Why does the desert freeze at night after a scorching day?

The desert is cold at night because there is almost nothing between the ground and space to hold the heat in. During the day the desert sky is usually clear, so most of the Sun's radiation reaches the ground. But as soon as the Sun sets, the desert cools quickly by radiating heat into space. Desert temperatures commonly swing by 20 to 30 degrees Celsius between day and night, and in hot deserts summer days can top 45 degrees while winter nights can plunge below freezing.

Humidity is the other half of the story. The higher the humidity, the smaller the swing between the day's high and the day's low. Pinnacles National Park can reach 38 degrees on a summer day and then fall to between 5 and 10 degrees. Washington D.C., which is much more humid, swings by only about 8 degrees. Water vapour in the air holds back some of the heat the ground radiates toward the sky, while dry, clear air lets that infrared heat pass straight through, so desert air makes a very thin blanket.

The coolest moment is not midnight but the hour around dawn, since heat is lost all night long. That rhythm shapes desert life. Reptiles cannot survive the heat of the Sahara's day, which can reach 50 degrees, so they shelter, and then come out in the first part of the night, as the ground gives back the heat it absorbed.

Even the rocks feel it. Repeated heating and cooling puts strain on exposed rock, and the flanks of mountains crack and shatter.