A Basic Introduction to Water Vapor Imagery

Water vapor comprises only 1-4% (by volume) of the atmosphere, yet it plays a critical role (along with carbon dioxide, ozone, and other so-called "greenhouse gases") in the Earth's energy balance. Water vapor absorbs and reradiates electromagnetic radiation in various wavelength bands, most notably the infrared 6-7 micron band. Such infrared radiation -- emitted by the Earth/atmosphere and intercepted by satellites -- is the basis for remote sensing of tropospheric water vapor.

Meteorological satellite payloads include scanning radiometers which are designed to detect upwelling terrestrial radiation in discrete wavelength bands. The GOES I-M series of satellites carry two separate radiometer packages -- an imager and a sounder. The imager has one water vapor channel (channel 3), while the sounder has three water vapor bands (bands 10, 11 and 12). These water vapor channels/bands employ mercury cadmium telluride (HgCdTe) detectors which are sensitive to radiation at various wavelength intervals within the 6.5-7.4 micron range. The output voltage from a particular radiometer detector is proportional to the energy striking that detector area per unit time (radiance). Given the intensity and wavelength of the radiance, the Planck function yields an equivalent brightness temperature.

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.

A Brief History of Remote Sensing of Water Vapor from Meteorological Satellites

Low Earth orbit satellites

Geostationary orbit satellites

Future satellites

Back to main page | Radiative transfer concepts | Forecasting applications | COMET case studies