You searched for: “psychrometer
psychrometer (s) (noun), psychrometers (pl)
A device for measuring app the humidity of the atmosphere by the difference in temperature between two thermometers: A psychrometer keeps the bulb of one thermometer to stay moist, the other one dry. Evaporation from the moist bulb lowers the reading of that thermometer and the greater the difference in readings, the drier the air. No difference indicates 100% relative humidity.