Desert climates can have very hot days and very cold nights. this is because

Clear Sky

Explanation: temperature during the day time rises to maximum as there is no moisture in the atmosphere. The moisture being extremely low leads to formation of no clouds/or very rare clouds . Due to no clouds, sky is clear and thus the heat from the sun does not stay close to the Earth surface and it is radiated back to the maximum extent causing the temperatures at night  falling to nearly freezing.