Definitions and Keywords

Active Remote Sensing
Remote sensing that involves emitting a signal and then measuring that signal when it is backscattered (or reflected) back to the instrument. Examples include radars and lidars (LIght Detection And Ranging Radars - similar to Radar but uses a laser).

This can refer to either a narrow spectral channel (see below) selected out of the electromagnetic spectrum, or to a larger portion of the spectrum.

A discrete portion of the spectrum measured by a satellite instrument, defined by a filter function (vs. wavelength). Satellite channels have a finite width, typically ranging from around 0.2 micrometers to greater than 1.0 micrometers in the infrared, or to greater than 10 micrometers for sounder infrared channels.

El Nino
A name given to the event when abnormally warm surface waters appear near the coast of equatorial South America.

Sometimes called geosynchronous - a characteristic of a satellite orbit in which the satellite circles the globe, over the equator, in synchronization with the earth's rotation. These satellites remain over the same earth location, allowing images of the scene below the satellite to be taken continuously, with little or no perceived movement.

Geostationary Operational Environmental Satellites (GOES) are the North American series of geostationary weather satellites. Currently there are two operational GOES satellites, GOES-8 (75W) and GOES-10 (135W), and two in-orbit replacements, GOES-11 and GOES-12. The GOES satellites provide almost continuous viewing of the Americas and surrounding oceans with spatial resolution up to 1 km in the visible channel, and 4 km in the infrared channels. The GOES have separate imaging and sounding systems to better suit the needs of the atmospheric science community for both their operational and research needs. The current imager provides routine coverage in five spectral channels (a visible, a shortwave window, a split window pair, and a mid-tropospheric water vapor band). The sounder provides a visible channel and 18 infrared channels for temperature and moisture profiling and cloud applications. This is a substantial improvement over the previous GOES VAS instrument which had 12-channels for all applications. Changes in channel locations and spatial resolutions will be in effect starting with GOES-12 onward.

Hydrologic Cycle
The movement of water among the reservoirs of the ocean, the atmosphere, and the land.

As applied to GOES satellites: a 5-channel instrument designed to measure the visible and infrared portions of the electromagnetic spectrum. Normal scheduling of the imager provides operational images every 15 minutes over most of the U.S.

Infrared Radiation
The portion of the electromagnetic spectrum with wavelengths ranging from longer than visible radiation, starting around 0.7 micrometers, to wavelengths shorter than those in the microwave portion of the spectrum. Satellite instruments typically measure infrared radiation between wavelengths of about 3 micrometers and 20 micrometers.

Long wave
When referring to the infrared portion of the electromagnetic spectrum, longwave is the region above about 10 micrometers.

Medium wave
When referring to the infrared portion of the electromagnetic spectrum, medium wave is the region between about 5 and 10 micrometers.

Meridional Wind
The wind, or wind component, in the north-south direction.

The Meteosat series of geostationary satellites is the European Community’s (operated by Eumetsat) equivalent to the U.S. GOES satellites and provides similar imaging capabilities in three channels.

The portion of the electromagnetic spectrum with much longer wavelengths than infrared radiation, typically above about one millimeter.

Moderate Resolution Imaging Spectroradiometer (MODIS) is the key instrument aboard the Terra (EOS AM-1) satellite. Terra MODIS is viewing the entire Earth's surface every to 2 days, acquiring data in 36 spectral bands, with spatial resolution of 250 m and 1 km for the visible and infrared channels, respectively.

Multispectral Imagery
The process of using combinations of imagery taken at different wavelengths to generate a product. (eg. vegetation classification, vertical temperature, moisture sounding, etc.)

Outgoing Longwave Radiation - A type of radiation emitted by the earth. Although the atmosphere obsorbs only a small percentage of the short-wave solar radiation, it absorbs much of the long-wave terrestrial radiation. Only when the earth's surface temperature is fairly high does the radiational loss through the transparent bands and from the top of the atmosphere equal the amount absorbed from the sun. This heat-retaining behavior is similar to that of a greenhouse.

Passive Remote Sensing
Remote sensing that involves measuring the natural emission of radiation. An example is weather satellites.

A characteristic of a satellite orbit that allows the satellite to circle the globe approximately over the poles of the earth. Polar-orbiting satellites have orbital inclinations, with respect to the equator, of close to 90 degrees. Typically, polar-orbiting weather satellites are also sun-synchronous.

Precipitable Water
The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending from the surface to the top of the atmosphere.

Remote Sensing
The process of measuring a parameter (such as temperature, moisture, or rainfall) from a distance (such as from space, or by using a radar). This is done by measuring natural emission of radiation or backscattered radiation (i.e. radiation that was reflected or bounced back to the observing instrument) from an emission source.

The size of the field-of-view (FOV) of a satellite picture element, often called a pixel, as measured on the earth in kilometers. Resolution can have a second meaning: as the distance between the centers of adjacent picture elements. The two resolutions can be different, resulting in either overlap of individual FOVs, or gaps between them.

As applied to GOES satellites: a 19-channel instrument designed to provide visible and infrared spectral radiances, used to vertically probe, or sound, the atmosphere. This is done by employing spectral bands with different amounts of atmospheric absorption, in order to measure temperatures and moisture at different depths in the atmosphere. Sounder data is typically available from GOES every hour, over the same locations.

A characteristic of a satellite orbit that allows the satellite's path to precess, or rotate slowly in synchronization with the earth's revolution about the sun. Sun-synchronous satellites view the earth at the same local time each pass; and, by necessity, are polar-orbiting, viewing the earth below during both a day-time and a night-time overpass, approximately twelve hours apart.

Television and InfraRed Observation Satellite - an old term used for the first polar-orbiting weather satellites. Currently, satellites in the series are called NOAA satellites.

The portion of the electromagnetic spectrum viewable by the naked eye, with wavelengths ranging from approximately 0.43 micrometers to 0.69 micrometers.

Water Vapor Transport
The movement or transport of moisture by the wind is an important aspect of the hydrologic cycle. Traditionally this transport has been calculated with the use of rawinsonde data. The availability of global atmospheric models has allowed for the calculations to be made over the globe with the initialized or forecasted modeled variables. Recent applications of satellite data allow for the determination of both wind and moisture fields, which, like the rawinsonde and model data, can be used to calculate the transport of water vapor in the atmosphere.

Water Vapor Winds
The application of a time sequence of co-registered satellite images can be used to estimate the motion of features in the images. When applied to cloud imagery, cloud motion vectors, or cloud drift winds, are generated. These winds capture the relative movement of the clouds with respect to the Earth. When applied to mid- and upper-tropospheric water vapor imagery, the winds are referred to as water vapor (tracked) winds. Water vapor winds are generally thought to represent the large-scale flow patterns in the mid- and upper-troposphere.

What is the wind and why is it important? Sunlight striking the earth's surface heats the air above, producing pressure differences. In an attempt to equalize these differences, the atmosphere responds by moving masses of air,which causes the wind. Additional forces come in to play that can influence the wind speed and direction as well. The distribution of the wind can tell us a great deal about the state of the atmosphere. The wind transports water vapor, heat, greenhouse gases, and aerosols, which interact with clouds and incoming/outgoing radiation. All of these factors work together to produce our weather and climate.

Zonal Wind
The wind, or wind component, in the east-west direction.




  • AVHRR - Advanced Very High Resolution Radiometer
  • CAMEX - Convection and Moisture Experiment
  • CaPE - Convection and Precipitation/Electrification Experiment
  • ENSO - El Nino Southern Oscillation
  • GMS - Geosynchronous Meteorological Satellite (Japanese)
  • GOES - Geostationary Operational Environmental Satellite
  • IWC - Integrated Water Content (same as PW)
  • LST - Land Surface Temperature
  • MAMS - Multispectral Atmospheric Mapping Sensor
  • MAS - MODIS Airborne Simulator
  • MODIS - Moderate Resolution Imaging Spectroradiometer
  • NCAR - National Center for Atmospheric Research
  • NCEP - National Centers for Environmental Prediction
  • NDVI - Normalized Difference Vegetation Index
  • NVAP - NASA Water Vapor Project
  • PW - Precipitable Water
  • TIROS - Television and InfraRed Observation Satellite
  • SSM/I - Special Sensor Microwave/Imager
  • SSM/T - Special Sensor Microwave/Temperature (sounder)
  • SST - Sea Surface Temperature
  • STORMFEST - Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test
  • UTH - Upper Tropospheric Humidity
  • VISSR - Visible Infrared Spin Scan Radiometer (an instrument aboard GOES-1 through 7)
  • VAS - VISSR Atmospheric Sounder (an instrument aboard GOES-4 through 7)
  • WVTI - Water Vapor Transport Index

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Last updated on: January 17, 2002