Microwave radio signals transmitted by GPS satellites are refracted by the atmosphere as they propagate to the Earth surface.
This delay, regarded as a nuisance parameter by geodesists, is directly related to the amount of water vapour in the atmosphere, and hence is a product of considerable value for meteorologists.
Measurement of atmospheric water vapour by Earth based GPS receivers was demonstrated in various international projects during the last few years.
H Derks, et al.. GPS water vapour meteorology : status report
KNMI number: WR-97-04, Year: 1997, Pages: 0