What is wet-bulb globe temp and why it’s more useful than heat index

You may have seen this recent tweet by our local National Weather Service in Omaha, referring to the ‘wet-bulb globe’ temperature instead of the heat index for this week’s heat wave.

Both are a measure of ‘heat stress' on the body. But, one is preferred over the other.

Heat index is calculated using air temperature and humidity in a shaded location.

The wet-bulb globe temperature (WBGT) is derived using air temperature, humidity, wind speed, angle of the sun and cloud cover.

-Tim Schmidt

Not all heat is the same.

Wet-bulb temp considers the caveats of, “it’s a dry heat,” or “yes, it’s humid, but there’s a breeze.”

You know how when you blow on your food to cool it off? Think of WBGT that way! It’s the temperature an object cools to when an evaporative process occurs!

In meteorology, we call this diabatic cooling. (Read below on why this matters in the winter).

While dew point is the temperature the air cools to reach saturation, wet-bulb temp is the temperature the air cools to after all water has been evaporated.

An air conditioner works similarly in this way, taking water out of the air through a series of condensers, thereby cooling it to a “wet-bulb” temperature.

Your cold drink works this way, forming condensation the the outside of the glass, cooling the air around the glass, but warming your drink over time.

When our bodies sweat, that sweat evaporates from our skin, cooling the skin to a ‘wet-bulb’ temperature.

Therefore, the wet-bulb temperature is considered a much more scientific measure of heat stress to the human body than heat index.

According to research done by the University of Connecticut, a wet-bulb globe temperature greater than 87 F degrees is considered a high risk for heat-related illness.

For example:

A temperature of 100 F with a dew point of 60 F only produces a WBGT of 86 F degrees, 10+ degrees cooler than the human body temp. (Heat index of 101)

A temperature of 100 F with a dew point of 78 F, like we experienced Monday, produces a WBGT of 93 F degrees, only 5 degrees cooler than the human body temp. (Heat index of 118)

If we only looked at the heat index, the dew point of 60 only produced a heat index of 101. Meanwhile, the dew point of 78 produced a heat index of 118.

This means our bodies must evaporate more sweat to cool our bodies, leading to higher heat stress and increased risk for heat-related illnesses, such as dehydration, or heat stroke.

OSHA uses wet-bulb globe temp in its guidelines for safe workplace climate conditions. School athletic programs use WBGT to regulate practice times and for scheduling extra rest breaks. The United States military also WBGT for heat safety at its basic training.

In the winter, we use the wet-bulb temperature to assess if we will receive snow, or just rain when temps are near freezing. Sometimes, it can snow even through the air temperature is nearly 36-39 F degrees!

For example, if snow falls into a very dry layer of air with a temperature of 36 F, it will melt, evaporate, and cool the air down to 31 degrees (the wet-bulb temperature), allowing for snow to reach the ground without melting...A problem for a few months from now.

Thanks for reading. Stay cool and safe this week,

-Tim Schmidt

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