is: nitric acid hydrates and sulfuric acid aerosol constituting nuclei for clouds. This kind of clouds is considered as having a major role in the ozone depletion, because of their absorption of odd nitrogen from the atmosphere.

Polar vortex / Polar low, Circumpolar whirl

Planetary scale cyclonic circulation, in the middle and upper troposphere or lower stratosphere, centered in the polar regions.

Pollen count

Analysis of the average quantity of pollen grains of various types, contained in a cubic yard or other determined volume of air, over a 24 hours lapse, at a specified time and place.

Pollutant

Substance present in the atmosphere, especially those that do not appear naturally, usually anthropogenic, which are generally contaminants that degrade or change its quality, altering the health of humans, animals and plants, deteriorating biological and material structures.

Post-mortem

Analysis or review of a finished or past weather event.

Powder avalanche

Mass of loosely packed snow sliding down a mountainside, triggered by a piece of falling rock or ice.

Precession

Tendency of the terrestrial axis, due to its tilt and Earth´s rotation, to wobble in the space, describing a closed circle, every 25800 years approximately (platonic year).

Precipitation

Hydrometeor, consisting of an ensemble of aqueous particles, in a solid or liquid phase, that originated in the atmosphere, falls to the Earth´s surface. The amount of the water substance, that has fallen during a specified period, at a given point. It is usually measured in millimeters or inches of liquid water depth.

Pressure

Pressure gradient

Rate of the change of pressure with distance. It is a vector, perpendicular to an isobaric line, being its magnitude equal to the rate of change of the pressure with distance. This vector is directed toward low pressure.

Pressure system

Individual cyclonic-scale feature of the atmospheric circulation. Generally, this term is used to refer either to a high or a low and more unusually, to denote a ridge or a trough.

Pressure tendency / Pressure trend

Character and amount of atmospheric pressure change for a three-hour period or other specified interval (over 24 hours in tropical regions).

Primary rainbow

Most often observed rainbow, characterized by its angular radius, color order and specially by being the brightest one. The bow shows between about 40° and 42° from the antisolar point, with violet in the inside and red to the outside.

Prominence

Protuberances, which are gas eruptions, from the chromosphere of the sun, that resembles filaments which are seen against the solar disk, in occasions such as during a total eclipse, or by the use of a coronagraph.

Proxy data

Data provided by phenomena such as glacier length, pollen deposits, tree rings, coral samples, etc, which are sensitive to climate; this data can be used to make an estimated reconstruction of past climate conditions.

Pyroclast