Wind chill: Why is it hard to measure but so easily felt?

They happen every winter: Weather alerts that include wind chill warnings.

You can certainly feel it, but technically can’t measure it.

So, what is wind chill, exactly?

“Wind chill is a really interesting measure that you can’t actually measure with a thermometer,” said Armel Castellan, a warning preparedness meteorologist with Environment Canada.

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“Because if you put out a thermometer and there’s zero wind or if it’s 50 km/h an hour and it’s -10 C, it will just say -10. But you, as a sentient being, will be able to feel that there is a big difference between standing there in a howling wind at -10, which makes it feel like it’s -25 C, or if it’s just -10 in calm conditions.”

Castellan said wind chill “is a felt measure. That’s why there’s no (Celsius or Fahrenheit) unit to wind chill.”

He continued, saying wind chill is a bit of an algorithm.

“There’s a boundary layer around your skin, but your temperature inside is around 37 C. There’s a layer built up around you, but if there’s a wind, it is sandwiching down that layer next to nothing and whisking away all of your heat. That’s essentially the idea of wind chill.”

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According to the federal government, more than 80 people die from over-exposure to the cold every year, with many more suffering hypothermia and frostbite.

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“Wind chill can play a major role in such health hazards because it speeds up the rate at which your body loses heat.”

The U.S. National Weather Service says “the faster the wind speed, the faster our body heat is taken away and the colder it feels. It is a similar process for when you blow on a hot bowl of soup to cool it down.”

So, when it comes to winter and heading outdoors, dress not only for what the thermometer shows, but also for wind chill if applicable.

“The call to action for being in the outdoor environment are, obviously, to be wearing many layers so that you’re insulated from the wind and cold,” Castellan said. “Moisture is going to play a big role if you’re sweating and exercising. You’re going to feel colder because you’re having evaporation loss from your body. You want to manage moisture.”

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Lastly, there’s also this: Disparities between communities along large bodies of water (wet cold) and those inland (dry cold).

“In some ways, as soon as you get down to -8 or -10 C, the human can no longer tell whether or not there’s a lot of humidity or very (little) humidity,” Castellan said. “When you get down to very cold temperatures, you cannot hold very much moisture. The difference between 100 per cent moisture at -20 C and a very dry -20 C is the equivalent of about a gram of water for a cubic metre. It’s so negligible that it doesn’t matter.”

One teaspoon will hold approximately five grams of water.

When that ‘wet cold’ will matter is when an inland resident visits a coastal community when it’s around 0 C and it’s extremely moist.

“That’s a different type of cold,” said Castellan. “But when we get to the types of cold (from cold snaps or arctic blasts), it’s not so much about the moisture content anymore, it’s about the cold and the wind.”

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