A resource of Biophilia: Pittsburgh, #bi oPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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I remember standing at a bus stop early one bitterly cold morning. My pink scarf was wrapped around my face, and my hood was pulled up to shield as much of my head of possible from the sharp wind. Through my watering eyes, though, I noticed one person down the sidewalk from me who seemed to be having a rougher morning than me: his neatly trimmed beard was frosted over from his own breath! That morning, the temperature was in the single digits, but the wind chill was well below zero. I remember at the time thinking that the wind chill was what the temperature “felt like,” but it was already plenty cold—what did the wind chill really mean? The answer is an intriguing blend of mathematics and human physiology; let’s explore!

First of all, what is wind chill? By definition, it’s an indicator of how cold the wind feels to us because of heat loss from skin. Though different meteorologists may use different formulas to calculate wind chill, our current National Weather Service calculation for wind chill is based on air temperature, wind speed, and a standardized measurement for heat loss from skin. It was originally developed as a metric to monitor frostbite potential, but it generally tells us to add an extra layer on windy days.

How does wind make us feel colder, though? We perceive cold and cold pain through specialized nerve cells called thermoreceptors. Sometimes, cold feels soothing, like when you’re icing a back injury or the comfort of menthol cough drops on a sore throat. Other times, though, the cold provides a sharp burn that either slowly creeps up through an extended exposure or the instantaneous needling sensation of touching cold metal. The differences in cold perception are managed by different chemical signals that those specialized nerve cells can receive and transmit. It’s a complex but important process.

Now imagine that you are outside on a cold day. You probably notice the cold, but your immediately exposed skin (like your face) is staying somewhat warm through a boundary layer of warm air just above the surface of your skin. This comes from the heat we naturally radiate. A gust of cold wind can quickly compromise that warm air boundary, though! Oddly enough, however, you will feel colder than the actual air temperature because of the way those specialized nerve cells process rapid changes in temperature (a phenomenon not completely understood). The experience is comparable to the feeling of jumping into a pool—the “cold” water shocks you at first, but after a few minutes, you realize it isn’t so bad. Your thermoreceptors were just responding to an abrupt, extreme change. That sensation of feeling colder than the actual air temperature what is being interpreted by the wind chill—it is a representation of how cold the air will feel to us.

It’s important to note, though, that wind chill doesn’t actually impact freezing. The air temperature is still what matters. If the actual temperature is 33° F, the wind chill may be in the 20’s, but water outside will still not freeze. However, a low wind chill can cool objects down faster because of heat being carried away; thus, if the actual temperature falls below freezing, objects could freeze faster than if the wind hadn’t been a factor.

There are also difference in the estimates of wind chill we see on our weather apps and on the news. Metrics like “RealFeel,” “feels like,” etc., can include cloud cover, humidity, and other conditions that go beyond simply the NWS version of wind chill. They are all just different ways to keep us informed about our weather!

Connecting to the Outdoors Tip: If you’re exploring weather concepts with the kids and you would like to make wind chill more tangible, try a few experiments at home that require nothing more than wet fabric scraps and a clothesline! (Hint: always be sure to talk through predictions before starting an experiment. The inquiry is what makes it an experiment and not just an activity.) First, to see if something will freeze when the actual temperature is above freezing, but the wind chill is below, hang a scrap of fabric in direct line of the wind, and another one out of the wind. Do either of them freeze? Or, to illustrate how the wind can cool down objects quickly so they can freeze faster, wait for a day with an actual temperature below freezing, and place two wet scraps of fabric outside—one in the wind and one blocked from the wind. Check on them at regular intervals to see which freezes faster (the timing of the intervals will depend on the temperature and thickness of the fabric, so use your best guess to get started.)

Resources

McKemy 2013: The Molecular and Cellular Basis of Cold Sensation

Scientific American— Fact or Fiction: Wind Chill is Real

Scientific American – Cold or Warm, Can We Really Tell?

National Weather Service—Wind Chill Chart 

National Weather Service—Wind Chill Questions

The Conversation – Health Check: why do some people feel the cold more than others?

BBC Archives—Wind Chill