Why is it so cold?
Usually because your air came from somewhere much colder, often Canada or the Arctic, a few days ago. The coldest nights add clear skies, calm wind and dry air, which let the ground lose its heat straight to space.
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Cold air travels
Air takes on the temperature of the ground it sits over. Air that spends a week over snow-covered northern Canada gets very cold and very dense. When the pattern shifts, that heavy air slides south behind a cold front and can drop temperatures 20 to 40°F in a day. Air Story's trail map shows this directly: on a cold day your air usually came down from the north or northwest.
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Why clear nights are the coldest
At night the ground radiates heat upward. Clouds and water vapor absorb some of that heat and send it back down, like a blanket. Take away the clouds, dry out the air and stop the wind, and the ground loses heat fast. The air touching it chills, and by dawn the coldest air is the layer right at the surface.
Why valleys and snow make it colder
Cold air is heavy, so on calm nights it drains downhill and pools in valleys and low spots. Hillsides a few hundred feet higher can be noticeably warmer. Snow cover makes it worse: it reflects sunlight during the day and gives off heat efficiently at night, so snowy ground stays colder.
Questions people ask
- Why does it feel colder than the temperature?
- Wind carries body heat away faster. The wind chill estimates how cold it feels on exposed skin: 30°F with a 15 mph wind feels like about 19°F.
- Why is it colder at the bottom of a hill?
- On calm, clear nights cold air drains downhill like water and collects in low ground. Frost often forms in valleys first.