Johnson-williams liquid water probe

From Glossary of Meteorology

Johnson–Williams liquid water probe

A hot-wire type instrument for measuring the liquid water content of clouds in situ.

The probe is most often used on research aircraft, but also occasionally at mountain-top installations and in wind tunnels. Resistivity changes, which occur as cloud droplets in the airflow that impinge on, and evaporate from, an exposed electrically heated wire, are sensed by an electric circuit. The liquid water content of the air is estimated from this signal using compensations for air temperature variations detected by a similar unexposed wire in the probe and knowledge of the airspeed.