Joan H. Branham, age 11, of Louisville, Ky., for her question:
WHY ARE THE DESERTS SO COLD AT NIGHT?
Many vacationing families will have crossed the delightful deserts of North America this summer. During the daytime, the dazzling sun beats down with a golden glare, the temperature soars higher and still higher. After a scorching day, visitors expect the night to be uncomfortably hot. But not at all. Soon after sunset the desert temperature drops, perhaps 60 degrees or more.
A desert is a dry region, where the average rainfall is limited to five or 10 inches a year. Rain comes from the clouds, which are created from gaseous water vapor mingled with the air above. There is always a certain percentage of vapor in the air, even above the driest desert. But the vapor content of desert air is very low.
This is why there are few clouds in the brilliant blue desert sky. At night the stars shine brighter, and we see many more of them. All this adds a razzle dazzle glory to the desert scenery. But in moister climates, the clouds overhead help to moderate the daytime and nighttime temperatures.
Everywhere the summer sun adds more heat to the earth during daylight hours. This accumulating heat naturally stops when the sun sinks below the western horizon. Then the heat from the ground begins to rise up into the atmosphere, expecting to escape. However, the clouds act like blankets. They prevent the heat from escaping, so the night becomes hot and humid.
In the dry air above the desert, there are no clouds and little or no hazy moisture to prevent the daytime heat from escaping. The accumulated daytime heat stops at sundown and immediately starts to escape. In a short time the desert air becomes chilly, and before morning it is downright cold.
Travelers in the desert must be prepared for these astonishing changes in the temperature. If you plan to camp outdoors, be sure to take along a couple of warm blankets or a very cozy sleeping bag.
The magnificent deserts of Southwestern North America become very hot on summer days, but winter days can be quite chilly. In parts of Asia and South America there are cold, dry desert regions. There the winters are bitterly cold, day and night. Summer days are mildly warm, followed by frosty nights. Here, too, there are few clouds to hold in the daytime warmth.