[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?
In: Earth Science
Basically, apparent temperature (i.e. “feels like”) is based on how a human body would be heated or cooled by the specific weather conditions. This is primarily effected by two processes that move heat into and out of our bodies: heat transfer and evaporation.
Heat transfer is caused by the difference between your body temperature and the air temperature. Hot air makes you gain heat, while cold air makes you lose heat. This results in your skin and the thin layer of air immediately around it eventually becoming the same temperature. If that layer stays put, it provides a little bit of insulation and slows the heat transfer. If that layer gets disrupted (i.e. by the air moving), then it will insulate you far less, as the skin temperature air disperses and gets replaced by air temperature air. In other words, wind makes temperatures below body temp feel colder (as you lose heat faster), but temperatures above body temp feel even hotter (as you gain heat faster).
Evaporation is how your body cools itself when it gets too hot. You sweat, and that sweat evaporates into the air. Since evaporation requires energy, it takes some of the heat from you, cooling you down. So the faster your sweat evaporates the cooler it will feel, while the slower your sweat evaporates the hotter it will feel. Humidity strongly effects the rate of evaporation, as the more water the air is holding, the harder it is to get more water to evaporate. At 100% humidity, water basically doesn’t evaporate at all since the air can’t hold any more.
Wind also has a similar effect on evaporation as on heat transfer. As your sweat evaporates, you end up with a thin layer of air near your skin that has a higher concentration of water than the rest of the air. If that stays put, it slows the rate of evaporation by essentially increasing the humidity right next to your sweat. If that layer is disrupted by the air moving, then it gets immediately replaced by air that has the same humidity level as the air around it, and the rate of evaporation increases. Therefore, wind can lessen the apparent temperature increase from humidity, and it can seriously lower apparent temperatures at low humidity.
The “feels like” temperature chart is basically combining these two effects using some math I won’t try to explain here to determine the speed of heat transfer (in or out) at each temperature, wind speed, and humidity. Then, you can compare those results to the rates you get at each temperature with 0 wind and 0% humidity and you get the heat index chart. So, something like “feels like 91°F” means that the actual temp, wind, and humidity lead to a rate of heat transfer equal to that at 91° with 0 wind and 0% humidity.
Hope that makes sense. Getting into much more detail would require a thermo/fluid dynamics course, but I think this should be accurate without being too complex.
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