Precipitable Water Vapor
10
H|0|alt|altimeter|102|110|
H|0|urad|radiometer|260|269|
H|0|ocean|ocean circulation|661|677|
H|0|climate|climate change|1043|1056|
G|0|32cm|32 centimeters|597|610|
G|0|5gsc|5 grams per square centimeter|521|549|
G|0|gg|greenhouse gas|864|877|
G|0|atmos|atmosphere|62|71|
G|0|atmoscirc|atmospheric circulation|784|806|
G|0|sshumid|sea-surface humidity|812|831|
    Water vapor is an ever-present ingredient of our earth's atmosphere. When TOPEX/Poseidon's radar altimeter measures sea level, water vapor in the atmosphere delays the return of the radar pulse and thereby produces a false reading. 
    Using a microwave radiometer, water vapor content is measured for a cylinder of air extending from the satellite to the surface. The measured water vapor content is used to correct the altimeter's signal. Without this correction, in the tropics, a typical water vapor content of 5 grams per square centimeter would cause an altimeter measurement error of 32 centimeters; such an error would interfere with the study of ocean circulation. 
    We measure water vapor not only to correct altimetry readings, but also because it plays a part in atmospheric circulation and sea-surface humidity. In addition, water vapor is a greenhouse gas that affects the earth's heat balance. Thus, monitoring the global water vapor content is an important task for understanding the role of the oceans in weather and climate change. 
