It seems that whenever it’s really cold in the winter, the skies are clear and sunny, and when the temperatures are mild, it’s cloudy. Why is that?—Joan Holmes, via email
First of all, excellent question! “And especially relevant this winter when we saw one of the cloudiest, gloomiest Decembers and Januarys on record,” says David Phillips, a senior climatologist with Environment and Climate Change Canada. “It’s unbelievable that in the first four days of February, we had more clear skies than in the previous two months in total.”
You’re correct that, in winter, the downright balmy temperatures are often associated with clouds, rain, and fog; frigid temperatures usually go hand-in-hand with bright sunshine. This phenomenon, in large part, has to do with where the air that blows into our region—and affects the temperature—originates from.
“Cold air and mild air come from different places,” says Phillips. “Cold air is polar air, northern air. It’s very dry. It doesn’t hold a lot of moisture.” You know this because the snow makes crunchy sounds when you walk on it, your skin feels like it’s about to crack open, and “your hair behaves in a different way.”
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Cold air is heavy and dense, says Phillips. “It sinks and hugs the ground.” That, combined with the lack of moisture in the air, means that “clouds simply don’t form on a very cold day.”
Warm air arrives from the south or the west. “It’s typically coming from areas of open water,” says Phillips. It can hold a lot of moisture, and it can eventually cool, condense, and form clouds. So, when that kind of air rolls in, “chances are it’s going to be mild and cloudy.” (There are exceptions, say, if the air coming from the south happens to be dry—then we get mild, sunny weather.)
Also keep in mind that heat is always emanating from the earth. A thick blanket of clouds above the earth acts like, well, a blanket, trapping that heat, making the air around you feel warmer. But if there’s no cloud cover, that heat dissipates into space, and the air is colder. And obviously, with no clouds blocking the sun, it’s sunny.
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There’s a third reason why we experience more clouds on mild days. If mild air causes existing snow to melt, but the ground is still frozen, the water just pools and ponds and turns into lakes of slush on the ground. Until it evaporates and becomes clouds, says Phillips. Usually these are stratus clouds, the long, grey, flat ones. “This forms a deck that stretches from one horizon to the next.” These might be the gloomiest clouds of all. “You can get a stretch of overcast days.”
Which is what happened for the first half of winter 2023-2024. December and January also brought more than 300 hours of fog, and two to three times more rain than normal.
“It’s been disappointing weather for outdoorsy people,” says Phillips. (Heck, it’s been disappointing weather even for indoorsy people.) “It’s not been a winter that they’d soon hope to repeat.”
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