Winter is in full swing. Who hasn’t shivered and tucked their nose under their scarf recently asking, “Why is it so cold?!” Yet, somehow, even if the temperature stays the same two days in a row, one of those days sometimes feel much colder to us. What explains this?
Staying Warm and Dry
The secret to keeping warm in cold winter weather is staying dry. We are much more likely to feel the cold when our skin is wet. That’s because body heat escapes much more easily through wet skin. That feeling you get when you step out of a hot shower is a great example. Same goes when wearing damp clothes; their dampness against our skin lets our body heat escape much more easily. The worse thing you can do during a deep freeze is to let yourself get sweaty under your winter coat before you step outside or to wear mittens that aren’t quite dry. Wearing damp clothes in freezing weather can lead you to get frostbite or hypothermia much more easily than when you are wrapped in dry clothes.
Relative Humidity
If dampness helps heat escape from our bodies, we might be tempted to think that high humidity during the winter means the weather will feel colder. Not necessarily so! The colder the air gets, the less water vapour it can hold. “Relative humidity” is a measure that tells us how much water vapour is actually in the air compared to the maximum amount it can hold. So, a room full of air with a temperature of 25°C and 75% relative humidity will hold more water vapour that if the room temperature was -10°C. Colder air holds less water vapour.
Too Little to Make a Real Difference
Seeing as very cold air cannot hold as much humidity, the amount of humidity found in cold dry air versus the amount found in cold humid air is not all that different and doesn’t much change how cold the air feels to us.
This explains, by the way, why we talk much more about humidity in the summer than in the winter. What meteorologists call the “humidex” (humidity index) is a measure of how intense and uncomfortable the heat can feel to us. In the summer, air temperature is higher and holds more water vapour, which raises the level of humidity. High humidity makes temperatures feel hotter, even stifling, because all the water vapour in the air makes it difficult for sweat to evaporate from our skin. This makes it hard for us to cool off. When the air is drier, our sweat can evaporate into the air, drying our skin and letting our body heat escape and our body cool off. The drier the air, the less it feels hot to us. The more humid the air, the more stifling the heat feels. Oh, but we’re still far off from those hot summer days right now!
100% Relative Humidity
So, what happens when cold winter air is REALLY damp? When cold air becomes saturated with water vapour (when it holds as much water vapour as it can, known as 100% relative humidity), the water vapour turns into water droplets, or what’s known as condensation. If those droplets come into contact with our clothes or our skin, they create a conduit that our body heat can use to escape. Remember, though, it’s not the amount of humidity in the air making us feel cold in this case, but the humidity turning into water droplets and creating the dampness that body heat uses to escape. Much like when it rains in the winter or when we get melting snow; the weather conditions make it harder for us to stay dry, which can make the temperature feel colder because our body heat is escaping more easily because we are damp.
Wind Chill
OK, so if the level of humidity in the air doesn’t really change how cold it feels to us in winter, what about the “wind chill” factor? Wind chill can considerably change how cold it feels! That explains why Grandma used to say, “Dress warm and wear a hat, it’s windy out there!”
It really does feel colder when the wind blows harder and that’s because the wind whisks away our body heat. The stronger the wind blows, the quicker our body heat is whisked away, even if the air temperature stays the same. For instance, when the wind is blowing at 30 km per hour, the wind chill factor will make 0°C temperature feel like -6°C (though heat from the sun can also change how cold it feels, which we ignored in our calculation here). So remember, keeping your eye on the wind chill factor can be particularly useful when trying to stay warm and avoid frostbite and hypothermia.
