Why do deserts get cold at night?
The short answer
Dry air and few clouds let heat escape from the ground easily after sunset, so desert temperatures can fall quickly.
How it works
Water vapor and clouds absorb some infrared radiation and send part of it back downward. Deserts have less of this effect, and shallow sand stores little heat, so the surface cools rapidly.
A familiar example
A clear, dry night usually cools more than a cloudy night in the same place.