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Words begining with "A"

Absolute Humidity

In a system of moist air, the ratio of the mass of water vapor present to the volume occupied by the mixture; that is, the density of the water vapor component. Absolute humidity is normally expressed in grams of water vapor in a cubic meter of air (25 g/m3).


	absolute humidity = mass of water vapor/volume of air


The process in which radiant energy is retained by a substance. A further process always results from absorption, that is, the irreversible conversion of the absorbed radiation into some other form of energy within and according to the nature of the absorbing medium. The absorbing medium itself may emit radiation, but only after an energy conversion has occurred.

Acid Rain

Acids form when certain atmospheric gases (primarily carbon dioxide, sulfur dioxide, and nitrogen oxides) come in contact with water in the atmosphere or on the ground and are chemically converted to acidic substances. Oxidants play a major role in several of these acid-forming processes. Carbon dioxide dissolved in rain is converted to a weak acid (carbonic acid). Other gases, primarily oxides of sulfur and nitrogen, are converted to stronq acids (sulfuric and nitric acids).

Although rain is naturally slightly acidic because of carbon dioxide, natural emissions of sulfur and nitrogen oxides, and certain organic acids, human activities can make it much more acidic. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas.

The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and from biological processes that occur on the land, in wetlands, and in the oceans. The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe. Principal human sources are industrial and power-generating plants and transportation vehicles. The gases may be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.

Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulfur oxides. These sources, plus the transportation sector, are the major originators of increased nitrogen oxides.

The problem of acid rain not only has increased with population and industrial growth, it has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. The same remote glaciers that provide evidence of natural variability in acidic deposition show, in their more recently formed layers, the increased deposition caused by human activity during the past half century.

Acquisition of Signal (AOS)

The time you begin receiving a signal from a spacecraft. For polar-orbiting satellites, radio reception of the APT signal can begin only when the polar-orbiting satellite is above the horizon of a particular location. This is determined by both the satellite and its particular path during orbit across the reception range of a ground station.

Active System (Active Sensor)

A remote-sensing system that transmits its own radiation to detect an object or area for observation and receives the reflected or transmitted radiation. Radar is an example of an active system. Compare with passive system.


Analog to Digital. Used to refer to the conversion of analoq data to its diqital equivalent.

Advanced Very High Resolution Radiometer (AVHRR)

A five-channel scanning instrument that quantitatively measures electromagnetic radiation, flown on NOAA environmental satellites. AVHRR remotely determines cloud cover and surface temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice. TIROS Automatic Picture Transmissions (APT) are derived from this instrument. See TIROS.


Particles of liquid or solid dispersed as a suspension in gas.


The act or process of establishing a forest, especially on land not previously forested.


Airborne Imaging Radar.

Air Mass

Large body of air, often hundreds or thou- sands of miles across, containing air of a simi- lar temperature and humidity. Sometimes the differences between air masses are hardly noticeable, but if colliding air masses have very different temperatures and humidity values. storms can erupt. See front.

Air Pollution

The existence in the air of substances in concentrations that are determined unacceptable to human health and the environment. Contaminants in the air we breathe come mainly from manufacturing industries, electric power plants, exhaust from automobiles, buses. and trucks.

Primary Air Pollutants

  • sulfur dioxide
  • carbon monoxide
  • nitrogen dioxide
  • ground-level ozone
  • lead
  • carbon particles

Air Pressure

The weight of the atmosphere over a particular point, also called barometric pressure. Average air exerts approximately 14.7 pounds (6.8 kg) of force on every square inch (or 101,325 newtons on every square meter) at sea level.


Also Known As.


The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.


A mathematical relation between an observed quantity and a variable used in a step-by-step mathematical process to calculate a quantity.

In the context of remote sensing, algorithms generally specify how to determine higherlevel data products from lower-level source data. For example, algorithms prescribe how atmospheric temperature and moisture profiles are determined from a set of radiation observations originally sensed by satellite sounding instruments.


Substance capable of neutralizing acid, with a pH greater than 7.0. See pH.


An active instrument (see active system") used to measure the altitude of an object above a fixed level. For example, a laser altimeter can measure height from a spacecraft to an icesheet. That measurement, coupled with radial orbit knowledge, will enable determination of the topoqraphy.


Height above the Earth's surface.

Ampere (Amp)

Standard unit to measure the strength of an electric current. One amp is the amount of current produced by an electromotive force of one volt acting through the resistance of one ohm. The ampere is .1 of the theoretical electromagnetic unit of current. Named for the French physicist Andre Marie Ampere. See ohm.


The magnitude of the displacement of a wave from a mean value. For a simple harmonic wave, it is the maximum displacement from the mean. For more complex wave motion, amplitude is usually taken as one-half of the mean distance (or difference) between maxima and minima.

Amplitude Modulation (AM)

One of three ways to modify a sine wave signal in order to make it "carry" information. The strength (amplitude) of a signal varies (modulates) to correspond to the transmitted information. As applied to APT, an audible tone of 2400 Hz is amplitude modulated, with the maximum signal corresponding to light areas of a photograph, the minimum levels black, and the intermediate strengths various shades of qray. See grayscale.


Transmission of a continuously variable signal as opposed to a discretely variable signal. Compare with digital. A system of transmitting and receiving information in which one value (i.e., voltage, current, resistance, or, in the APT system, the volume level of the video tone) can be compared directly to the information (in the APT system, the white, black. and gray values) in the image.

Ancillary Data

Data other than instrument data required to perform an instrument's data processing. Ancillary data includes such information as orbit and/or attitude data, time information, spacecraft engineering data, and calibration information.


Instrument used to measure wind speed, usually measured either from the rotation of wind-driven cups or from wind pressure through a tube pointed into the wind.


  1. The deviation of (usually) temperature or precipitation in a given region over a specified period from the normal value for the same region.


  2. The angular distance of an Earth satellite (or planet) from its perigee (or perihelion) as seen from the center of the Earth (sun). See Keplerian elements for examples of use.


A wire or set of wires used to send and receive electromagnetic waves. Two primary features must be considered when selecting antennas: beamwidth or the "width" of the antenna pattern (wide beamwidth suggests the ability to receive signals from a number of different directions) and gain or the increase in signal level. Generally beamwidth or gain can be increased only at the expense of the other. Gain can be increased by multiplying the number of antenna elements, although this adds "directionality" that reduces beamwidth.

Important antenna considerations

  1. The physical size of antenna components is determined by the frequency of the transmissions it will receive - the higher the frequency, the shorter the elements. At high frequencies, use of a satellite dish will compensate for the reduced amount of energy intercepted by shortened components.


  2. The antenna design should fit the type of radio frequency (RF) signal polarization it will receive. The orientation of radio waves in space is a function of the orientation of the elements of the transmitting antenna. A circularly polarized wave rotates as it propagates through space. Antennas can be designed for either right or left-handed circular polarization. Earth-based communication antennas are either vertical or horizontal in polarization, and not suited for space communication. Police and cellular phone transmissions use vertical polarization because a simple vertical whip antenna is the easiest sort of omnidirectinal antenna to mount on a vehicle.


  3. The antenna needs to produce sufficient signal qain to produce noise free reception.


  4. The antenna should be clear of conductive objects such as power lines, phone wires, etc., so height above the ground becomes important.

Basic antenna components are:

  1. Driven element - the parts connected to and receiving power from the receiver/ trsnsmitter


  2. Parasitic elements - the parts dependent upon resonance rather than connection to a power source


    • A director or parasitic element that rein forces radiation on a line pointing to it from the driven element


    • A reflector or parasitic element that rein forces radiation on a line pointing from it to the driven element.

A fundamental form of antenna is a single wire whose length approximately equals half the transmitting wavelength. Known as a dipole antenna, it is the unit from which many more complex forms of antennas are constructed.

One of the most common forms of VHF antenna is the Yagi/beam, named for the Japanese scientist who first described the principles of combining a basic dipole (driven element) and parasitic elements. A common TV antenna is an example of this type. A Yagi/beam antenna is directional and therefore includes a rotator to aim (direct) the antenna. See yagi.

An omnidirectional antenna has a wide beamwidth and consequently does not require "tracking" (aiming the antenna toward the signal source). An example of an omnidirectional antenna is the turnstile antenna, a variation of the standard dipole antenna well suited for space communications. The quadrifilar helix antenna is omnidirectional and an inherently excellent antenna for ground station use. Quadrifilars are also used on NOAAs polarorbiting environmental satellites.

The parabolic reflector or satellite dish antenna collects RF signals on a passive dishshaped surface. A feedhorn antenna a simple dipole antenna mounted in a resonant tube structure (cylinder with one open end) - transfers the RF energy to a transmission line. The bigger the dish, the greater the amount of RF energy intercepted, and therefore the greater the gain from the signal.

Antenna Array

An ordered assembly of elementary antennae spaced apart and fed in such a manner that the resulting radiation is concentrated in one or more directions.

Antenna Beam

The focused pattern of electromagnetic radiation that is either received or transmitted by an antenna.


A high pressure area where winds blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. See cyclone, wind.

Apogee (aka Apoapsis or Apifocus)

On an elliptical orbit path, point a which a satellite is farthest from the Earth. See Perigee Diagram.


Layer of water-bearing permeable rock, sand, or gravel capable of providing significant amounts af water


French random-access Doppler data collection system. Used on NOAAs Polar-Orbiting Environmental Satellites (POES), ARGOS receives platform and buoy transmissions on 401.65 MHz. This data collection system now monitors more than 4,000 platforms worldwide, outputs data via VHF link, and stores them on tape for relay to a central processing facility.

Argument of Perigee (aka ARGP or w)

One of the six Keplerian elements, it gives the rotation of the satellite on the orbit. The argument (argument meaning angle) of perigee - perigee is the point on an orbital path when the satellite is closest to the Earth - is the angle (measured from the center of the Earth) from the ascending node to perigee. Example:

When ARGP = 0 degrees, the perigee occurs at the same place as the ascending node. That means that the satellite would be closest to Earth just as it rises up over the equator.

When ARGP = 180 degrees, apogee would occur at the same place as the descending node. This means that the satellite would be farthest from Earth just as it rises over the equator.

Arctic Circle

The parallel of latitude that is approximately 66.5 degrees north of the equator and that circumscribes the northern friqid zone.

Artificial Intelligence (AI)

Neural networks. The branch of computer science that attempts to program computers to respond as if they were thinking - capable of reasoning, adapting to new situations, and learning new skills. Examples of artificial intelligence programs include those that can locate minerals underground and understand human speech.

Ascending Node

The point in an orbit (longitude) at which a satellite crosses the equatorial plane from south to north.

Aspect Ratio

The ratio of image width to image height. Weather Facsimile (WEFAX) images have a 1:1 aspect ratio (square); a conventional TV aspect ratio is 4:3 (rectangle).

Astronomical Unit (AU)

The distance from the Earth to the sun. On average, the sun is 149,599,000 kilometers, or 93,440,974 miles from Earth.

ATLAS (Atmospheric Laboratory for Applications and Science) mission

The focus of ATLAS is to study the chemistry of the Earth's upper atmosphere (mainly the stratosphere/mesosphere) and the solar radiation incident on the Earth system (both total solar irradiance and spectrally resolved radiance, especially ultraviolet). Science operations onboard ATLAS 1 (March 1 992) and ATLAS 2 (March-April, 1993) began a comprehensive and systematic collection of data that will help establish benchmarks for atmospheric conditions and the suns stability.


The air surrounding the Earth, described as a series of shells or layers of different characteristics. The atmosphere, composed mainly of nitrogen and oxygen with traces of carbon dioxide water vapor, and other gases, acts as a buffer between Earth and the sun. The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the qlobe and in response to seasonal chanqes. Troposphere stems from the Greek word tropos, which means turning or mixing. The troposphere is the lowest layer of the Earth's atmosphere, extending to a height of 8-15 km (5 - 9 mi), depending on latitude. This region, constantly in motion, is the most dense layer of the atmosphere and the region that essentially contains all of Earth's weather. Molecules of nitrogen and oxygen compose the bulk of the troposphere.

The tropopause marks the limit of the troposphere and the beginning of the stratosphere. The temperature above the tropopause increases slowly with height up to about 50 km (31 mi). The stratosphere and stratopause stretch above the troposphere to a height of 50 km. It is a region of intense interactions among radiative, dynamical, and chemical processes, in which horizontal mixing of gaseous components proceeds much more rapidly than vertical mixing. The stratosphere is warmer than the upper troposphere, primarily because of a stratospheric ozone layer that absorbs solar ultraviolet energy. The mesosphere, 50 to 80 km above the Earth, has diminished ozone concentration and radiative cooling becomes relatively more important. The temperature begins to decline again (as it does in the troposphere) with altitude. Temperatures in the upper mesosphere fall to -70 to - 140 degrees Celsius, depending upon latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with some of the billions of gas particles contained in that layer. The collisions create enough heat to burn the falling objects long before they reach the ground. The stratosphere and mesosphere are referred to as the middle atmosphere. The mesopause, at an altitude of about 80 km, separates the mesosphere from the thermosphere - the outermost layer of the Earth's atmosphere. The thermosphere, from the Greek thermo for heat, begins about 80 km above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures can rise to 2,000 degrees Celsius. Radiation causes the scattered air particles in this layer to become charged electrically, enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000 km above the Earths surface, the atmosphere blends into space. The few particles of gas here can reach 4,500 degrees F (2,500 degrees C) during the day.

Atmospheric Infrared Sounder

Advanced sounding instrument selected to fly on the EOS-PM I mission (intermediate sized, sun-synchronous, morning satellite) in the year 2000. It will retrieve vertical temperature and moisture profiles in the troposphere and stratosphere. Designed to achieve temperature retrieval accuracy of 1 degree C with a 1 km vertical resolution, it will fly with two operational microwave sounders. The three instruments will constitute an advanced operational sounding system, relative to the TIROS Operational Vertical Sounder (TOVS) currently flying on NOAA polar-orbiting satellites. See Earth Observing System, TIROS-N/NOAA Satellites.

Atmospheric Pressure

The amount of force exerted over a surface area, caused by the weight of air molecules above it. As elevation increases, fewer air molecules are present. Therefore, atmospheric pressure always decreases with increasing height. A column of air, 1 square inch in cross section, measured from sea level to the top of the atmosphere would weigh approximately 14.7 Ib/in2. The standard value for atmospheric pressure at sea level is:

  • 29.92 inches of mercury
  • 760 mm of mercury
  • 1013.25 millibars (mb)
  • 101,325 pascals (Pa}

Atmospheric Radiation Measurements Program (ARM)

U.S. Department of Energy program for the continual, ground-based measurements of atmospheric and meteorological parameters over approximately a ten-year period. The program will study radiative forcing and feedbacks, particularly the role of clouds. The general program goal is to improve the performance of climate models, particularly general circulation models of the atmosphere.

Atmospheric Response Variables

Variables that reflect the response of the atmosphere to external forcing (e.g., temperature, pressure, circulation, and precipitation).

Atmospheric Window

The range of wavelengths at which water vapor, carbon dioxide, or other atmospheric gases only slightly absorb radiation. Atmospheric windows allow the Earth's radiation to escape into space unless clouds absorb the radiation. See greenhouse effect.


A coral island consisting of a ring of coral surrounding a central lagoon. Atolls are common in the Indian and Pacific Oceans.


The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals.

Audio Frequencies

Frequencies that the human ear can hear (usually 30 to 20,000 cycles per second).

Automatic Picture Ttansmission (APT)

System developed to make real-time reception of satellite images possible whenever an APT- equipped satellite passes within range of an environmental satellite ground station. Transmission (analog video format) consists of an amplitude-modulated audible tone that can be displayed as an image on a computer monitor when received by an appropriate ground station.

APT images are transmitted by polar-orbiting satellites such as the TIROS-N/NOAA satellites, Russia's METEOR, and the Chinese Feng Yun, which orbit 500-900 miles above the Earth, and offer both visible and infrared images. An APT image has thousands of squares called picture elements or pixels Each pixel represents a four-km square.


The direction, in degrees referenced to true north, that an antenna must be pointed to receive a satellite signal (compass W direction). The angular distance is measured in a Clockwise direction.


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

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