wetbulb

Wetbulb temperature, often written as wet-bulb temperature, is a thermodynamic property of moist air that represents the lowest temperature that can be reached by evaporative cooling of a parcel of air at constant pressure. It is determined by the balance between heat transfer to the air and the latent heat absorbed during evaporation of water. A wet-bulb thermometer has a bulb wrapped in a wick that is kept moist; as air passes over the wick, water evaporates and cools the bulb, yielding the wet-bulb reading.

The measurement depends on the ambient dry-bulb temperature and the humidity of the air. At relative humidity of 100 percent, no evaporation occurs and the wet-bulb temperature equals the dry-bulb temperature. In drier air, evaporation is more vigorous, and the wet-bulb reading can be several degrees lower than the dry-bulb temperature. In practice, wet-bulb measurements are obtained with a sling psychrometer or an aspirated psychrometer, which provide paired dry- and wet-bulb temperatures used to derive humidity metrics.

In meteorology and engineering, the wet-bulb temperature is used to characterize evaporative cooling potential and to compute humidity, dew point, and heat index values via psychrometric relations. It is also used in the design of cooling towers, ventilation systems, and agricultural practices, as well as in assessing human heat stress, because it reflects the limit of evaporative cooling from the skin.

Limitations include the need for steady-state conditions, controlled airflow, and properly prepared wick and water; measurement errors can arise from evaporation rate, contamination, or poor ventilation.