Climate change is driving the extreme heat baking France’s Olympics

Amid all this hotness, it’s important to remember that the thermometer doesn’t always indicate how hot our bodies feel. That brings us to this week’s reader question from Robert Ball:

What is the heat index?

The heat index, or apparent temperature, considers humidity along with temperature to indicate how we really feel in the heat. That’s why it’s often called the “feels like” temperature in forecasts.

Humidity is important because the body’s primary mechanism for cooling in hot weather is sweating (SN: 7/27/22). When sweat evaporates from the skin, it carries heat away from the body. But if the air holds a lot of moisture, the evaporation of sweat slows down. In 100 percent humidity, little to no sweat evaporates.

It’s important to note that the heat index has a major limitation: It was devised for shady conditions with little wind. If you’re working or exercising outdoors, a more useful indicator would probably be the wet bulb globe temperature, or WBGT, which provides the apparent temperature in direct sunlight.

Getting the WBGT is a bit more complicated than calculating the heat index. You first measure the air temperature with a thermometer that is shielded from sunlight and moisture. Then, you measure the “wet bulb” temperature with a thermometer that’s wrapped in a wet cloth, mimicking the cooling effect of sweating. Lastly, you need a thermometer inside a copper globe that’s painted black and positioned in direct sunlight, to measure the heat stress from sunlight. Add 10 percent of the air temperature, 70 percent of the wet bulb temperature and 20 percent of the black globe temperature to calculate the WBGT.

The takeaway here is that when compared to temperature alone, the heat index and WBGT can help provide a more complete picture of how we feel the heat, and when we need to take measures to avoid heat stroke and exhaustion. These are vital tools in a world where heat waves are becoming more common and more intense (SN: 4/3/18).