Wet-bulb temperature: What is it, and how is it linked to human survival in extreme heat?

A public safety officer in a yellow high-vis jacket and white shirt is shown sweltering in the heat. He is holding a red cloth to this forehead to wipe away sweat. .

Extreme heat waves can cause cases of dehydration, exhaustion, heat stroke and even death. But the air temperature isn't the only factor in how dangerous a hot day is; there's also another measure, called the wet-bulb temperature.

The wet-bulb temperature accounts for both the temperature of the air — known as "dry-bulb temperature" — and the amount of water vapor, or humidity, in it.