The wet-bulb temperature is the lowest temperature that may be achieved by evaporative cooling of a water-wetted, ventilated surface.īy 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.įor a parcel of air that is less than saturated (i.e., air with less than 100 percent relative humidity), the wet-bulb temperature is lower than the dry-bulb temperature, but higher than the dew point temperature. ( July 2021) ( Learn how and when to remove this template message)
Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources. When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.
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The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. 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. This is part of the cause of apparent temperature in humans. 0% means the air is completely dry, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source). This is measured by comparing how much water is in the air to the maximum that could be in the air-the relative humidity. Water can evaporate only if the air around it can absorb more water. The faster water evaporates, the lower the thermometer's temperature will be relative to air temperature. The drier and less humid the air is, the faster the water will evaporate.
If a thermometer is wrapped in a water-moistened cloth, it will behave differently. A reading of 35 ☌ (95 ☏) – equivalent to a heat index of 71 ☌ (160 ☏) – is considered the theoretical human survivability limit for up to six hours of exposure. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.Įven heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 ☌ (90 ☏), equivalent to a heat index of 55 ☌ (131 ☏). A wet-bulb thermometer indicates a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel.
At 100% relative humidity, the wet-bulb temperature is equal to the air temperature ( dry-bulb temperature) at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling. The wet-bulb temperature ( WBT) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed.
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