How is Wind Chill Calculated?

What does it actually mean when my weatherman says it feels like minus-20 in Chicago? Is there a specialized thermometer for wind chill, or is it all based on a mathematical formula? Let's dive into these and other frequently asked questions about the widely discussed winter measurement.

What causes the sensation of coldness when the wind blows?

When wind moves across our skin, it strips away heat from our body. As the wind speed increases, it accelerates the rate at which heat is lost. So, if your skin is exposed to the wind, you will lose heat faster than you would on a calm day.

Who first introduced the concept of calculating wind chills?

Paul Siple, an American explorer and geographer, along with his fellow explorer Charles Passel, pioneered wind chill research during an Antarctic expedition in 1940. They suspended water bottles outside their base station and timed how long it took for the water to freeze under various wind conditions. After gathering hundreds of data points, they gained valuable insights into the rate at which heat was lost under different wind speeds.

What exactly does 'wind chill temperature' refer to?

When Siple and Passel conducted their research, they weren’t aiming to create a temperature equivalent for dramatic weather reports. Initially, their measurements expressed heat loss in a more abstract unit: watts per square meter.

The concept of presenting wind chill as an equivalent temperature—the familiar 'feels like' term we hear on the news—wasn't introduced until the 1970s. Prior to that, weathermen reported wind chill using three- or four-digit numbers, which were hard for viewers to understand. To make the concept more relatable, American meteorologists began translating wind chills into temperature equivalents.

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

No. While strong winds may make us feel colder, they don’t have the same impact on inanimate objects. Lower wind chills cause inanimate objects to cool to the surrounding air temperature more quickly, but even the fiercest winds can’t bring an object’s temperature below that of the air. So, in the example above, although it might feel like 28°F on your skin, your water pipes will remain at a comfortable 40°F.

Is there an actual wind chill formula?

Absolutely. In case you're ever caught with a thermometer, anemometer, and calculator but without access to weather forecasts, here’s the equation for wind chill in Fahrenheit:

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

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

Wait, shouldn’t how cold the wind feels depend on a variety of factors, like your body type or whether you’re walking into the wind?

Those factors do influence how quickly the wind cools your skin. The formula above makes certain assumptions to simplify its calculations. Essentially, the wind chill value you hear reported assumes that your exposed face is about five feet above the ground, it's nighttime, and you’re walking directly into a 3 mph wind in an open field.

Are those conditions really that realistic?

If you're in a profession where night-time walking through fields is common, then maybe. Otherwise, not so much. Critics of wind chill reporting point out that many variables can reduce the actual wind chill you feel. Your weatherman might say it feels like 50 below outside, but if you're dressed warmly, standing in the sun, or sheltered by buildings and trees, you'll feel much warmer.

If the formula is somewhat questionable, can we just ignore wind chill reports as irrelevant statistics?

Not so fast. While the method behind wind chill calculations is still debated in some circles, that doesn’t mean the measurements are entirely useless. Keep in mind, the core idea behind wind chill is that stronger winds make exposed skin cool faster. The quicker the skin cools, the faster frostbite can occur. When wind chills drop below -50°F, frostbite can set in as quickly as five minutes, so it’s still important to monitor the wind chill—even if the idea of your skin 'feeling like' a certain temperature may not be entirely accurate.

Has the formula always been the same?

The formula used today is actually a relatively recent development introduced by the National Weather Service in late 2001. In 2000, both the National Weather Service and its Canadian counterpart began reevaluating the wind chill formula, partly because they suspected the previous formula overstated the severity of the cold. While this may seem like a minor issue, the weather services were concerned that it might create a false sense of security, making people believe they could endure colder conditions than they actually could.

Knowing the old wind chill formula was flawed—'feels like -50°F' actually felt much warmer than standing outside on a windless -50°F day—the two weather agencies recruited volunteers to walk on treadmills inside a refrigerated wind tunnel. With sensors on the participants' skin, scientists developed 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 the changes made to the formula over time, it’s hard to say for sure, but here's a truly cold one: on July 4, 2003, a remote weather station in eastern Antarctica recorded a day with a temperature of minus-94°F. That’s already extremely cold, but the wind was also gusting at 75 mph, which would result in a wind chill of about minus-150°F.

This post was first published in 2010.