In the 1940s, two explorers in Antarctica noticed that in windy conditions, water froze at temperatures higher than 32°F. After multiple experiments, they published a table for estimating the “feels like” temperature by combining the air temperature with the wind speed. But it’s still not an exact science: since the early 20th century, people have devised more than 100 formulas to quantify how hot or cold it really feels outdoors. Here are the basics.
Factors in the “Feels Like” Temperature
A thermometer tells you the air temperature, but adding relative humidity and wind speed to the mix will tell you the apparent or perceived temperature — what the conditions actually feel like against your skin. Weather organizations have their own proprietary formulas for calculating the perceived temperature: AccuWeather touts the accuracy of its “RealFeel” technology, which also takes cloud cover, sun intensity, and the angle of the sun into consideration. The Weather Channel’s “Feels Like” index crunches the air temp, relative humidity, wind chill, and other factors to come up with a numerical value for what the weather really feels like. The National Weather Service provides much of the data that feeds into these formulas.
Summertime “Feels Like” Temps Calculated with the Heat Index
Perhaps you’ve heard (or said) the phrase, “it’s not the heat, it’s the humidity.” There’s some truth to that statement: the higher the relative humidity in the air, the hotter it feels outside. (In high humidity, sweat doesn’t evaporate from your skin as easily, so you don’t cool off as efficiently.) When temps are high but humidity is low, you experience pleasantly warm temperatures. When both air temperature and the humidity are high, it will feel like a steamy, sticky jungle. Think Florida in August.
The relationship between air temperature and humidity is called the heat index. Many meteorologists use the heat index chart calculated by the National Weather Service to forecast the “feels like” temperature in summer. For example, when the air temp is 90°F but the humidity is at 35%, the dry conditions will make the “feels like” temperature roughly equal to the air temp. The higher the temp and humidity go, the steamier it feels. Even if the air temp stays at 90°F, if the relative humidity doubles to 70%, it will feel like a sweltering 106°F. In conditions like that, people are at risk of heat exhaustion or potentially life-threatening heat stroke.
Wintertime “Feels Like” Temps Determined by Wind Chill
Wind chill is basically the winter version of the heat index. Cool weather can’t retain moisture as well as warm weather, so humidity becomes less important and wind speeds gain prominence for winter forecasts. Meteorologists combine the air temperature and the wind speed to come up with the wind chill, or how cold the air feels on your skin. To get an accurate measure of the wind’s effect, wind speed readings are taken 5 feet above the ground — which represents the average height of a standing adult’s face. When air temp and wind speed are low, the “feels like” temperature will be about the same as the thermometer’s reading. But with lower temps and higher wind speed, the greater wind chill will make you feel progressively more like a human popsicle.
As with its heat index, the National Weather Service provides a wind chill chart to allow meteorologists to forecast potentially dangerous conditions. On a 40°F day with winds at just 15 miles an hour, the “feels like” temperature will drop to the freezing point, for instance. But with a temperature of 5°F and winds at 35 miles an hour, the apparent temp plummets to -21°F. In those conditions, you can experience frostbite — an injury in which the skin and underlying tissues freeze — in 30 minutes. The colder and windier it becomes, the sooner frostbite will set in, particularly on your nose, cheeks, ears, fingers, and toes. Cold and windy weather can also cause hypothermia.
Calculating the “Feels Like” Temperature
Calculating these apparent temperatures takes some serious math, but thankfully the National Weather Service has made it easier on non-mathematicians by offering online calculators to determine the heat index or wind chill of any location. Popular weather apps from AccuWeather, The Weather Channel, and Weather Underground will give you the apparent temperature forecast along with air temperature, precipitation, and wind speed in their hourly forecasts.