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researches launching aerosonde balloon
An aerosonde balloon run (precursor to the radiosonde), on the summit of Mount Washington. Circa mid 1930's. Joe Dodge Photo Collection

Water vapor is one of the most significant constituents of the atmosphere since it is the means by which moisture and latent heat are transported to cause "weather". Water vapor is also a greenhouse gas that plays a critical role in the global climate system.

This role is not restricted to absorbing and radiating energy from the sun, but includes the effect it has on the formation of clouds and aerosols and the chemistry of the lower atmosphere. Despite its importance to atmospheric processes over a wide range of spatial and temporal scales, water vapor is one of the least understood and poorly described components of the Earth's atmosphere.

Primarily, water vapor is observed through the use of radiosondes. The radisonde is a small, lightweight box equipped with weather instruments and a radio transmitter. It is attached to a cord that has a parachute and a gas filled balloon. As the balloon rises, the attached radiosonde measures air temperature, humidity and air pressure. Special radar tracking equipment converts the continuously changing altitude and distance of the radiosonde into wind velocity and direction data.

GPS-IPW instrument
Ground-based GPS-IPW instrument in Bartlett, NH.

There are presently about 900 stations worldwide that send up radiosondes, twice a day every day at Noon and Midnight, Greenwich Mean Time.

Even with all that data, meteorologists don't have enough information to improve their forecasting accuracy. However, scientists at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory have been working to change that.

The Ground-based Global Positioning System Integrated Precipitable Water Vapor Instrument (GPS-IPW) consists of a GPS antenna in a fixed position. The sensitivity of the system is such that it can determine its distance from the Earth's center to better than 10 mm. This project, a collaboration between NOAA and the Mount Washington Observatory, uses the instrument as part of a GPS network to analyze water vapor content in the atmosphere. This is done by measuring the delay in the reception of a signal from the network of GPS satellites in orbit to the antenna on the ground. By the system knowing its precise location on the Earth, analysis of the time delay in the reception of the signals from space can determine the amount of water vapor in the atmosphere.

The ability of forecasters to have "real-time" data of moisture at the upper-levels, instead of the current system of twice daily soundings via balloon, will dramatically increase forecasting accuracy.

Additional information about the NOAA Ground-Based GPS-IPW project

NOAA-ESRL Ground-Based GPS Meteorology Network website

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