The Desert

It is common to think of deserts as hot sandy places, but the real definition of a desert is based solely on the amount of rainfall that an area receives (less than 10 inches (25 cm) of precipitation a year). Some deserts are rocky, some are sandy and some can be very cold. However, let’s focus on the stereotypical hot sandy desert for the moment.

Although we think about these deserts as hot, they can get quite cold at night. In fact it is not uncommon for the temperature in a desert region to fluctuate more than 55^oF between day and night. By contrast, the temperature in a forest rarely fluctuates more than about 25^oF between day and night. The reason behind this is simply water.

The soil and plants in a forest contain a large amount of water, so that the heat capacity of the forest is huge. That means that it takes a long time, and a lot of energy to change the temperature very much. Thus, even though the sun beats down throughout the day on the trees and grasses of a forested area, and even though much of that energy is absorbed, the temperature changes very little.

At night, after the sun sets and the air begins to cool, the plants and wet soil give heat to the air, preventing it from becoming too cold.And, even though they may give away a great deal of heat, the temperature will still not change a great deal.

By contrast, when the sun rises over a desert, the heat is absorbed by the land. However the land (containing very little water) has a low heat capacity and therefore warms up quickly. At night, after the sun goes down and the air cools, the land has very little heat to give up and the temperature drops significantly.