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Words begining with "P"


The study of ancient or prehistoric qeoqraphy.


Climate as it existed in the distant past, particularly before historical records.


Sensitive to all or most of the visible spectrum.

Parasitic Element

See antenna.


The addition of one or more redundant bits to information to verify its accuracy.


Types of parity

  • Even Parity - The additional bit added to a 7 bit byte to ensure that there are an even number of 1's in the byte of information.
  • Odd Parity - The additional bit added to a 7 bit byte to ensure that there are an odd number of 1's in the byte of information.

Pascal (Pa)

Unit of atmospheric pressure named in honor of Blaise Pascal (1632-1662), whose experiments greatly increased knowledge of the atmosphere. A pascal is the force of one newton acting on a surface area of one square meter. It is the unit of pressure designated by the International System.

100,000 Pa = 1000 mb = 1 bar See atmospheric pressure, millibar.

Passive System

A system sensing only radiation emitted by the object being viewed or reflected by the object from a source other than the system. See active system.


The instruments that are accommodated on a spacecraft.


Personal computer.

Perigee (aka periapsis or perifocus)

On an elliptical orbit path, the point where a satellite is closest to the Earth. See Keplerian elements.


The point in the orbit of a planet or comet which is nearest the Sun (as opposed to the aphelion, which is the point in the orbit farthest from the Sun).


Time required for a satellite to make one complete orbit.

Period Decay (aka Decay)

The tendency of a satellite to lose orbital velocity due to the influence of atmospheric drag and gravitational forces. A decaying object eventually impacts the surface of the Earth or burns up in the atmosphere. This parameter directly affects the satellites mean motion.


See cryosphere.


Minor corrections to the Keplerian model of a satellite orbit as an ellipse of constant shape and orientation. Since satellite orbits are affected by Earth's gravity and drag caused by the Earths atmosphere (causing satellites to spiral downward), minor adjustments must be made to the orbit.


A symbol for the degree of acidity of alkalinity of solution. Expressed as a negative logarithm of the hydrogen ion concentration in a solution, pH = -log[H+]. If the hydrogen ion concentration of a solution increases, the pH will decrease, and vice versa. The value for pure distilled water is regarded as neutral, pH values from 0 to 7 indicate acidity, and from 7 to 14 indicate alkalinity.

Phase Interval

In direct readout, the time between the end of a satellite image start tone and start of the actual frame data. The phase interval represents white level video, interrupted by a black level pulse marking the start of each line and is used to set up phasing prior to image display.


Subdiscipline of agriculture, a science that treats relations between climate and periodic biolgical phenomena that are related to or caused by climate conditions, such as the budding of trees and migration of birds.

Photochemical Smog

A type of smog that forms in large cities when chemical reactions take place in the presence of sunlight, its principal component is ozone. Ozone and other oxidants are not emitted into the air directly but form from reactions involving nitrogen oxides and hydrocarbons. Because of its smog-making ability, ozone in the lower atmosphere (troposphere) is often referred to as "bad" ozone.


A quantum (smalles unit in which waves may be emitted or absorbed) of light.

Photosynthetically Active Radiation

Electromagnetic radiation in the part of the spectrum used by plants for photosynthesis.

Physical Climate System

The system of processess tht regulate climate, including atmospheric and ocean circulation, evaporation and percipitation.


Smallest par (addressable element) of an electronically-coded image, such as a computer display. Pixel is a contraction of "picture element."

Plaentary Albedo

The fraction of incident solar radiation that is reflected by a planet and returned to space. The planetary albedo of the Earth-atmosphere system is approximately 30%, most of which is due to backscatter from clouds in the atmosphere.


A fourth state of matter (in addition to solid, liquid, and gas) that exists in space. In this state, atoms are positively charged and share space with free negatively-charged electrons. Plasma can conduct electricity and interact strongly with electric and magnetic fields. The solar wind is actually hot plasma blowing from the sun. See magnetosphere

Plate Tectonics

Concept that the Earth's crust is composed of rigid plates that move over a less rigid interior.


A satellite that can carry instruments. See bus. The same term is applied to automatic weather data transmitters installed on bouys, ballons, ships and planes, and mounted in remote areas.

POES (Polar-orbiting Operational Environmental Satellite)

Operated by the National Oceanic and Atmospheric Administration, they are designated "NOAA satellites." Included in this group are the current series of TIROS-N satellites, the third-generation polar-orbiting environmental spacecraft operated by NOAA.

Polar Orbit

An orbit with an orbital inclination of near 90 degrees where the satellite ground track will cross both polar regions once during each orbit. The term is used to describe the near-polar orbits of spacecraft such as the USA's NOAA/TIR0S and Landsat orbit path satellites.


The comparatively slow torquing of the orbital planes of all satellites with respect to the Earth's axis, due to the bulge of the Earth at the equator which distorts the Earths gravitational field. Precession is manifest by the slow rotation of the line of nodes of the orbit (westward for inclinations less than 90 degrees and eastward for inclinations greater than 90 degrees).


Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, their water droplets slowly being pulled down by gravity. Because their water droplets are so small and light, it can take 21 days to fall 1,000 feet and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.

When ice crystals move within a very cold cloud ( 10 dgrees F and -40 degrees F) and enough water droplets freeze onto the ice crystals, snow will fall from the cloud. If the surface temperature is colder than 32 degrees F the flakes will land as snow.

Precipitation Weights:

  • one raindrop .000008 Ibs
  • one snowflake .0000003 Ibs
  • one cumulus cloud 10,000,000 Ibs
  • one thunderstorm 1 0, 000, 000, 000 Ibs
  • one hurricane i 0, 000, 000. 000, 000 Ibs

Prevailing Westerlies

Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east. The subtropical high pressure regions at the horse latitudes (30 degrees) forces surface air poleward, and the rotation of the Earth causes these winds to bear to the right (east) in the Northern Hemisphere and to the left (east) in the Southern Hemisphere (see Coriolis force). This is, to some extent, an idealized picture of the atmospheric circulation. The actual circulation on individual days includes modifications and variations due to the migratory cyclones and anticyclones of middle latitudes, causing rapid and often violent weather changes, as warm semi-tropical air from the horse latitudes meets cold polar air from the high latitudes. See wind.

Prime Meridian

An imaginary line running from north to south through Greenwich, England, used as the reference point for longitude.

Printed Circuit

A fiber card on which integrated circuits and other electronic components can be mounted. Connections between the components are etched in the correct circuit patterns.


An association of phenomena governed by physical, chemical, or biological laws. An example of a process is the vertical mixing of ocean waters in the so-called surface-mixed layer; the state variables for this process include temperature, salinity in the water on a vertical scale of tens of meters, and heat flow and wind stress at the sea surface. Other examples include the volcanic deposition of dust and gases into the atmosphere, eddy formation in the atmosphere and oceans, and soil development.

Process Study

An organized, systematic investigation of a particular process designed to identify all of the state variables involved and to establish the relationships among them. Process studies yield numerical algorithms that connect the state variables and determine their rates of change; such algorithms are essential ingredients of Earth system models.

Prograde Orbit

Orbits of the Earth in the same direction as the rotation of the Earth (west-to-east).


An instrument designed to measure dew point and relative humidity, consisting of two thermometers (one dry bulb and one wet bulb). The dew point and humidity levels are determined by drying the wet bulb (either by fanning or whirling the instrument) and comparing the difference between the wet and dry bulbs with preexisting calculations. See hygrometer.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 13 2021

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