The Heat that Kills. Wet Bulb Temps explained

July 21, 2021

“It’s a dry heat” describes some real physics.
Appreciate this from Salon.


What is wet bulb temperature anyway?

The normal temperature of a healthy human is about 98 °F (37 °C). So why don’t we feel most comfortable when that’s the temperature of the air around us? Because our bodies are engines, burning food to do work, and engines need to lose heat to their surroundings, or they overheat and stop working. As in, die. 

Fortunately, even if our surroundings are warmer than our bodies, we still lose heat if our sweat evaporates, by the miracle of the energy required to convert water from liquid to vapor. In dry air, this evaporative cooling system works great, even at air temperatures that sound really hot, because evaporation makes our skin feel cooler. But as moisture in the air increases, evaporation and heat removal slows, as anyone who has worked outside on a hot humid day knows.

But when do things go from uncomfortable to dangerous — and when will climate change take us there? 

Scientists have come up with more than 120 ways to quantify heat stress. One of the most useful is wet bulb temperature — the temperature that a wet thermometer in the shade measures as water evaporates freely off it. This temperature will be lower than the temperature on a dry thermometer in the same place (which is known as dry bulb temperature), and the difference between the two is a measure of humidity. So to get wet bulb temperature we can either measure it directly with a wet thermometer, or calculate it from dry bulb temperature and humidity. 

The usefulness of wet bulb temperature is it makes it clear how close conditions are to lethal. The closer wet bulb temperature gets to our body temperature, the less heat is lost and the closer we are to heat death.

It has been known for more than a century that wet bulb temperatures higher than 88 °F (31 °C) make it impossible to do physical labor, and a wet bulb temperature of 95 °F (35 °C) kills healthy humans within a few hours. Interestingly, reconstructions of wet bulb temperatures in hothouse periods of Earth’s past have also been used to interpret possible geographic limits and body temperatures of ancient mammals. 

Wet bulb temperature also explains why sky-high temperatures in places like Arizona are typically not lethal. Even a warm 100 °F (38 °C) May day in Tucson with relative humidity of 20% is a relatively comfortable wet-bulb temperature of 70 °F (22 °C). But the same dry bulb temperature with the average relative humidity in Jacksonville (75%) leads to a wet bulb temperature of 94 °F (34 °C), close enough to human body temperature to cause severe and potentially fatal heat stress. And while extreme temperatures usually get the headlines, globally, humidity dominates wet bulb temperature. 


One Response to “The Heat that Kills. Wet Bulb Temps explained”

  1. grindupbaker Says:

    I don’t know what the humidity was Sunday June 16, 2002 but it was a huge mistake to cut back on Powerade for sure. 33 degrees whatever humidity and air from Illinois & Ohio no more breathable than wildfire smoke. 14 litres of sweat and my best day of life ruined. Still it’s better than having a pre-disposition to heatstroke because at least there’s the choice to just stop.

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