You Should Use WetBulb Temperature to Measure Heat Outside – Popular Mechanics

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The direct sunlight approach to measuring heat considers how the body reacts when humidity, wind speed, and other factors change.
Think about this all too common scenario: The heat is rising, and so is the humidity. You’re clamoring for indoor air conditioning or at least some outdoor shade. As temperatures rise all over the world, recent research shows that our bodies can get dangerously hot at lower temperatures than previously believed, given high enough humidity. The typical temperature reading isn’t enough anymore. You really should use the WetBulb Globe Temperature to get a better idea of how badly you’ll need to cool off.
A more holistic approach to measuring heat stress, the WetBulb Temperature takes into account not only temperature, but also humidity, wind speed, sun angle, and solar radiation (cloud cover) to “measure the heat stress in direct sunlight,” according to the National Weather Service (the agency has a “prototype” reading still under development.)
This approach is far different than a typical heat index, which calculates temperature and humidity in a shady area. Military agencies, OSHA, and even entire nations have used this technique for years to gauge the stress level of direct sun. Understanding the WetBulb Temperature may provide a more accurate depiction of the stress you could feel if you’re outside, especially if you’re working or engaged in sports.
As heat waves prove common across both the United States and much of the world during the Northern Hemisphere’s summer months, the WetBulb Temperature could become a more widely recognized tool.
An Example of How Atmospheric Conditions Change the Temp
A National Weather Service example provides three different 100-degree Fahrenheit days with 39 percent relative humidity and a dew point of 70 degrees. The WetBulb reading varies from 90 or 91 to 94, thanks to differences in just wind and solar radiation.
On a day with 10 percent cloud cover and a 5 miles per hour wind, the WetBulb Temperature is at 94. Keep the cloud cover the same and push the wind to 13 miles per hour and you get a 90. Now, drop the wind back down to 5 miles per hour and push the cloud cover up to 65 percent and the WetBulb Temperature is 91.
By tweaking the humidity numbers, the WetBulb reading can really take its biggest swing, showing how the body struggles to cope with the heat most when in direct sunlight and high humidity.
A recent Penn State University study showed that what we thought the body could handle in the heat may not be accurate. A decade-old study had claimed that 95 degrees Fahrenheit at 100 percent humidity was the “critical environmental limit,” where the body’s core temperature rises continuously, and risk of heat-related illnesses increase.
The Penn State Noll Laboratory tested the limits of healthy young men and women to show that really, a WetBulb temperature of 88 degrees Fahrenheit at 100 percent humidity is a more accurate critical limit number.
The WetBulb Temperature, then, is more commonly used for folks regularly doing activities—whether work or sport—outside. OSHA suggests plenty of shade breaks, avoiding direct sunlight, ample water to cool the body, and sun protection to avoid burns.
You know it’s hot when the temperature rises, but you’ll know just how your body may react by a glance at the WetBulb Temperature.


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