Why do deserts get cold at night?

Answer: Dry air doesn't retain heat

Cold underground water rises — Wrong. Underground temperature doesn't cause surface temperature swings. Desert cooling happens because dry air poorly retains daytime heat.

Dry air doesn't retain heat ✓ — Correct! Water vapor in air acts like a blanket, trapping heat (greenhouse effect). Deserts have very dry air with little water vapor. During the day, sun heats the ground intensely. At night, without moisture to trap heat, infrared radiation escapes rapidly to space. Temperatures can drop 40°F (22°C) or more! Humid areas stay warmer at night.

Sand generates cold at night — Wrong. Sand doesn't generate cold—nothing generates cold. Sand simply loses heat rapidly at night because dry air lacks water vapor to trap infrared radiation like a greenhouse blanket.