Wet-bulb temperature: What is it, and how is it linked to human survival in extreme heat?

Extreme heat waves can cause cases of dehydration, exhaustion, heat stroke and even death. But the air temperature isn't the only factor in how dangerous a hot day is; there's also another measure, called the wet-bulb temperature.

The wet-bulb temperature accounts for both the temperature of the air — known as "dry-bulb temperature" — and the amount of water vapor, or humidity, in it.

Wet-bulb temperature is measured using a thermometer that is wrapped in a cotton cloth that has been soaked in water. If the humidity of the air around the bulb is less than 100%, some of the water from the cloth will evaporate and cool the thermometer, Kathleen Fisher, a doctoral student in exercise physiology at Penn State, told Live Science.

In a more humid environment, less water can evaporate from the cloth because the air is already saturated. Consequently, the thermometer can't cool down as much as it would in a drier climate. The higher the humidity, the higher the wet-bulb temperature, and vice versa.

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Wet-bulb temperature is calculated using a specific equation that considers dry-bulb temperature and relative humidity. It can also be easily deduced by reading a Psychrometric Chart, such as the one shown below. For example, an air temperature of 104 degrees Fahrenheit (40 degrees Celsius) and a relative humidity of 30% would result in a wet-bulb temperature of around 78.8 F (26 C). However, a lower air temperature of 77 F (25 C) but a higher humidity level of 90%, would produce a similar wet-bulb temperature of around 75.2 F (24 C).

Wet-bulb temperature is useful for understanding how hot weather affects the body because a similar evaporative process occurs when people sweat. How hot you feel at a given temperature can vary depending on whether you're in a humid or dry environment.