National Oceanic and Atmospheric Administration

Water vapor is one of the most important constituents of the free atmosphere since it is the principal mechanism by which moisture and latent heat are transported and cause “weather. ” The measurement of atmospheric water vapor is essential for weather and climate research as well as for operational weather forecasting. An important goal in modern weather prediction is to improve the accuracy of short-term cloud and precipitation forecasts, but our ability to do so is severely limited by the lack of timely water vapor data. Most of the water vapor in the atmosphere resides in the troposphere, which ranges from an altitude of approximately 9 kilometers at the poles to more than 16 kilometers at the equator. The National Weather Service primarily uses balloon-borne radiosondes, launched at 12-hour intervals at approximately 100 sites in the United States, to measur