Deserts are classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.

Trade wind deserts

Africa's Sahara Desert    The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world's largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert.

The Sahara of Africa is the world's largest desert. It contains complex linear dunes that are separated by almost 6 kilometers. (Skylab photograph).

What to do if Lost in the Desert

Earth Science

It is in the papers yet again. Someone has died in the Mohave. This time, a mother and child took a weekend trip to the desert. They got lost and their car got stuck, and one of them died. In a horrifying reversal, the child died, while the mother is expected to survive.

If you suddenly find yourself lost in the wilds, remember to think, not feel, and plan, not panic. However, almost no one suddenly finds himself or herself lost in the desert. Generally, people decide to visit, and then don’t properly plan and prepare. Some who are lost in the wilderness are unlucky, but most are unprepared.

Preparation

or more sturdy lighters. A knife may feel silly, but it is appropriate. GPS is good, but maps are better, more trustworthy and easier to pore over. The woman who lost her son in the desert was counting on her GPS. Incidentally, cell phones don’t work in some parts of the desert. Satellite phones work anywhere, if you can afford one. Keep any phone turned off, to spare its battery.

Make sure your car is in good shape. Have it maintained, and top off the fluids. If you’re car camping, you can easily take plenty of extra supplies, like water, a box of garbage bags, extra food, and extra clothing. You could even bring tarps, and a water purification kit if you’re driving. Why not?

Before you leave on your adventure, tell people. Tell your family, tell friends, tell a trusted coworker your precise destination. When you arrive at a desert park, tell a ranger, or, if you will not see one, leave an email or note. If help is needed, it is important that rescuers look in the right place. The skilled searchers looking for the lost mother and child were scouring the wrong end of Death Valley.

Water

Ordinary people can survive three weeks without food. They are likely to die after three days without water. Water is the essential for desert survival. You should have a half gallon per day per person. If your time in the desert might stretch though, you should ration. You can probably survive on four ounces of water a day or so. Delirium indicates a need for more water.

Even if you have sufficient food, eat very lightly if you have little water. Water is required for digestion, and you can go three weeks or so without food. If you catch insect food, remember not to eat anything colorful or pungent. Insects like these are advertising that they are poison.

To conserve water, avoid sweating as much as possible. Stay in the shade, if there is any. Use the shade of your car, or beside your car, or even beneath it. Throw fabric or garbage bags over a bush if there is one, leaving the open side to the north, or towards any breeze. Or use a rock to support your shelter. Also insulate yourself from the hot ground, with anything you have, before you lie down in the shade.

Viking spacecraft image of Mars

This Viking spacecraft image of Mars shows alternating layers of ice and windblown dust near the north polar cap. Annual and other periodic climatic changes due to orbit fluctuations may occur on Mars (courtesy of USGS Image Processing Facility, Flagstaff, Arizona).

Image taken at the

Viking 2 landing site on Mars    Viking image of crescent-shaped

dunes on Mars

One of the first images taken at the Viking 2 landing site on Mars shows the pink sky over Utopia and the desert pavement on the ground (courtesy of NASA).    Some of the crescent-shaped dunes in this Viking image of Mars are more than a kilometer wide. The dark material that streaks from the horn-shaped features may be dust recently blown from the dunes (courtesy of NASA).

Scan the territory immediately around you for water. Then decide whether the exertion is worth the unlikely chance of water. Low places can mean water. Animal tracks are clues to water. Vegetation in any quantity is nurtured by water. Dig in low places on the outside of curves in dry riverbeds for water, but remember why they are called washes. People die of dehydration in the desert, but they also drown.

Shelter

Washes are dangerous places to seek shelter, because of the danger of flash floods. Water can rise in an arroyo because of a thunderstorm miles away. It can roar down a dry watercourse faster than anyone can run. Yes, there are summer storms.

The base of a cliff might seem like a good place to shelter, but cliffs crumble and shed rocks. Do not add an injury to your other troubles. If people have been told your destination, and you are near it, or on a logical route to it, your best shelter is nearby, and your best plan is to stay put and wait for help.

Signals

Signals can help searchers find you. It’s a big wilderness. Signals should be sent in threes, if possible. A single noise can be misheard. Random noises can be hunters, ranchers, miners, or happy campers. Three signals show intention.

If you even suspect someone might hear you, blow your whistle three times. Or clang on metal, or toot your car horn, if you can, in bursts of three. Mirrors work as signaling devices too.

Burning car tires are an excellent signal, making a thick dark column of smoke. Do not burn them in threes however, save your fuel. Fire in the desert draws interest.

Travel

Polar deserts

The Dry Valleys of Antarctica    Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10° C. Polar deserts on the Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This "freeze-thaw" alternation forms patterned textures on the ground, as much as 5 meters in diameter.

The Dry Valleys of Antarctica have been ice-free for thousands of years (courtesy of USGS Image Processing Facility. Flagstaff. Arizona).

Paleodeserts

Data on ancient sand seas (vast regions of sand dunes), changing lake basins, archaeology, and vegetation analyses indicate that climatic conditions have changed considerably over vast areas of the Earth in the recent geologic past. During the last 12,500 years, for example, parts of the deserts were more arid than they are today. About 10 percent of the land between 30? N. and 30? S. is covered now by sand seas. Nearly 18,000 years ago, sand seas in two vast belts occupied almost 50 percent of this land area. As is the case today, tropical rain forests and savannahs were between the two belts.

Fossil desert sediments that are as much as 500 million years old have been found in many parts of the world. Sand dune-like patterns have been recognized in presently nonarid environments. Many such relict dunes now receive from 80 to 150 millimeters of rain each year. Some anci

If you have good reason to believe that no one knows where to look for you, if you are not safe where you are, or if someone in your party is injured, you may want to try to find you way out.

If you are on or near a road, follow the road. Train tracks, if they are in use, and high-line pylons are good guides as well. Do not cut across country unless you must. If you believe you know where you are, there are ways to navigate by the sun and moon.

If you don’t have a compass, or don’t know how to use one, wait for noon. When the sun is highest in the sky, if you walk towards it you are walking south (in the Northern Hemisphere). If you turn your back to the sun, you are walking north.

If you mark north and south on the ground, you can see where east and west are. However, you do not want to walk in the desert at noon. Instead, at noon, fix your eyes on a landmark in the direction that you want to travel. Then, in late afternoon or at first light, walk toward it.

Seek the easiest path. Five miles of sand is harder than ten miles of decent going. Luckily, there is not much sand in many deserts. Do try to pick out a route that is level and on good ground.

When the moon rises before sunset, the bright side of the moon is toward the west, but if it rises after midnight, the eastern side is lit. Again, after determining a direction, walk towards a distant landmark. This keeps you from being led astray by contour changes.

In most circumstances, do not travel in full darkness. You cannot afford an injury. The best times of day for travel are early morning and late afternoon.

Conclusion

Many intelligent and thoughtful people have been lost in the desert, often because they were not prepared. Anyone who runs into trouble should think carefully about the situation, and decide as calmly as possible what to do. Almost always, it is better for lost hikers to stay where they are. Th

Make sure your fitness level is high enough for the outing you anticipate. The desert is a tough environment. Toughen up before you go. While you’re at it, why not learn how to make a fire, follow a compass, and read a contour map. Shorten your planned route. This leaves time for leisured enjoyment, and margin for error. Check the weather report.

Dress properly. A tank top, tight capris, and mules will not serve you in the desert. On the other hand, you don’t need the latest from REI, either (though their gear is excellent). Wear loose-fitting light clothing, with long sleeves and long pant legs. You can roll them up. Take fleece, a top layer, and a sleeping bag. Wear protective shoes, not sandals, not heels. Take extra socks.A hat is essential. So are a phone, maps, a whistle, and twoey should usually seek shade, send signals, and conserve their water, while they wait for rescue.

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Midlatitude deserts

Tengger Desert of China    Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical highpressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert.

A rare rain in the Tengger, a midlatitude desert of China, exposes ripples and a small blowout on the left. Winds will shortly cover or remove these features.

Rain shadow deserts

Landsat image of the Turpan

Depression of China    Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside "shadow" of the range.

This Landsat image shows the Turpan Depression in the rain shadow desert of the Tian Shan of China. A sand sea is in the lower center on the right, but desert pavement, gray in color, dominates this desert. The few oases in the desert and the vegetation in the mountains at the top are in red. A blanket of snow separates the vegetation in the Tian Shan from the rain shadow desert.

Coastal deserts

Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama of South America, is the Earth's driest desert. In the Atacama, measurable rainfall--1 millimeter or more of rain--may occur as infrequently as once every 5-20 years.

Crescent-shaped dunes in Namib, 

Africa

Crescent-shaped dunes are common in coastal deserts such as the Namib, Africa, with prevailing onshore winds. Low clouds cover parts of the Namib in this space shuttle photo.

High dunes of the Namib near

Sossus Viei    Morning fog in the Namib

High dunes of the Namib desert near Sossus Viei (photograph by Georg Gerster).    Morning fog moistens the dunes of the Namib coastal desert (photograph by Georg Gerster).

Monsoon deserts

Landsat image of the Thar desert    "Monsoon," derived from an Arabic word for "season," refers to a wind system with pronounced seasonal reversal. Monsoons develop in response to temperature variations between continents and oceans. The southeast trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Rajasthan Desert of India and the Thar Desert of Pakistan are parts of a monsoon desert region west of the ranqe.

The Indus River floodplain, lower left, is the western border of the Thar Desert. This Landsat image of the monsoon desert shows small patches of sand sheets in the upper right, with three types of dunes; some dunes are almost 3 kilometers long.ent dunes are in areas now occupied by tropical rain forests.

The Nebraska Sand Hills is an inactive 57,000square kilometer dune field in central Nebraska. The largest sand sea in the Western Hemisphere, it is now stabilized by vegetation and receives about 500 millimeters of rain each year. Dunes in the Sand Hills are up to 120 meters high.

Nebraska Sand Hills    Vegetation in the

 Nebraska Sand Hills

This aerial photograph of the Nebraska Sand Hills paleodesert shows a well-preserved crescent-shaped dune (or barchan) about 60 to 75 meters high. (Photograph by Thomas S. Ahlbrandt)    A dry community of vegetation grows among the dunes of the Nebraska Sand Hills. (Photograph by N.H. Darton)

Extraterrestrial deserts

Mars is the only other planet on which we have identified wind-shaped (eolian) features. Although its surface atmospheric pressure is only about one-hundredth that of Earth, global circulation patterns on Mars have formed a circumpolar sand sea of more than five million square kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest sand sea on our planet. Martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north polar area. Smaller dune fields occupy the floors of many large craters in the polar regions.