On a water vapor image, each pixel is assigned a gray shade according to the measured brightness temperature. Typically, white indicates a very cold brightness temperature (radiation from a moist layer or cloud in the upper troposphere), and black indicates a warm brightness temperature (radiation from the Earth or a dry layer in the middle troposphere).

The "moist" and "dry" features seen on water vapor imagery result from various combinations of vertical motion, horizontal deformation and moisture advection within the middle and upper troposphere (generally the 4-12 km altitude range). Such features exhibit spatial and temporal continuities which are evident using image animation loops. Water vapor is therefore a "passive tracer" which can be used to represent three-dimensional atmospheric motions on the meso and synoptic scale.

• TIROS -- The first meteorological satellites to measure radiation emitted within the water vapor absorption portion of the IR spectrum were the U.S. TIROS series. TIROS 2, 3 and 4 (launched in 1960, 1961 and 1962) carried scanning radiometers which were sensitive to the 6.0-6.5 micron wavelength band (Bandeen, 1961; Moller and Raschke, 1964; Raschke and Bandeen, 1967).

• Nimbus -- The next U.S. satellites to image tropospheric water vapor were Nimbus 2 through Nimbus 7 (launched between 1966 and 1978). Nimbus 2 and 3 carried an instrument called the Medium-Resolution Infrared Radiometer (MRIR), which sensed moisture in the 6.4-6.9 micron band (Nordberg, 1966; Holub and Shenk, 1972). Nimbus 4 through Nimbus 7 carried a Temperature Humidity Infrared Radiometer (THIR), which measured wavelengths between 6.5 and 7.2 microns (Allison, 1972).

• TIROS N/Advanced TIROS N -- This series (also known as the NOAA polar orbiter satellites) includes a High Resolution Infrared Radiation Sounder (HIRS) as part of its payload. HIRS/1 actually flew on the Nimbus 6 satellite, while HIRS/2 has flown on every TIROS/NOAA satellite since TIROS N (launched in 1978). HIRS/2 has 3 water vapor channels within the 6.7 - 8.2 micron range. NOAA 12 and NOAA 14 are the two current operational polar orbiters.

• Meteosat -- The first geostationary orbit satellite with water vapor imaging capability was the European Meteosat-1 (launched 1977). The Meteosat series carries an imaging radiometer with a 5.7-7.1 micron water vapor channel (Morel, Desbois, and Szejwach, 1978). Meteosat-6 is currently operational at the prime meridian (0 degrees longitude). Meteosat-7 was launched September 1997, and is undergoing evaluation.

• GOES -- In 1980, the U.S. GOES-4 satellite introduced the Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS), which made water vapor measurements in the 6.7 and 7.3 micron channels. VAS was part of the GOES payload through GOES-7 (launched 1987). The first three satellites in the GOES I-M series were launched in 1994 (GOES-8, operational at 75W longitude), 1995 (GOES-9, on-orbit storage) and 1997 (GOES-10, operational at 135W longitude). These satellites carry separate imager and sounder instruments which offer significant improvements over the VAS instrument on the previous generation of GOES (Menzel and Purdom, 1994).

• GMS -- The Japanese GMS-5 was launched in 1995. The GMS-5 satellite (operational at 140E longitude) carries a VAS payload which is similar to the GOES VAS.

• Feng Yun-2b (FY-2b) -- The first Chinese geostationary satellite (FY-2a) was lost in a pre-launch accident. FY-2b (launched in 1997) includes a 6.3-7.6 micron water vapor channel, and will soon be operational at 105E longitude.

• GOMS (Elektro) -- The Russian Geostationary Operational Meteorological Satellite (GOMS) N1 was launched in 1994, and is semi-operational at 76E longitude. While GOMS N1 has no water vapor imaging capability, an improved imager is planned for the GOMS N2 satellite (proposed 1998 launch) which would include a water vapor channel.

• Meteosat Second Generation -- Scheduled for launch in the year 2000, MSG-1 is slated to have two water vapor channels (6.2 and 7.3 microns).