[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?
In: Earth Science
I think you’re confusing “objectively quantify” and “objective feeling”. All you need to objectively quantify something is a math formula. If I define my heat index as *h* = T(H) where *h* is my heat index, T is temperature in celcius, and H is relative humidity as a percentage, then I have an objective means of quantifying heat index.
Feeling is inherently subjective though. There is no objective way to quantify how something feels. Instead, heat index attempts to incorporate what we know about how humans perceive temperature into a formula that is far more elaborate than the overly-simplistic example in my first paragraph.
When you sense and perceive heat, most of what you’re feeling isn’t the absolute temperature, but rather the amount of heat leaving your body. That’s why 26°C (80°F) water feels cooler than 26°C air. Water is a better thermal conductor, so it draws heat out of your body more quickly.
Once you start sweating, your sensation of heat is a combination of the actual temperature countered by the rate at which your sweat is evaporating, which cools your skin. Since sweat evaporates more slowly in humid conditions, you’ll feel hotter as the humidity goes up. That is what heat index does.
Humans don’t sweat at the same temperature and at the same rate though. I’m sure you’ve noticed that some of your friends sweat more quickly than others. This makes it impossible to “objectively” rate how hot someone feels.
Instead, the formula for heat index considers the effect of evaporative cooling at a level that most people sweat at. It’s not a matter of black & white though. It’s a curve with multiple inputs. The [National Weather Service actually has a web page that lays out the formula](https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml).
Latest Answers