[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?
In: Earth Science
A simple answer since this is ELI5: Your body feels cooler on a hot day because your sweat evaporates. More humid days mean more water is already in the air and less of your sweat is evaporating and cooling you down.
In Canada we use the humidex system, which is a combination of the temperature in Celsius and the dew point. Humidex tells you basically how uncomfortable you’ll be on a given day based on how much your sweat can evaporate and cool you down.
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.
You know how a 90 degree hot tub feels hotter than a 90 degree day outside? Thats because water transfers heat better than air does. Air with water in it (humidity), thus, will transfer heat better than air with no humidity. And it does this in a predictable way that you can calculate. So to answer your question, “base humidity” is okay essentially zero. Then you add more depending on the level of humidity
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.
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!
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).
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