The dry-bulb temperature is the temperature of a perfectly-dry surface when exposed to convective heating of a humid gas stream. This temperature is higher than that of a wetted surface exposed to the same gas conditions of humidity, temperature and velocity. If the surface receives only convective heating from the gas, being isolated from other heating sources through conduction or radiation, then the dry-bulb temperature is the temperature of the gas itself. A dry-bulb thermometer is one of two temperature sensors which make up a dry- and wet-bulb hygrometer or psychrometer used to measure the moisture content (or humidity) of a moist gas. In a properly-designed psychrometer, both sensors are shielded from thermal radiation and exposed to a flow of the moist gas at a sufficient rate to ensure fully-developed forced-convective heat transfer. Normally, a gas velocity of 5 m s−1 is adequate [Wiederhold (1995)].
Wiederhold, P. (1995) Humidity Measurements, Chapter 42 of Handbook of Drying Technology, (Ed. A.S. Mujumdar), 2nd ed. Marcel Dekker, New York and Basel.