A. Characteristics Rainfall less than 25 cm (10 inches) per year Sparse, widely spaced, mostly low and

vegetation Cover 30 % of the Earth’s surface and are

situated mainly between tropical and subtropical regions north and south of the equator

The baking sun warms the ground in the desert during the day. At night, most of the heat radiates quickly into the atmosphere because desert soils have little vegetation and moisture to help store heat. This is why you roast during the day and shiver at night.

Types of deserts Tropical deserts –

southern Sahara in Africa

Temperate deserts – Mojave in Southern California

Cold deserts – Gobi in China

Why are deserts located only in certain areas?1. Convection cells create desertsAir rising at the equator is warm and moist. As the warm, moist air rises, the air cools and the moisture condenses then falls as rain. The air currents spread out to the north and south, and descend around 30 degrees north or south of the equator. The descending air warms and dries the land, creating deserts.

2. The Rain Shadow effect creates desertsAir pushed up over mountains cools and the moisture condenses and falls as precipitation. The sinking air on the other side of the mountain warms and dries the land creating deserts on the leeward side of mountains.

3. Interior of continents far away from oceans

Geologic Processes in Arid Climates

Weathering – much of the weathered debris in deserts has resulted from mechanical weathering. Chemical weathering is very slow due to the lack of moisture and scarcity of organic acids.

Ephemeral streams – streams that only carry water after it rains, problem of flash flooding

A Desert Landscape – Most desert erosion is from water erosion

Alluvial fan – cone of debris at the mouth of a canyon

Playa lake – shallow lake that lasts only a few weeks

Disappearing streams – streams that dry up before they reach the ocean

Wind Erosion – winds pick up, transport and deposit great quantities of fine sediment

Deflation – lifting and removal of loose particles such as clay and silt,

creates depressions called blowouts creates a stony surface called desert

pavement Saltation – sand particles roll or skip

along the surface Abrasion – wind blown sand cuts and

polishes exposed rock surfaces

Wind Deposits Loess – windblown silt that

blankets the landscape Thickest most extensive

deposits in western and northern China

Found in the United States Midwest

Sand dunes – sand deposited in mounds or ridges Begin near obstruction such as a clump of

vegetation or rock. As the wind loses speed, the sand particles drop out.

Once the sand starts to mound up, it serves as its own obstruction and traps more sand.

Dunes are steeper on the sheltered side and more sloping on the inclined side facing the wind.

As the sheltered side becomes steeper, the sand eventually slides down the slope. This forms cross-bedding

Dunes migrate downwind.

Types of Sand Dunes – what forms sand dunes assume depends on the wind direction and speed, how much sand is available, and the amount of vegetation.a. Barchan dunes – solitary sand dunes shaped like

crescentsb. Tranverse dunes – long ridges of sand perpendicular to

the direction of the windc. Barchanoid dunes – intermediate between Barchan

and tranversed. Longitudinal dunes – long ridges of sand that form

parallel to the direction of the winde. Parabolic – crescent shaped with tips pointing into the

windf. Star dunes – develop in areas of variable wind

direction

Environmental impact of humans on deserts

Large desert cities Soil salinization from irrigation Depletion of underground water

supplies Large land disruption from mining Storage of toxic and radioactive

waste Livestock overgrazing and off-road

vehicles destroys vegetation

Plant and Animal Adaptations

Plant adaptations Waxy-coated leaves Using deep roots or widely spread

shallow roots to collect water Dropping leaves during dry periods or

becoming dormant during dry periods Animal adaptations Hiding in cool burrows Thick skin to conserve water Getting water from dew Become formant during periods of

extreme heat or drought