The word “desert” comes from a Latin word meaning “abandoned.” How appropriate, because when people think about deserts, they mostly conjure up images of endless sand dunes and desolate stretches of barren land. These images are accurate in some places in the world, but there are different types of deserts. Principally, what makes a desert is an inherent lack of regular rainfall, but scientists use a variety of additional factors to classify deserts into categories (hot and dry, semiarid, coastal, and cold). These factors include temperature, humidity, geology, soil and mineral composition, and the flora and fauna that inhabit the region.
Types of Deserts
Hot and Dry
Hot and dry deserts are classified as being hot throughout the year and very hot in the summer. They receive minimal amounts of rainfall in the winter. Soils tend to be shallow and rocky with good drainage. Plant life includes shrubs, small trees, and cactuses.
Seasons in semi-arid deserts are more defined than in hot and dry deserts. Summers are reasonably hot and dry, and rainfall in winter is minimal. Semi-arid deserts support an assortment of shrubby and grassy plant life.
These deserts are found in areas that are moderately warm to cool. Winters typically are cool and brief, and summers are long and warm. Coastal deserts support a wider variety of plant life than hot and dry or semi-arid deserts.
In cold deserts the main source of precipitation is not rain as in the other types of deserts. Here, moisture comes in the form of snow, ice, or fog. Many of these deserts are found at high elevations and support little life, if any.
Why are Deserts so Hot?
In most places around the world, moisture in the air prevents the sun’s rays from completely penetrating the atmosphere, allowing a ‘moisture blanket’ to protect the Earth from the sun’s intense heat. But in deserts, humidity is low, and the moisture content in the air is minimal, so the sun’s rays are able to penetrate the Earth, creating dangerously high temperatures. This lack of humidity also accounts for deserts’ extreme temperature differences. Since moisture in the air is low, heat accumulated during the daylight hours dissipates quickly, causing temperatures to plummet once the sun goes down. Some deserts experience temperature differences as great as 50°F or more in a single day!
Deserts of the World
• North America: North American Desert
• South America: Atacama, Patagonian
• Africa: Sahara, Arabian, Namib, Kalahari
• Asia: Turkestan, Takla Makan, Gobi, Iranian, Indian
• Australia: Australian
Most of these deserts are separated further into regions, each with a specific name. The North American Desert is divided into four regions: the Sonoran, Mojave, Great Basin, and Chihuahuan.
Surviving in the Desert
Deserts are some of the most inhospitable places on Earth, but despite the callous, arid environment, some creatures have managed to eek out a way of life. Desert-dwelling plants and animals have astonishing adaptations that allow them to survive harsh desert conditions.
Desert Plant Adaptations
Every living organism on Earth requires some form of moisture in order to survive, and in deserts, this is the most limiting factor. Deserts get an extremely limited amount of precipitation each year, which makes supporting plant life an extreme challenge. There are two basic strategies plants use to deal with this ever-present dilemma: drought evasion or drought resistance.
The Drought Evaders: A Sit-and-Wait Strategy
Many plants have circumvented a lack of moisture by altering their life strategy to generate, unfold, and complete in the time span of a single rain event. Certain grasses and weeds avert drought by remaining in a dormant stage as a seed until the seasonal rains come. A tough seed coat prevents them from desiccation (drying out) while they lay dormant. These plants have a short life span and dedicate their meager existence to perpetuating the next generation of desert-dwellers.
The Drought Resistors: Moisture Misers
Other plants, like shrubs and bushes, have developed many creative strategies to work around the perpetual water shortage in the desert. Some, like the creosote bush, have shallow roots that spread just beneath the surface to absorb surface moisture from rain as well as deep tap roots that explore deeper soil layers for moisture. Some plants have sparsely distributed, waxy leaves and some even shed them, both in order to prevent transpiration (evaporation through the leaves). The paloverde (“green tree”) has a chlorophyll-laden stem, enabling photosynthesis to occur without the additional threat of water loss through leaves. Succulents, like cactuses, conserve water in their tissues and release it in limited amounts over long periods of time to nourish the plant; sharp spines and irritating hairs prevent other organisms from obtaining the precious fluid. An interesting plant known as the night-blooming cereus stores water in a specialized underground bulb-like structure.
Desert Animals: Extreme Survivors
While most desert plants survive the arid climate through modifications of their structure, many desert animals alter their behavior to cope with the stressors of desert life. The majority of desert denizens limit their periods of activity to the night, when temperatures are cooler. Amphibians, such as the spadefoot toad, aestivate (a form of summer hibernation) during the dry season, emerging only during the rainy season. There are many animals, however, that have special adaptations for surviving the desert heat. Jack rabbits, for example, have large, upright ears with large blood vessels; this allows blood traveling through the ears to be cooled by the wind and re-circulated throughout the body, cooling the rest of the blood. Reptiles have roughly-scaled skin to prevent water loss.
The Rainy Season- A Temporary Eden
Even the driest, hottest, most arid deserts in the world have to get rainfall at some point in order to sustain life, and when it rains, it pours! Once a year (or once every several years in some regions) the desert sky opens up and healing drops of liquid life rejuvenate the landscape. Almost overnight the desert transforms from a desolate wasteland to a thriving garden of lush vegetation. Dormant seeds germinate; bare branches extrude verdant, green leaves; fruits and flowers blossom in luxuriant radiance. For a few short weeks, the desert is a tropical paradise, and this is when all the action happens. Plants generate the seeds that will perpetuate the next generation of desert survivors, and animals gorge themselves on the abundant vegetative resources. Then, almost as suddenly as the rains appeared, the fruits of their labor begin to vanish. Leaves wither and crumble under the scorching sun; pools dry up; animals burrow underground; and desert life fades back to its melancholy dormancy.
Desert Fun Facts
- The Sahara Desert in northern Africa is the largest desert in the world, encompassing over 3.5 million square miles; that’s almost as large as all of America’s 50 states!
- The Gobi Desert, in Mongolia, is actually cold for most of the year.
- There are some deserts in the world that are adjacent to tropical rain forests! These deserts are bordered by mountains that block the rain from entering; this is called the rain shadow effect.
Links for Additional Information on Deserts
- Desert USA: http://www.desertusa.com/glossary.html
- University of CA Museum of Paleontology: http://www.ucmp.berkeley.edu/exhibits/biomes/deserts.php