Weather Words: Radiational Cooling

Have you ever noticed how some nights feel much colder than others, even without a strong cold front moving through? That’s often due to radiational cooling, a process where the Earth rapidly loses heat after sunset.

During the day, the sun warms the ground, which in turn warms the air above it. But at night, when the sun sets, the Earth’s surface radiates heat back into space. On clear, calm nights, this process happens more efficiently because clouds and wind play a big role in temperature retention. Cloud cover acts like a blanket, trapping heat near the Earth’s surface. On cloudfree nights, that heat is allowed to escape, dropping temperatures quickly. This can lead to frost or even freezing conditions in colder months.

Radiational cooling is strongest under clear skies, dry air, and light winds. This is why desert areas, which often have low humidity and little cloud cover, experience extreme temperature swings between day and night. It’s also why rural areas with open fields and valleys tend to be colder at night than cities. This is because buildings and pavement retain heat longer, keeping the city warmer (even on cloudless nights) than rural locations very close by. It’s a phenomenon known as the urban heat island effect.

On especially cold nights, radiational cooling can lead to black ice on roads and even record-breaking temperature drops. Meteorologists often look at overnight forecasts and sky conditions to predict just how much cooling will occur.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.