Microwave refractometer

From Glossary of Meteorology

microwave refractometer

An instrument that measures the refractive index of air by determining changes in humidity.

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.