Science Puzzle
Why Is the Desert Freezing at Night?
The Sahara can reach 45 degrees Celsius in the afternoon. That same night, the temperature can plunge below freezing. A swing of nearly fifty degrees in a single day.
Deserts are famously hot. Why do they get so brutally cold at night?
The Answer
Because deserts have no water vapour, and water vapour is what normally keeps the night warm.
Everything warm glows with invisible heat radiation, and after sunset the ground pours the day's heat back out toward the sky. In a humid place, water vapour in the air absorbs a great deal of that outgoing heat and radiates much of it straight back down. The moisture acts like a blanket, and the night stays mild. Cloud does the same job even better, which is why a cloudy night is warmer than a clear one.
Desert air is bone dry and usually cloudless. There is nothing to catch the escaping heat, so it streams unimpeded out into space. On top of that, sand is a poor heat store: it warms only a thin surface layer, which cannot keep the night going. So the desert sheds almost everything it gained in the afternoon, and by dawn the ground can be covered in frost.
The principle: Radiative cooling and the greenhouse effect. Water vapour absorbs and re-emits outgoing heat radiation, so dry air allows far faster night-time cooling.