Question

why are deserts cold at night and really hot during the day?

Answer 1

Because of the lack of water, deserts cannot support much vegetation. So during the night when the sun is not shining the sand loses all its heat, making the desert night cold. Sand is the biggest reason why deserts are hot during the day and cold during the night.

Answer 2

Sand cannot hold the heat. It acts like a mirror to the sun. During the daytime, it stays warm, and when the Sun is absent it loses all its heat making the nights colder. There's nothing in the desert that can either absorb heat from the sun or hold it on the surface when the sun is set.