Lack of moisture keeps clouds away throughout world's deserts

"Why are there barely ever clouds in the desert?," asked Haley Hauptman, 10, a fourth-grader at Diamond Lake School in Mundelein.

Deserts are places that year-round are too dry to grow enough crops to feed a community.

There are two kinds of deserts -- hot and cold. Hot deserts are located at low elevations and have extreme high temperatures during the day. Cold deserts are found at higher altitudes and have very low temperatures. Both kinds of deserts have very little moisture -- typically less than 10 inches of rain each year.

Moisture acts like a blanket and retains heat. In ecosystems that have very little moisture -- either hot or cold deserts -- temperatures can become very cold at night.

"There's little moisture in the air to begin with," said Blake Hamilton, a junior at Iowa State University majoring in meteorology. Without moisture, temperatures in a hot desert become extreme, ranging from above 100 degrees in the day and dropping to as low as 32 degrees at night. Cold desert temperatures can sink to as low as -70 degrees.

The southwestern U.S. has four major hot desert regions -- the Sonoran, the Chihuahuan, the Great Basin and the Mohave. Barrow, Alaska, sits north of the Arctic Circle and has an average of 4.5 inches of precipitation each year. Around the world, deserts cover about 20 percent of the Earth.

Clouds form when rising air, called a parcel, cools. Air pressure drops as the parcel continues to rise, forcing moisture out of the parcel. Moisture escapes in the form of condensation, changing the moisture into a vapor called a cloud.

Hamilton explains that the lack of moisture in a desert ecosystem makes it nearly impossible for clouds to form. "The warmer air rises and has to go so much higher to condense. When the air parcel condenses, that's when clouds form. In the Arctic deserts, colder air can't hold as much moisture as warmer air, so it's drier." Low moisture levels make low levels of snowfall in the Arctic, but because of low temperatures, the snow remains much longer.

Because of the lack of rain, plants and animals that exist in the barren desert have developed unique characteristics. In a hot desert, plants and trees are scarce. Plants often have a waxy coating and spikes that are the plant's leaves. Trees can be spiny and slow-growing like the Joshua tree. Cold deserts can only sustain small grasses and mosses. Arctic plants have to be able to withstand very low temperatures and a short growing season.

Animals in hot deserts are mainly nocturnal and many burrow underground to beat the heat. Snakes, lizards and insects can survive the temperature extremes in hot deserts. Animals that can survive the Arctic temperatures are caribou, musk ox, arctic foxes, arctic hares, snowy owls, lemmings and, in extremely cold temperatures, polar bears.

Even deserts, with so little moisture, can suffer from drought conditions. Some scientists believe that global warming and wildfires will drive desert plants like the Joshua tree to extinction within 100 years.