[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?

Question

[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?

In: Earth Science

Answer 1

There’s all these answers with complicated stuff, but I didn’t see anyone talk about wet bulb and dry bulb thermometers.

You know humans cool down by sweating, then the sweat evaporates. In low humidity areas this really works great! In the desert, you don’t feel really uncomfortable in the shade until the air temperature gets close to your body temperature and the sweat evaporation just isn’t keeping up.

By comparison, in high humidity areas, you sweat and it builds up on your body, soaking your clothes. Since there is already humidity in the air, evaporation is slower. You just get soggy and feel hot because evaporation is slowing down, like how a car starts to overheat if the radiator is clogged. Can’t pump the heat out as fast, temperature goes up.

Now, there is a really cheap way to check how humidity impacts the “feel” of the air. It’s called a wet bulb thermometer.

Basically take a glass thermometer, put some wet gauze around the end of it. Put it in the shade. Put a similar thermometer next to it with no gauze.

If the air isn’t humid, the wet bulb thermometer will show a lower temperature than the dry bulb because evaporative cooling is working. This means, at a given temperature, lower humidity will feel cooler.

Now, as the humidity goes up, the wet bulb thermometer will get closer and closer to the dry bulb thermometer. Evaporation isn’t working as fast, so it’s not removing heat, so this leads to a “feels like” temperature that is hotter than the air temperature because sweat evaporation isn’t cooling the body. There is still cooling happening, it’s just not as fast as evaporation, so it feels hotter than it actually is.

Yes, there are formula and there are charts. That stuff is just ways of speeding up the wet/dry bulb test, instead of sitting there watching thermometers for 15 minutes you check your dry thermometer and your humidity sensor, do some math, and have your “feels like” temperature. Those formulas mean the weather programs are doing the math for the meteorologists.