Why is there such a thing as a “feels like” temperature? For example, in NYC, it’s 91 degrees today but the weather app says it “fees like” 99 degrees

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Why is there such a thing as a “feels like” temperature? For example, in NYC, it’s 91 degrees today but the weather app says it “fees like” 99 degrees

In: Earth Science
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The raw temperature is one thing, but when you include humidity, it often feels hotter.

91+ humidity will feel more like 99, basically.

Its to give you a better idea of what humidity and other potential factors like wind etc all combined, will feel like.

A large portion of that has to do with the humidity in the air. 90⁰ with 10% humidity feels way different than 90⁰ with 95٪ humidity.

The air can only absorb so much moisture based on its temperature. If it’s hot and humidity, it’s hard for the sweat on your body to evaporate. Since it’s not evaporating and cooling you, it feels hotter than it would if the air was dry.

Because what it “feels like” to you depends on your ability to sweat and cool off when the sweat evaporates. Literally your skin gets wet with water from the sweat, and the wind blowing “forces” it to evaporate, and the water “sucks” the heat out of your body to use as energy to go from liquid phase to vapor phase. That’s how you can be “ok” in 100+ degree temperature outside, which is hotter than your body temperature of 98 (Fahrenheit).

So basically a 91 degree humid air with no wind will feel equivalent to a 99 degree dry+windy day.

It’s because the human body cools itself off via sweating, which can be much less effective in high humidity. Our sweat needs to evaporate off our skin to cool our body, but if there is a ton of water already in the air the sweat doesn’t evaporate as readily and we can overheat easier.

So scientists did a bunch of studying and modeling on the human body, how sweating works, etc, and came up with a general model for how humidity effects the “average person”, it’s not super futuristic science and needs to be taken with a grain of salt but we basically have a scale that says from humidity 20-40%, add 5 degrees F to the thermometer temperature, for 40-60% at 10 degrees, etc.

So if in NYC the thermometer reads 91, and the humidity is 50%, the scale says add 8 degrees for a “feels like” temperature.

It is a calculated temperature based on humidity and wind in addition to the actual measured temperature. This is because we get cooled down by wind and when we sweat. But if the wind is calm and the humidity is high we do not get cooled down in the same way. So even though it is 91 degrees your body will experience it the same way as if you would in a 99 degree low humidity indoor environment where your body is allowed to sweat effectively.

Your body does not sense temperature. Your body senses the rate of heat transfer to your surroundings. This is why grabbing a bag of ice from the freezer feels colder than a freezer meal in cardboard packaging. Your body feels heat leaving your hands and finger tips and tells you about it via the sensation of cold. It’s a function of how fast the heat is leaving. Touching 30F cardboard is almost negligible because cardboard is a poor conductor of heat. Touching 30F ice feels very cold because ice conducts heat extremely well, and has a large specific heat, so it’ll absorb lots of heat before changing its temperature much.

So for the “feels like” temperatures, it’s a function of how quickly your body sheds heat to the air around you. In areas with very low humidity, sweating is a very efficient process. If you increase the humidity percentage, your body can’t shed heat as well because sweat doesn’t evaporate as quickly. In addition, the air itself now holds more moisture, which holds more thermal energy. Surprisingly, humid air actually transfers heat less efficiently than dry air (even though water transfer heat better than air) and this is because water vapor changes the thermal properties of dry air. Because the air transfer heat more poorly, your body can’t shed heat as quickly. The end result is your body feeling like it’s hotter outside because heat isn’t leaving your body as quickly, which is perceived as hotter ambient temperatures.

External temperature is the main variable affecting how hard it is for the human body to cool itself. Which is what gives us perceptions of a hot / cold environment.

It’s not the only variable though.

High humidity makes it feel hotter because it makes sweating less effective. Wind makes it feel colder because it’s constantly replacing warm-ish air next to you with cold air, and also makes sweating more effective.

So the “feels-like” temperature is the actual temperature plus or minus some number representing the wind / humidity conditions.