Why do some deserts get cold at night even after scorching days?

Water vapor and clouds act like insulation, trapping heat near the ground. In dry deserts, the atmosphere contains little water vapor, so heat is radiated away rapidly once the Sun sets. Bare rock and sand also heat up and cool down faster than moist soil or vegetation-covered land. That is why deserts such as the Sahara or the Gobi can have very hot afternoons and chilly nights. The large day-night temperature swing is one of the defining features of arid climates.