Why is it that deserts get so cold at night?


The Milky Way shines over the Sahara Desert in the night sky. (Image credit: Shutterstock)

In the Sahara, temperatures will drop overnight by an average of 75 degrees Fahrenheit (42 degrees Celsius).

If you're taking a day trip to North Africa's Sahara Desert, you'll want to bring plenty of water and plenty of sunscreen. But if you're planning to stay overnight, it's safer to bring a snug sleeping bag with you too.

This is because, according to NASA, temperatures in the Sahara will drop once the sun goes down, from an average high of 100 degrees Fahrenheit (38 degree Celsius) to An average low of 25 degrees Fahrenheit (minus 4 degrees Celsius) at night during the day.

Then why, in arid deserts like the Sahara, does this dramatic temperature change happen? And how do such wild extremes cope with native animals and plants?

Heat and humidity

The reason why arid deserts get so hot, and subsequently so cold, is a combination of two main factors: sand and humidity: dry regions covering around 35 percent of the earth's ground.

Sand doesn't hold heat very well, unlike a thermos. According to a 2008 study from NASA's Jet Propulsion Laboratory in Pasadena, California, as heat and sunlight reach a sandy desert, sand grains in the top layer of the desert absorb and even release heat back into the air. The sand's radiation from the sun's energy overheats the air during the day and causes temperatures to soar. But, much of the heat in the sand radiates rapidly into the air at night, and there is no sunshine to heat it up again, making the sand and its surroundings colder than before.

This phenomenon alone does not, however, account for such a dramatic temperature drop. After all, you don't need to don a winter coat when the sun goes down on a tropical beach.

The primary explanation for the sudden shift in temperature is that the desert air is exceptionally dry. The humidity, the amount of water vapor in the air, is virtually zero in arid deserts such as the Sahara and the Atacama Desert in Chile, and water has an enormous capacity to store heat, unlike sand.

Changing climate

Researchers are still working out how arid areas and species can be influenced by climate change, but "we're definitely going to see changes," DeNardo said. We are expecting an average temperature increase of 3 to 4 degrees Fahrenheit [1.7 to 2.2 C] for most deserts.

Instead, the real issue is that the amount of annual precipitation that desert creatures depend on can influence climate change. It's going to get less stable, you're going to have relatively wet years, DeNardo said, and relatively dry years. But even though most of them are wet enough, it's going to take just one very dry year to cause major problems.

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