It’s an sunny February morning and you turn on the TV to watch the local weather report. The meteorologist’s forecast says 30 degrees Fahrenheit, which doesn’t seem too bad. It’s winter after all! But then, you notice that the “feels-like” temperature is closer to 20 degrees. Suddenly, that extra cup of warm coffee and a fuzzy hat sound really nice.
That feels-like temperature can come in the form of wind chill or heat index, so similar scenes can play out during the dog days of summer. The amount of moisture in the air–or humidity–can make 80 degrees feel closer to 90.
“It’s basically just how the temperature feels different to our skin,” meteorologist Cyrena Arnold tells Popular Science. “We have sensors in our skin and our skin is made up of water. So our skin actually behaves differently based upon the evaporation of that water.”
Feels-like: wind chill
When the air temperature itself is cold and the winds are moving at a higher speed, it can pull more heat away from the body and cause the water on our skin to evaporate faster.
“In order for moisture to evaporate off of your skin, it takes heat. It has to absorb heat in order for those little water molecules on your skin to go from liquid to a gas,” says Arnold. “So when there’s a wind chill, it doesn’t take a lot of energy because of that wind and that cold for the liquid to go into the gas form.”
This increased evaporation speed and energy expenditure is also why hikers, skiers, first responders, or anyone staying outside in the cold for extended periods of time should avoid cotton clothing.
[ Related: Why 60 degrees in fall feels different than in the spring. ]
“Cotton is known for causing quick evaporation,” explains Arnold. “It’s a breathable fabric where wools and synthetics prevent some of that evaporation and help keep you warmer when it’s cold.”
While meteorologists use a tried and true mathematical formula to calculate the wind chill, charts can also be helpful to make a guess at a glance.
Feels-like: heat index
While the old adage “it’s not the heat, it’s the humidity,” sounds a bit clichè, it is true. Sweating is like the body’s natural air conditioner and helps it cool down when it is warm. However, if there is a lot of moisture in the air, it takes a lot more physical energy for that water to evaporate off our skin.
“It doesn’t allow the evaporation, because you have a combination of heat and humidity together that doesn’t allow for evaporative cooling on your body,” says Arnold. “It’s that evaporation process that cools us and if the evaporation process can’t happen, it feels lousy.”
A mathematical formula is also used to calculate the heat index.
Dealing with the heat and cold
Feels-like temperature can be frustrating, but planning ahead for it can help prevent heat stroke or frostbite. Too much exertion during high heat indexes can be deadly. Planning exertion around these heat indexes and trying to stay in the shade when possible can be helpful. Unlike in the winter, wearing light-colored clothing made of cotton is great for the summer since it allows for more evaporation.
“That white cotton t-shirt is fantastic in the summertime. When you sweat, it’ll help cool you off,” says Arnold.
Hydration is also critical since your body is going to be working extra hard to evaporate the water off of your skin to help cool you down.
During the coldest months, the opposite advice is key. Dress in loose and insulating layers that are not too tight and can trap pockets of warm air.
“You could have five layers on it could be the most insulating fabric ever, but if it is absolutely pressed against each other and super tight, you won’t have air in there to actually warm up between those layers and keep you warm,” explains Arnold.
Covering up exposed bits of skin like the cheek bones or tips of the ears is also critical for extended time outside. Frostbite can settle in in as little as 10 minutes and will make the affected parts of the skin permanently susceptible to infection.
“One of my favorite sayings is ‘there’s no such thing as bad weather, only bad gear.’ We can survive extreme heat and extreme cold as long as we have the ability and the technology to prepare,” says Arnold.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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