In the deserts the sunrays reach the surface of the earth at almost right angles during summertime. The heat is therefore concentrated over small areas of ground which gives great heat. Daytime temperatures are high due to no cloud or humility to help block the sun rays The absence of vegetation and water bodies allows sun rays to be absorbed into the desert surface. Then the surface returns this heat to the atmosphere causing high atmospheric temperatures.
Deserts are cold by night as the cloudless skies allows the daytime temperatures to escape rapidly after nightfall.