[ELI5] How do meteorologists objectively quantify the “feels like” temperature when it’s humid – is there a “default” humidity level?
In: Earth Science
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Here’s a link to the National Weather Service’s heat index chart.
https://www.weather.gov/ama/heatindex
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“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).
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.
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.
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