Wet-bulb temperature

Wet-bulb temperature is the lowest temperature attainable by evaporative cooling of air at a given pressure and humidity. It measures how strongly evaporation can cool a moist surface.

Evaporative cooling: Cooling that occurs when a liquid absorbs heat as it changes into vapor. Evaporation removes heat from the wetted surface until air and water reach equilibrium.
Heat index: An apparent-temperature measure combining air temperature and humidity to estimate heat stress in shade. Unlike the heat index, wet-bulb temperature has a direct thermodynamic definition.
Dry-bulb temperature: The air temperature measured by a thermometer shielded from radiation and moisture. It is the uncooled air temperature against which wet-bulb depression is measured.
Psychrometer: An instrument that estimates air moisture by comparing wet-bulb and dry-bulb temperatures. It provides the classic direct measurement of wet-bulb temperature.
Psychrometrics: The study of the thermodynamic properties of moist air and their measurement. Wet-bulb temperature is one of psychrometrics’ defining state variables.
Wet-bulb globe temperature: A heat-stress index combining natural wet-bulb, globe, and air temperatures. It uses a natural wet-bulb measurement as one component of outdoor heat-risk assessment.
Dew point: The temperature to which air must cool at constant pressure for water vapor to condense. Dew point marks saturation by cooling, whereas wet-bulb temperature reflects evaporative equilibrium.
August psychrometer: A stationary psychrometer with wet and dry thermometers exposed to ambient air. Its design illustrates the early method for measuring evaporative cooling.
Relative humidity: The ratio of water vapor present in air to the maximum amount possible at the same temperature. It determines how much additional water can evaporate and how far cooling can proceed.
Cooling tower: A heat-rejection device that cools water through contact with moving air and evaporation. The achievable cooling-water temperature is constrained by the entering air’s wet-bulb temperature.

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