Abstract: An aircraft system intended for gliders and low power-to-weight aircraft, to aid the pilot in locating updrafts associated with the buoyancy of humid air in the convective layer of the atmosphere, namely moist thermals. The system comprises a pair of laterally spaced hygrometers, generally at the wingtips. Four types of hygrometer units are described; resistive, piezoelectric, spectrometric, and capacitive. Humidity data is transmitted to the cockpit by telemetry links, of which four types are also described; by dedicated wiring, by carrier current using wingtip position lights wiring, by modulated infrared beams, and by low-power radio waves. The system monitors the difference of humidity between left and right sensors, and also the rate of change of humidity of one or both sensors as a function of time.