You know how sometimes you’ll open up a weather app and it’ll say that it’s 80°F, feels like 92°F, or when it’s super cold it’ll say 10°F, feels like -2°F?
How do they determine what a certain temperature feels like? Is it based on humidity, wind, or pressure? I figured that ‘feels like’ would be more subjective, which is why I’m always confused when it says that.
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It depends on wind and, to a lesser extent, humidity. There are several formulas to do it, here’s one that works just on wind: [https://www.lenntech.com/calculators/wind/wind-chill.htm#:~:text=To%20calculate%20the%20wind%20chill%20temperature%2C%20we%20use,this%20formula%20does%20not%20give%20very%20reliable%20results](https://www.lenntech.com/calculators/wind/wind-chill.htm#:~:text=To%20calculate%20the%20wind%20chill%20temperature%2C%20we%20use,this%20formula%20does%20not%20give%20very%20reliable%20results).
Humans don’t actually sense temperature, we sense how fast heat is entering/leaving us. When it’s cold, heat leaves us faster. When it’s windy, heat also leaves us faster, so it feels colder than the true air temperature.
It is a combination of the actual (dry-bulb) temperature, humidity, and wind.
As a rule, for low actual temperatures, wind is what matters most, known as wind-chill. The basis for this calculation is looking at how fast a bottle of water froze at differing wind speeds, though it has since been refined. The calculation is designed to account for how wind will strip away the thin layer of warm air around a warm object, such as a human.
For high actual temperatures, humidity matters most, and can be measured as a wet-bulb temperature. That is, they wrap the bulb of the thermometer in a wet cloth and wait for it to stabilize in temperature. This process basically accounts for sweat production and how humidity reduces how sweat evaporates.
As both are based on actual physical features, the “feels like” temperature is purely objective.
Naturally, there will be some variation between people: different people sweat differently or have differing set body temperatures, and both will vary from day to day. For cold weather, slight movements will also strip that layer away, though clothing will help preserve that layer.
Other commenters have pointed out a few but also worth noting is that solar intensity is often factored in to these sorts of numbers, so a cloudy day will feel colder than an identical sunny day.
There’s two types of temperature measurements. Wet bulb and dry bulb. Dry bulb is exactly that. You take a dry mercury thermometer, put it outside and it will tell you the temperature.
Wet bulb is a little more complicated. But basically you wrap a damp cloth around the mercury bulb and allow any natural breeze to hit it. The water in the cloth will absorb heat from the bulb and then blow away on the breeze leading to potentially cooler temperatures.
Why do this? Well it simulates out ability to self regulate temperature. IE sweat. If you’re able to sweat you’re able to cool off and wet bulb attempts to consider how sweating plays into perceived temperature.
If it’s hot and there’s no breeze and it’s humid, sweating isn’t going to be very effective. There’s already moisture in the air so the sweat cant evaporate off after absorbing the body heat. A breeze helps cool you by convection, your body heat slowly heats the air around you, so that air around you gets warmer. A breeze will blow away that warmer air and bring in ambient air which will be cooler.
A wet bulb measurement is a rough estimate of how those natural effects will influence how the human body perceives the temperature.
For example, if it’s 100% humidity and stagnant, as low as 90° F can be lethal temperatures. Conversely, in 0% humidity with a breeze 120°F is something you can survive for hours. Just be sure to hydrate.
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