Winds and deserts

Winds in hot deserts. Dry, bare desert floors heat up fast; the hot air above rises with turbulence, eddies, whirlwinds, updrafts and downdrafts, and winds sweep along the floor at great speed. Wind works by:

• Deflation: lifting and removing dust and small particles.
• Abrasion: sand and silt carried by wind scour the surface.
• Impact: the sheer force of blown sand striking rock, like sand-blasting.

Running water in deserts. Rain is scarce but falls torrentially in short bursts. Bare rocks, weakened by big daily temperature changes, break down fast, and sheet floods carry the debris away. So wind moves fine material, but most mass erosion is by sheet floods (sheet wash). Desert channels are broad, smooth and short-lived.

Erosional landforms:

• Pediments: gently sloping rocky floors at the foot of mountains, with little or no debris, cut by lateral stream erosion and sheet floods. The steep slope and cliff above them retreat backwards (parallel retreat through backwasting), so pediments grow at the mountain's expense, leaving a remnant hill, an inselberg. Finally the high relief becomes a low, featureless pediplain.
• Playas: in basins ringed by hills, drainage flows to the centre, building a level plain that holds a shallow lake after rain; it soon evaporates, often leaving salts. A salt-covered playa is an alkali flat.
• Deflation hollows: shallow depressions where wind blows loose material away. Blow outs are shallow hollows in rock faces; some deepen into caves.
• Mushroom, table and pedestal rocks: resistant remnants polished by wind, with a slender stalk and broad cap, a flat table top, or standing like pedestals.

Depositional landforms. Wind is a good sorting agent: grains move by rolling, saltation (bouncing) or suspension according to size, and settle in order as the wind slows. Sand dunes need a sand source, steady winds and obstacles to start them:

• Barchans: crescent dunes with their wings pointing downwind; steady, moderate wind over an even surface.
• Parabolic dunes: crescent dunes reversed (wings pointing upwind), where sand is partly held by vegetation.
• Seif: like a barchan with only one wing, from shifting winds; the wing can grow very long and high.
• Longitudinal dunes: long, low ridges where sand is scarce and wind direction steady.
• Transverse dunes: perpendicular to the wind, where the sand source is a long feature at right angles to it.
• With plenty of sand, dunes merge; most dunes shift, though some are fixed near settlements.

Example (new): in the Thar desert, villagers plant grasses and shrubs on dunes near their fields; holding the sand with plants slows the dunes and helps stop them burying the crops.