Why is it so hot at night?
Normally the ground loses its heat to space after sunset. Nights stay hot when something blocks that: humid air or clouds holding the heat in, wind stirring warm air down from above, or air that arrived hot from somewhere else.
See your air right nowWhere it was three days ago, live. No sign-up.→How a normal night cools
On a clear, calm, dry night the ground radiates heat straight up to space and cools quickly. The air touching it cools too, so a thin layer of cold air settles near the surface while warmer air sits on top. That's why deserts can swing 30 or 40°F between afternoon and dawn.
What stops it
- Water vapor and clouds. Both absorb the heat the ground gives off and send part of it back down. A humid night, or a cloudy one, cools slowly.
- Wind. A steady breeze keeps mixing the warmer air from a few hundred feet up down to the ground, so the cold layer can't form. Windy nights stay warm.
- The air itself is warm. If the air mass arrived from a hot desert, or sank and heated up on the way down, the whole column is warm before the night even starts. That's a Santa Ana night: hot, dry and breezy at 3 AM.
- Cities. Pavement and buildings soak up heat all day and release it all night.
Two kinds of hot night
A muggy hot night has a high dew point: the air is full of water and can't cool. A dry hot night, with a low dew point and wind, means warm air is being pushed down to you. Air Story tells them apart on its first screen and shows where tonight's air came from.
Questions
Why is it hotter at night than during the day sometimes?
When a warm air mass or downslope wind arrives after sunset, temperatures can rise overnight. It's common at the start of a Santa Ana event and ahead of some cold fronts.
Are warm nights dangerous?
They can be. Bodies recover from daytime heat overnight; when nights stay above the mid-70s°F that recovery doesn't happen, which is why heat advisories often mention overnight lows.