What is wet-bulb temperature?

Wet-bulb temperature (WBT) is the lowest temperature to which something can be cooled through water evaporation at a constant barometric pressure. It is measured using a wet-bulb thermometer, which has a bulb covered in wet cloth that is subjected to moving air. The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in temperature. The wet-bulb temperature is nearly always lower than the dry-bulb temperature, except when the air is already saturated with water. WBTs are used to determine relative humidity and dew-point temperatures and are useful indicators of whether it is safe to engage in outdoor activities. The evaporative cooling that occurs mimics the cooling of the human body from perspiration, so a WBT can be read as the minimum temperature at which sweating can cool the body.

How is wet-bulb temperature measured?

Wet-bulb temperature is measured using a wet-bulb thermometer . This thermometer has a bulb covered in wet cloth that is subjected to moving air. The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in temperature. To ensure airflow, the thermometer is rapidly rotated, often as part of a psychrometer , or exposed to a fan. A sling psychrometer is whirled until the wet-bulb thermometer's reading reaches a constant value. The difference between the dry-bulb and wet-bulb temperatures is known as the wet-bulb depression and can determine relative humidity.

How does wet-bulb temperature differ from regular (dry-bulb) temperature?

A dry-bulb thermometer measures ambient air temperature, whereas a wet-bulb thermometer measures the temperature of a surface cooled by water evaporation into ambient air. The wet-bulb thermometer's bulb is covered in wet cloth, and air flows across it. The drier the air, the more moisture it can absorb, increasing water evaporation and lowering the wet-bulb temperature. The wet-bulb temperature is nearly always lower than the dry-bulb temperature, and the difference is called wet-bulb depression. When the air is saturated with water, the values are equal because the air cannot accept more moisture. Wet-bulb temperatures help determine relative humidity and dew-point temperatures and assess the safety of outdoor activities.

Why is wet-bulb temperature important in weather forecasting?

Wet-bulb temperature (WBT) is important in weather forecasting because it indicates the lowest temperature to which a person or object can be cooled by water evaporation at constant barometric pressure. WBTs help determine relative humidity and calculate dew-point temperatures. The evaporative cooling that occurs when air passes over a wet-bulb thermometer mimics the cooling of the human body from perspiration. Therefore, WBT can be read as the minimum temperature at which sweating can cool the body in a particular environment. Awareness of WBTs has grown with global warming, which has increased humidity and global temperatures. A critical WBT for humans is around 35 °C (95 °F), beyond which healthy people can survive for only six hours.

How does humidity affect the wet-bulb temperature?

The wet-bulb temperature (WBT) is the lowest temperature to which something can be cooled through water evaporation at constant barometric pressure. Humidity affects the wet-bulb temperature because the rate of evaporation depends on how much moisture the surrounding air can absorb. The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in the wet-bulb thermometer's reading. When the air is already saturated (100% relative humidity), no further evaporation occurs, and the wet-bulb and dry-bulb temperatures are equal. WBTs help determine relative humidity and dew-point temperatures. They also indicate the minimum temperature at which sweating can cool the body, which is useful for gauging the safety of outdoor activities.

Why is a high wet-bulb temperature dangerous for humans and animals?

A high wet-bulb temperature (WBT) is dangerous because it impairs the body's ability to cool itself through sweat evaporation. The wet-bulb temperature measures how effectively water evaporates from a surface, like skin. When the WBT rises, the air is already saturated with moisture, reducing the amount of sweat that can evaporate. With less evaporative cooling, the body is at risk of overheating, which can lead to heatstroke, a condition marked by an extremely high body temperature that can damage the central nervous system. A WBT of 35 °C (95 °F) is considered a critical threshold, beyond which even healthy individuals can only survive for approximately six hours. Some studies suggest that the survivable threshold WBT for humans may be as low as 31.5 °C (88.7 °F).

Wet-bulb temperature

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Images

Illustration of two psychrometers: on the left, a sling psychrometer with two thermometers and a handle for swinging; on the right, a stationary psychrometer with labeled dry-bulb and wet-bulb thermometers, wicks, and a water container.

A wet bulb dry bulb thermometer, or hygrometer, with two glass thermometers mounted on a yellow board labeled dry and wet, used to measure humidity.

A table showing relative humidity values dry-bulb temperature values on the y-axis and the difference between the dry-bulb and wet-bulb temperatures on the x-axis.

Quick Facts

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Wet-bulb temperature

physics

Also known as: WBT

Written by

Adam Volle

Fact-checked by

Britannica Editors

Last updated

Nov. 10, 2026 •History

Top Questions

What is wet-bulb temperature?
Wet-bulb temperature (WBT) is the lowest temperature to which something can be cooled through water evaporation at a constant barometric pressure. It is measured using a wet-bulb thermometer, which has a bulb covered in wet cloth that is subjected to moving air. The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in temperature.

The wet-bulb temperature is nearly always lower than the dry-bulb temperature, except when the air is already saturated with water. WBTs are used to determine relative humidity and dew-point temperatures and are useful indicators of whether it is safe to engage in outdoor activities. The evaporative cooling that occurs mimics the cooling of the human body from perspiration, so a WBT can be read as the minimum temperature at which sweating can cool the body.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How is wet-bulb temperature measured?
Wet-bulb temperature is measured using a wet-bulb thermometer. This thermometer has a bulb covered in wet cloth that is subjected to moving air. The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in temperature.

To ensure airflow, the thermometer is rapidly rotated, often as part of a psychrometer, or exposed to a fan. A sling psychrometer is whirled until the wet-bulb thermometer's reading reaches a constant value. The difference between the dry-bulb and wet-bulb temperatures is known as the wet-bulb depression and can determine relative humidity.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does wet-bulb temperature differ from regular (dry-bulb) temperature?
A dry-bulb thermometer measures ambient air temperature, whereas a wet-bulb thermometer measures the temperature of a surface cooled by water evaporation into ambient air. The wet-bulb thermometer's bulb is covered in wet cloth, and air flows across it. The drier the air, the more moisture it can absorb, increasing water evaporation and lowering the wet-bulb temperature.

The wet-bulb temperature is nearly always lower than the dry-bulb temperature, and the difference is called wet-bulb depression. When the air is saturated with water, the values are equal because the air cannot accept more moisture. Wet-bulb temperatures help determine relative humidity and dew-point temperatures and assess the safety of outdoor activities.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is wet-bulb temperature important in weather forecasting?
Wet-bulb temperature (WBT) is important in weather forecasting because it indicates the lowest temperature to which a person or object can be cooled by water evaporation at constant barometric pressure. WBTs help determine relative humidity and calculate dew-point temperatures.

The evaporative cooling that occurs when air passes over a wet-bulb thermometer mimics the cooling of the human body from perspiration. Therefore, WBT can be read as the minimum temperature at which sweating can cool the body in a particular environment. Awareness of WBTs has grown with global warming, which has increased humidity and global temperatures. A critical WBT for humans is around 35 °C (95 °F), beyond which healthy people can survive for only six hours.
1 Britannica Source
1. wet-bulb temperature
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does humidity affect the wet-bulb temperature?
The wet-bulb temperature (WBT) is the lowest temperature to which something can be cooled through water evaporation at constant barometric pressure. Humidity affects the wet-bulb temperature because the rate of evaporation depends on how much moisture the surrounding air can absorb.

The drier the air, the more moisture it can absorb, leading to faster evaporation and a greater decrease in the wet-bulb thermometer's reading. When the air is already saturated (100% relative humidity), no further evaporation occurs, and the wet-bulb and dry-bulb temperatures are equal.

WBTs help determine relative humidity and dew-point temperatures. They also indicate the minimum temperature at which sweating can cool the body, which is useful for gauging the safety of outdoor activities.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is a high wet-bulb temperature dangerous for humans and animals?
A high wet-bulb temperature (WBT) is dangerous because it impairs the body's ability to cool itself through sweat evaporation. The wet-bulb temperature measures how effectively water evaporates from a surface, like skin. When the WBT rises, the air is already saturated with moisture, reducing the amount of sweat that can evaporate.

With less evaporative cooling, the body is at risk of overheating, which can lead to heatstroke, a condition marked by an extremely high body temperature that can damage the central nervous system. A WBT of 35 °C (95 °F) is considered a critical threshold, beyond which even healthy individuals can only survive for approximately six hours. Some studies suggest that the survivable threshold WBT for humans may be as low as 31.5 °C (88.7 °F).
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Wet-bulb temperature (WBT), lowest temperature to which a person or an object can be cooled solely by the evaporation of water, given a constant barometric pressure. It is so named because its approximate value is obtained from a wet-bulb thermometer. Whereas a normal, dry-bulb thermometer measures the temperature of ambient air, a wet-bulb thermometer measures the temperature of a surface from which water has evaporated into a stream of ambient air. The bulb of a wet-bulb thermometer is covered in cloth, usually muslin, that has been soaked in water at ambient temperature and then subjected to a source of moving air.

The air flow is achieved by rapidly rotating the thermometer, often as part of a psychrometer, or by exposing the thermometer to a fan. The drier the air that flows across the wet-bulb thermometer is, the more moisture that air is capable of absorbing and hence the more rapidly the water in the cloth evaporates. The faster the water evaporates, the greater the decrease in the temperature registered by the wet-bulb thermometer is. This phenomenon occurs because evaporation is an endothermic process, meaning that it absorbs heat (see also hygrometer). Consequently, in a given situation, the wet-bulb temperature (WBT) is nearly always lower than the dry-bulb (or ambient-air) temperature. The difference between these temperature values is known as the wet-bulb depression. An exception to this circumstance occurs when the air is already saturated with water (that is, when the relative humidity of the air is 100 percent), so the air cannot accept more moisture from the cloth. In this situation, the wet-bulb thermometer value and the dry-bulb thermometer value will be equal.

Uses

WBTs from wet-bulb thermometers are used primarily in determining the relative humidity of a volume of air or to calculate dew-point temperatures. WBTs are also useful indicators in determining whether it is safe to engage in outdoor activities. The evaporative cooling that occurs when air passes over the wet-bulb thermometer’s cloth mimics the cooling of the human body from perspiration, so a WBT can be read as the minimum temperature at which sweating can cool the body in a particular environment, given that environment’s current air temperature and humidity. The body’s ability to tolerate the conditions in that environment can be extrapolated from this information.

In the early 1950s, surges of heat illness that occurred during training exercises in the United States military led to the refinement of the WBT as a monitoring measure for exercise environments. Military scientists invented the wet-bulb globe thermometer, a black spherical device that can be added to a wet-bulb and a dry-bulb thermometer. The wet-bulb globe thermometer measures the wet-bulb globe temperature (WBGT)—a value that takes into account the angle of sunlight and the amount of cloud cover (in other words, the amount of solar radiation from direct sunlight the instrument is exposed to), in addition to the temperature, humidity, and wind speed. Consequently, the WBGT is a useful measurement for judging temperature stress inflicted in direct sunlight. Since its creation, the WGBT has been used by government agencies, companies, and other organizations around the world to assist with the management of strenuous outdoor activities.

WBTs and climate change

Public awareness of WBTs has grown with the effects of global warming, which is responsible for increases in global and regional average temperatures (which in turn have resulted in increased humidity in several locations). Particular attention has been paid to the “threshold” or “critical” WBT for human beings, beyond which healthy people could live for only six hours. Beginning in 2010 this WBT was thought to be 35 °C (95 °F)—which is equal to a temperature of 35 °C with 100 percent humidity, or 46.1 °C (115 °F) at 50 percent humidity. Such sensible temperatures have been briefly reached on multiple occasions in some regions, particularly the Middle East, during heat waves. However, some experimental studies involving the 35 °C threshold suggest that the survivable threshold WBT for human beings varies with the type of environment people have been exposed to, and it may be as low as 31.5 °C (88.7 °F).

Adam Volle

Wet-bulb temperature

Uses

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WBTs and climate change