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.
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 deserts
Air 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 deserts Air 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
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
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. Barchan dunes – solitary sand dunes shaped like crescents Tranverse dunes – long ridges of sand perpendicular to the direction of the wind Barchanoid dunes – intermediate between Barchan and tranverse Longitudinal dunes – long ridges of sand that form parallel to the direction of the wind Parabolic – crescent shaped with tips pointing into the wind
Star dunes – develop in areas of variable wind direction
Environmental impact of humans on deserts 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 Using deep roots or widely spread shallow roots to collect water Dropping leaves during dry periods or becoming dormant during dry periods Thick skin to conserve water
Become formant during periods of extreme heat or drought