Wet-bulb temperature

Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F), equivalent to a heat index of 55 °C (131 °F).

A reading of 35 °C (95 °F) – equivalent to a heat index of 71 °C (160 °F) – is considered the theoretical human survivability limit for up to six hours of exposure.

This is measured by comparing how much water is in the air to the maximum that could be in the air—the relative humidity.

The drier the air, the more moisture it can take up beyond what is already in it, and the easier it is for extra water to evaporate.

The result is that sweat evaporates more quickly in drier air, cooling down the skin faster.

By contrast, the dew point is the temperature to which the ambient air must be cooled to reach 100% relative humidity assuming there is no further evaporation into the air; it is the temperature where condensation (dew) and clouds would form.

The lower the relative humidity (the drier the air), the greater the gaps between each pair of these three temperatures.

Lower wet-bulb temperatures that correspond with drier air in summer can translate to energy savings in air-conditioned buildings due to: The thermodynamic wet-bulb temperature is the temperature a volume of air would have if cooled adiabatically to saturation by evaporation of water into it, all latent heat being supplied by the volume of air.

The temperature of an air sample that has passed over a large surface of liquid water in an insulated channel is the thermodynamic wet-bulb temperature—the air has become saturated by passing through a constant-pressure, ideal, adiabatic saturation chamber.

This is not necessarily true at temperatures and pressures that deviate significantly from ordinary atmospheric conditions, or for other gas–vapor mixtures.

"[6] Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with distilled water via wicking action.

The thermometers are attached to a swivelling handle, which allows them to be whirled around so that water evaporates from the sock and cools the wet bulb until it reaches thermal equilibrium.

At some point the air becomes saturated with water vapor (and has cooled to the thermodynamic wet-bulb temperature).

In this case we can write the following balance of energy per mass of dry air:

Experiment 2 For the case of the wet-bulb thermometer, imagine a drop of water with unsaturated air blowing over it.

As long as the vapor pressure of water in the drop (function of its temperature) is greater than the partial pressure of water vapor in the air stream, evaporation will take place.

Furthermore, the evaporation rate depends on the difference of concentration of water vapor between the drop-stream interface and the distant stream (i.e. the "original" stream, unaffected by the drop), and on a convective mass transfer coefficient, which is a function of the components of the mixture (i.e. water and air).

The precise relative humidity is determined by reading from a psychrometric chart of wet-bulb versus dry-bulb temperatures, or by calculation.

A wet-bulb thermometer can also be used outdoors in sunlight in combination with a globe thermometer (which measures the incident radiant temperature) to calculate the Wet Bulb Globe Temperature (WBGT).

[8] Such cooling may occur as air pressure reduces with altitude,[clarification needed] as noted in the article on lifted condensation level.

Comparisons indicate, however, that the two temperatures are rarely different by more than a few tenths of a degree Celsius, and the adiabatic version is always the smaller of the two for unsaturated air.

When the ambient temperature is excessive, many animals cool themselves to below ambient temperature by evaporative cooling (sweat in humans and horses, saliva and water in dogs and other mammals); this helps to prevent potentially fatal hyperthermia due to heat stress.

The effectiveness of evaporative cooling depends upon humidity; wet-bulb temperature, or more complex calculated quantities such as wet-bulb globe temperature (WBGT) which also takes account of solar radiation, give a useful indication of the degree of heat stress, and are used by several agencies as the basis for heat stress prevention guidelines.

Given the body's vital requirement to maintain a core temperature of approximately 37°C, a sustained wet-bulb temperature exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, semi-nude in the shade and next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it.

[11] [12] A 2022 study found that the critical wet-bulb temperature at which heat stress can no longer be compensated in young, healthy adults mimicking basic activities of daily life strongly depended on the ambient temperature and humidity conditions, but was 5–10°C below the theoretical limit.

[13][14] A 2015 study concluded that depending on the extent of future global warming, parts of the world could become uninhabitable due to deadly wet-bulb temperatures.

[15] A 2020 study reported cases where a 35 °C (95 °F) wet-bulb temperature had already occurred, albeit too briefly and in too small a locality to cause fatalities.

[16] Severe mortality and morbidity impacts can occur at much lower wet-bulb temperatures due to suboptimal physiological and behavioral conditions; the 2003 European and 2010 Russian heat waves had values no greater than 28 °C (82 °F).

[16] In 2018, South Carolina implemented new regulations to protect high school students from heat-related emergencies during outdoor activities.

[24] (Weather stations are typically at airports, so other locations in the city may have experienced higher values.)