From AMS Glossary
(Redirected from Radar hygrometer)
The refractive index of the moist air is measured either by measuring its dielectric constant using special capacitors or by measuring frequency shifts. For the latter method a measuring cell containing the moist air is compared with a reference cell containing dry air or an inert reference gas. The frequencies used are about 10 GHz, which corresponds to a wavelength of about 30 cm.