[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?
In: Earth Science
Hey there!
I haven’t seen this mentioned as a top-level comment yet, but something to note is the origin of the heat index.
Before we get to that, for a true answer and ELI5 of your actual question, there is no default humidity level since it directly affects the outcome of the “feels like” temperature, and they determine it using math that a guy I’m going to talk about below pioneered.
Details:
Years ago (~1979, though people had considered this earlier), there was a guy named Robert G. Steadman who pulled together a [study](https://journals.ametsoc.org/view/journals/apme/18/7/1520-0450_1979_018_0861_taospi_2_0_co_2.xml?tab_body=pdf) because he was incredibly interested in the relationship between humidity, clothing choice, physiology, and temperature, and how that may affect perception and bodily response. This person was much like yourself, and wanted to know how we could quantify that kind of answer.
All those equations you’re seeing posted originated from that study and ones like it, most of which used as a baseline a roughly 5’7” (1.7m) adult of either sex who weighed about 148 pounds (67kg) wearing light clothing standing in limited sunlight. Steadman used human physiological data from 1949, so it was less than perfect, but pretty good at the time. If that sounds odd, you’d be right, but he didn’t want to do a million trials and leave the answers up to subjectivity with live subjects, so he picked a model he could test against reliably and ran with it.
The linked study above breaks down the methods and shows how they got to their conclusions. I’d recommend reading it if you’re a natural sciences geek like me.
Since then, the US National Weather Service, the Canadian Atmospheric Environment Service, other international agencies, and nameless nerds like me have used and helped refine those equations to reflect what it may seem like when you’re outside during a heat wave and you know a few objective meteorological measurements. You’ve seen comments about the wet bulb and dew point, and those answers are great and worth further investigation if you’re interested. They’re also helpful for determining other things you may want to know. Unfortunately, those two alone don’t cover everything you need if you want a truly accurate measure of how you’ll respond to the weather. You’d have to build your own mathematical model of yourself and then run the numbers, and you still might be off because, to the dismay of physicists everywhere, people are not uniform spheres.
As we’ve advanced our understanding of meteorology and human physiology, we’ve tried adjusting those equations to better reflect how variable conditions might determine the perceived temperature. Unfortunately, many things still elude us, and you’ll notice the equations tend to take things like wind as a constant (5kts or 9.3km/h, in this case). It simplifies things and helps us come to an answer quicker that generally gets the job done. At the end of the day, that’s what practical meteorology is about – getting info to people so they know what’s going to happen when they step outside.
It’s less than perfect, as others have mentioned, but if you’re really interested in learning about it and attempting to find your own way to account for it, take a look at the original study and the information that followed it. You may find additional answers to questions you didn’t know you had!
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