The heat index

Ah, GW, the existential disaster of our own making, continues to gather steam as the notion of wet bulb temperature, aka, the heat index, becomes yet another essential item to understand as the world moves further into the 21st century. 

When the idea of wet bulb temperature was created, it meant this: “Wet-bulb temperature is read by a thermometer with a wet wick over its bulb and is affected by humidity and air movement. It represents a humid temperature at which the air is saturated and holds as much moisture as it can in the form of water vapor; a person’s sweat will not evaporate at that skin temperature.” In other words, at that temperature, the human body can’t cool itself anymore. The brain swells, the organs stop working, and shutdown happens — it’s ugly.

So wet bulb temperature is like an index: it’s like the temperature as we think of it, with humidity factored in. Maybe one way to think about is it how “it feels,” or as “heat stress.” Now we have to think of two temperatures to understand the environment — “the” temperature, and it’s real equivalent on human functioning.

Factoid.

It surprises many people to learn that the heat index values in the chart above are for shady locations.  If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F.  As shown in the table below, heat indices meeting or exceeding 103°F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.

In closing ...

Places like Arizona have recently had stretches of days running above 110 degrees Fahrenheit. That’s about 43 degrees C. How dangerous is that, really? The chart tells us that temperature, at just 35 percent humidity, human beings begin to enter the danger zone.