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Earth Observatory Glossary
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  Pacific Disaster Center A Federal information processing center designed to provide value added information processing support to Federal, State, local, and regional emergency managers to support mitigation, preparation, response and recovery within the Pacific region. In addition, the PDC is being developed as an organizational and technological model for global, national, and local initiatives in disaster management. In particular, the PDC serves as a nodal model for the Global Disaster Information Network (GDIN) proposed by Vice President Gore. Currently, the area of operations of the PDC includes Hawaii, Alaska, and the pacific insular States of Guam, American Samoa, Marshall Islands, Northern Marianas, Palau, and Micronesia. Pacific Disaster Center web site
  paleoclimate Climate as it existed in the distant past, particularly before historical records.
  paleoclimatology The study of past climates, throughout geological history, and the causes of the variations among.
  paleogeography The study of ancient or prehistoric geography.
  panchromatic Sensitive to all or most of the visible spectrum.
  parameter A constant whose values determine the specific form or characteristics of an expression.
  particulates Very small pieces of solid or liquid matter such as particles of soot, dust, fumes, mists or aerosols. The physical characteristics of particles, and how they combine with other particles, are part of the feedback mechanisms of the atmosphere.
  parts per billion A unit of measure used for very small quantities, it is equal to the ratio of the weight or volume of one component of a mixture to a billion weights or volumes of the mixture. When based on weight (ppbw), it is equal to the weight or mass of the component divided by the total weight or mass in a given volume, multiplied by one billion. When based on volume (ppbv) it is equal to the volume of the component divided by the total volume of the mixture, multiplied by one billion.
  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 microwave A system sensing only microwave radiation emitted by the object being viewed or reflected by the object from a source other than the system.
  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.
  payload The instruments that are accommodated on a spacecraft.
  perigee (aka periapsis or perifocus) On an elliptical orbit path, the point where a satellite is closest to the Earth.
  perihelion 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).
  1. Time required for a satellite to make one complete orbit.
  2. A division of geologic time, delimited by full-scale withdrawal of the sea from land masses and by limited crustal, climatic, and volcanic upheaval in a localized area. Two or more periods are required to make up a geologic era, and each period is comprised of two or more geologic epochs.
  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 satellite's mean motion.
  permafrost Perennially frozen ground that occurs wherever the temperature remains below 0ш C for several years.
  perturbations 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 Earth's atmosphere (causing satellites to spiral downward), minor adjustments must be made to the orbit.
  pH A symbol for the degree of acidity or alkalinity of a solution. Expressed as a negative logarithm of the hydrogen ion concentration in a solution, pH = -log10[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.
  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.
  photochemistry The study of the impact of light on certain chemical molecules.
  photodissociation A chemical reaction involving sunlight in which molecules are split into their constituent atoms. Also known as photolysis.
  photon A quantum (smallest unit in which waves may be emitted or absorbed) of light.
  photoreceptor A sensor sensitive to light.
  photosynthesis The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.
  photosynthetically active radiation Electromagnetic radiation in the part of the spectrum used by plants for photosynthesis.
  physical climate system The system of processes that regulate climate, including atmospheric and ocean circulation, evaporation, and precipitation.
  physics The scientific study of matter, energy, motion, and force. (From a Greek term meaning "the science of nature.")
  phytoplankton Microscopic marine organisms (mostly algae and diatoms) which are responsible for most of the photosynthetic activity in the oceans.
  pixel 'Picture element' is the ground area corresponding to a single element of a digital image data set.
  planetary 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 percent, most of which is due to backscatter from clouds in the atmosphere.
  planetary boundary layer the turbulent layer of atmosphere occupying the lowest few hundred meters of the atmosphere.
  planetesimals small bodies that formed from the solar nebula
  plasma 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.
  platforms A satellite that can carry instruments. See bus. The same term is applied to automatic weather data transmitters installed on buoys, balloons, 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/TIROS and Landsat satellites.
  polar stratospheric clouds (PSCs) High altitude clouds that form in the stratosphere above Antarctica during the Southern Hemisphere winter. Their presence seems to initiate the ozone loss experienced during the ensuing Southern Hemisphere spring.
  polar vortex A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion.
  pollutant Strictly, too much of any substance in the wrong place or at the wrong time is a pollutant. More specifically, atmospheric pollution may be defined as the presence of substances in the atmosphere, resulting from man-made activities or from natural processes that cause adverse effects to human health, property, and the environment.
  polynya An area of open sea surrounded by ice.
  positive feedback An interaction that amplifies the response of the system in which it is incorporated.
  precession 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 Earth's 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).
  precipitation 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 degrees 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 lbs

one snowflake .0000003 lbs

one cumulus cloud 10,000,000 lbs

one thunderstorm 10,000,000,000 lbs

one hurricane 10,000,000,000,000 lbs

  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.
  primary productivity The rate at which new plant biomass is formed by photosynthesis. Gross primary productivity is the total rate of photosynthetic production of biomass; net primary productivity is gross primary productivity minus the respiration rate.
  process 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).
  psychrometer 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.
  pycnocline In the ocean, a region where the water density increases rapidly with depth.
  pyroclastic flow a fast-moving, fluid-like cloud of hot ash, pumice, rock fragments, and gas that occurs during explosive volcanic eruptions or the collapse of a lava dome
  pyrogenic Resulting from fire activities. Usually used in the context of emissions that are produced by fires -- e.g., smoke from fires.

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