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

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

In: Earth Science

14 Answers

Anonymous 0 Comments

Your body does not feel temperature at all. What it feels is how quickly it is gaining or losing heat.

How much humidity is in the air affects how quickly we gain or lose heat, and it does so in predictable ways that you can just punch into an equation and get a result. If it is a particularly wet and hot day and you are gaining heat as quickly as you would if it was 10゚ hotter and dry, then they say it feels like it is 10゚ hotter.

Anonymous 0 Comments

The actual formula they use is very complicated. It needs to be because they are trying to model how well a human being is able to shed body heat under different conditions, which is not a simple thing to describe.

There is not a default percentage humidity, but there is a default vapor pressure. This means the amount of water in the air, but that will be a different “percentage” depending on the air temperature and the air pressure.

But BASICALLY, if the temperature is less than 90 F, “feels like” temp will be the same as the real temp at about 40% humidity. As you get hotter, you need a lower and lower humidity for them to be the same. For example at 100 F the feels like and real temp are the same at about 25% humidity.

Anonymous 0 Comments

Here’s a link to the National Weather Service’s heat index chart.

https://www.weather.gov/ama/heatindex

>
“It’s not the heat, it’s the humidity”. That’s a partly valid phrase you may have heard in the summer, but it’s actually both. The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This has important considerations for the human body’s comfort. When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body’s temperature. When the atmospheric moisture content (i.e. relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases. The body actually feels cooler in arid conditions. There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase (decrease), the heat index increases (decreases).

Anonymous 0 Comments

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Anonymous 0 Comments

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).

Anonymous 0 Comments

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!