How Do You Calculate the Wind Chill?

iStock/HelenL100
iStock/HelenL100

What does it really mean when my weatherman says that it feels like minus-20 in Chicago? Is there a wind chill thermometer somewhere, or is he just using a mathematical formula? Let's answer these and some of the other pressing questions about the ubiquitous winter statistic.

Why does wind make us feel cold?

When the wind blows across the exposed surface of our skin, it draws heat away from our bodies. When the wind picks up speed, it draws more heat away, so if your skin is exposed to the wind, your body will cool more quickly than it would have on a still day.

Who came up with the idea of calculating wind chills?

American explorer and geographer Paul Siple and his fellow explorer Charles Passel made the first breakthroughs in wind chill research while on an expedition in the Antarctic in 1940. Siple and Passel suspended bottles of water outside a hut at their base station and measured how long it took the water to freeze under various wind conditions. After taking hundreds of these readings, the pair had a good idea of how rapidly heat was lost at different wind speeds.

What exactly is a wind chill temperature?

When Siple and Passel did their research, they weren't really trying to develop a temperature equivalent that alarmist weathermen could trot out. In fact, their original measure expressed the heat loss in a more esoteric unit: watts per square meter.

The idea of expressing wind chills in terms of an equivalent temperature—the "feels like" language we hear on the news—didn't start until the 1970s. Before the switch, weathermen would report the wind chill in three- or four-digit numbers which were a bit difficult for viewers to wrap their heads around. American weathermen started translating wind chills into temperature equivalents in order to give viewers a more familiar term.

If the air temperature is 40 degrees but the wind chill is 28 degrees, will water freeze?

Nope. Although high winds can make those of us with a pulse feel more miserable than normal, they don't have the same effect on inanimate objects. Lower wind chills mean that inanimate objects cool to the air temperature more quickly, but even high winds can't force the object's temperature below the air temperature. That means that in the above example even though your skin might think it's 28 degrees, your water pipes will still be a balmy 40 degrees.

So is there an actual formula for wind chill?

You bet. Just in case you ever find yourself with a calculator, thermometer, and anemometer but without access to The Weather Channel, the Fahrenheit version of the equation looks like this:

Wind Chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)

T is the air temperature in degrees Fahrenheit, and V is the wind speed in miles per hour.

Wait, shouldn't how cold the wind makes you feel depend on all sorts of variables like your body type, and whether you're walking into the wind?

Those things certainly affect how quickly the wind cools a person's skin. The above formula makes some simplifying assumptions to get its numbers. Basically, the wind chill factor you hear reported assumes that your exposed face is roughly five feet off the ground, it's night, and you're walking directly into the wind in an open field at a clip of about 3 mph.

Are those conditions really all that realistic?

If you're in a profession that involves a lot of night-field-walking, sure. Otherwise, maybe not. Critics of wind chill reports note that lots of factors can mitigate the reported wind chill. Your weatherman may tell you that it feels like 50 below outside, but if you're dressed warmly, standing in the sun, or in an area with cover like buildings and trees that block the wind, you will feel significantly warmer.

So if the formula is arguably somewhat dubious, can we just disregard wind chill reports as frivolous statistics?

Not so fast. While the methodology concerning wind chill calculations is still being debated in some quarters, that doesn't mean that the measurements are altogether useless. Remember, the basic concept behind wind chill is that stronger winds will cause exposed skin to cool more quickly. The faster skin cools, the faster frostbite can set it. As wind chills drop south of -50 or so, the onset of frostbite can take as little as five minutes, so it's worth keeping an eye on the wind chill even if the notion of your skin "feeling like" a certain temperature may be a bit misleading.

Has the formula always been the same?

The above formula is actually a fairly new development that the National Weather Service introduced in late 2001. During the year 2000, the National Weather Service and its Canadian counterpart had independently started looking for ways to improve the wind chill formula, partially because they had a sneaking suspicion the old formula overstated just how cold it was. This overstatement may sound innocuous, but the weather services worried that it would lull citizens into a false sense of security if it led people to believe they could withstand colder temperatures than they realistically could.

Since the collaborating weather services knew that the old wind chill formula was broken—"feels like -50" actually felt much warmer than standing around on a windless 50-below day—they recruited a group of volunteers to walk on treadmills in a refrigerated wind tunnel. Using sensors on the subjects' skin, scientists calculated a more accurate formula. You can read one of the test subjects' thoughts on the experiments here.

What's the lowest wind chill ever recorded?

With all of the tweaks in the formula over the years, it's tough to say definitively, but how's this for chilly: on July 4, 2003, a remote weather station in east Antarctica picked up a minus-94 degree day. That would be plenty frigid on its own, but the wind was also blowing at 75 miles per hour, which would be good for a wind chill of about minus-150.

This post originally appeared in 2010.

Why Does Humidity Make Us Feel Hotter?

Tomwang112/iStock via Getty Images
Tomwang112/iStock via Getty Images

With temperatures spiking around the country, we thought it might be a good time to answer some questions about the heat index—and why humidity makes us feel hotter.

Why does humidity make us feel hotter?

To answer that question, we need to talk about getting sweaty.

As you probably remember from your high school biology class, one of the ways our bodies cool themselves is by sweating. The sweat then evaporates from our skin, and it carries heat away from the body as it leaves.

Humidity throws a wrench in that system of evaporative cooling, though. As relative humidity increases, the evaporation of sweat from our skin slows down. Instead, the sweat just drips off of us, which leaves us with all of the stinkiness and none of the cooling effect. Thus, when the humidity spikes, our bodies effectively lose a key tool that could normally be used to cool us down.

What's relative about relative humidity?

We all know that humidity refers to the amount of water contained in the air. However, as the air’s temperature changes, so does the amount of water the air can hold. (Air can hold more water vapor as the temperature heats up.) Relative humidity compares the actual humidity to the maximum amount of water vapor the air can hold at any given temperature.

Whose idea was the heat index?

While the notion of humidity making days feel warmer is painfully apparent to anyone who has ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, an academic textile researcher. In a 1979 research paper called, “An Assessment of Sultriness, Parts I and II,” Steadman laid out the basic factors that would affect how hot a person felt under a given set of conditions, and meteorologists soon used his work to derive a simplified formula for calculating heat index.

The formula is long and cumbersome, but luckily it can be transformed into easy-to-read charts. Today your local meteorologist just needs to know the air temperature and the relative humidity, and the chart will tell him or her the rest.

Is the heat index calculation the same for everyone?

Not quite, but it’s close. Steadman’s original research was founded on the idea of a “typical” person who was outdoors under a very precise set of conditions. Specifically, Steadman’s everyman was 5’7” tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at just over three miles per hour into a slight breeze in the shade. Any deviations from these conditions will affect how the heat/humidity combo feels to a certain person.

What difference does being in the shade make?

Quite a big one. All of the National Weather Service’s charts for calculating the heat index make the reasonable assumption that folks will look for shade when it’s oppressively hot and muggy out. Direct sunlight can add up to 15 degrees to the calculated heat index.

How does wind affect how dangerous the heat is?

Normally, when we think of wind on a hot day, we think of a nice, cooling breeze. That’s the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it’s that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.

When should I start worrying about high heat index readings?

The National Weather Service has a handy four-tiered system to tell you how dire the heat situation is. At the most severe level, when the heat index is over 130, that's classified as "Extreme Danger" and the risk of heat stroke is highly likely with continued exposure. Things get less scary as you move down the ladder, but even on "Danger" days, when the heat index ranges from 105 to 130, you probably don’t want to be outside. According to the service, that’s when prolonged exposure and/or physical activity make sunstroke, heat cramps, and heat exhaustion likely, while heat stroke is possible.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This article has been updated for 2019.

Is the Heat Index Real?

MarianVejcik/iStock via Getty Images
MarianVejcik/iStock via Getty Images

Complaining about the humidity is a mainstay of small talk. “It’s not the heat that gets you, it’s the humidity” is a common refrain around the South, just as “it’s a dry heat” is a go-to line in the desert Southwest. The clichés aren’t wrong on this one—a hot and humid day can have a dramatic effect on both your comfort and your health. We can measure this very real impact on your body using the heat index. 

The heat index is the temperature it feels like to your body when you factor in both the actual air temperature and the amount of moisture in the air. If the heat index is 103°F, that means that the combination of heat and humidity has a similar physical impact on your body as it would if the actual air temperature were 103°F. Even though it’s tempting to think of the heat index as an exaggerated temperature that only exists to make the heat sound worse than it really is, scientists came up with the measurements after decades of medical and meteorological research devoted to studying the impact of heat and humidity on the human body. It’s the real deal.

The dew point is an important component of the heat index. The dew point is the temperature at which the air would reach 100 percent relative humidity, or become fully saturated with moisture like on a foggy morning. Since cooler air can’t hold as much moisture as warmer air, lower dew points reflect lower moisture levels and higher dew points indicate higher moisture levels. Dew points below 60°F are comfortable, while readings reaching 70°F and even 80°F range from muggy to downright oppressive.

Measuring humidity on a hot day is important because moisture is how your body naturally cools itself off. Your sweat cools the surface of your skin through a process known as evaporative cooling. If the air is packed with moisture, it takes longer for your sweat to evaporate than it would in more normal conditions, preventing you from cooling off efficiently. The inability to lower your body temperature when it’s hot can quickly lead to medical emergencies like heat exhaustion or heat stroke, which is why the heat index is such an important measurement to pay attention to during the summer months.

The heat index is generally considered “dangerous” once the value climbs above 105°F, and your risk of falling ill increases the higher the heat index climbs.

Dry climates can have the opposite effect on your body, with the distinct lack of moisture in the air making it feel cooler to your body than it really is. Summers get oppressively hot in places like Arizona and Iraq, but the heat doesn’t affect residents as severely because the air is extremely dry. Dew points in desert regions can hover at or below 32°F even when the air temperature is well above 100°F, which is about as dry as it can get in the natural world.  

In 2016, a city in Kuwait measured the all-time highest confirmed temperature ever recorded in the eastern hemisphere, where temperatures climbed to a sweltering 129°F during the day on July 21, 2016. The dew point there at the same time was nearly 100 degrees cooler, leading to a heat index of just 110°F, much lower than the actual air temperature. That’s not necessarily a good thing. Extreme heat combined with extreme aridity can make your sweat evaporate too efficiently, quickly dehydrating you and potentially leading to medical emergencies similar to those you would experience in a much more humid region of the world. 

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This story has been updated for 2019.

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