Why do deserts get so cold at night?

Water vapor is the atmosphere's most important greenhouse gas, and deserts have almost none. During the day the sun heats sand and rock efficiently, and with no clouds or humidity to scatter incoming radiation the surface gets extremely hot. After sunset that heat radiates straight back to space as infrared, and with no water vapor or cloud cover to absorb and re-emit it downward, it simply leaves. Humid regions stay warm overnight precisely because their moist air acts as a blanket, which is why a muggy summer night in Georgia barely cools while a Saharan night can approach freezing.

Sand's physical properties amplify the effect. It has low heat capacity compared with water, so it both heats and cools rapidly and stores little energy to release overnight, unlike a coastal region where the ocean moderates temperature in both directions. Dry sand also conducts heat poorly, so the daytime warmth stays concentrated in a shallow surface layer that sheds quickly rather than penetrating deep. Diurnal swings of 40 degrees Fahrenheit are common and swings above 70 have been recorded. This is a genuine safety issue for desert travel, since people prepare for the heat and get caught by the cold, and hypothermia in a desert at night is a real and regularly underestimated risk.