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Most places on Earth receive more than 25 cm of annual precipitation, with coastal mountains exposed to moist winds receiving at least 10 times that amount. However, there is considerable variation between different latitudes and between coastal or continental locations.

In temperate regions, where depressions are the major source, rainfall is usually spread throughout the year, with a modest increase in winter (when the low‐pressure systems are deeper and more frequent).

At the equator, rainfall is also found throughout the year, though the maximum coincides with the time of strongest solar heating and convective uplift. This typically results in thunderstorm activity during the afternoon.

Between these two wet zones, the rainfall is more seasonal and unreliable. Many of the driest places occur at 20–30° latitude, where dry, sinking air results in a belt of hot deserts (see Zonal Winds). Other deserts occur in the center of Asia, in rain shadow regions which are cut off from moist, maritime air by high mountains.¹¹

Deserts are characterized by cloudless skies, high daytime temperatures, low nighttime temperatures, and an annual precipitation of less than 25 cm. Some places may experience several years with no rain at all. This inevitably results in an absence of vegetation. When rain does fall, it usually comes in the form of short‐lived, but torrential storms, which often lead to flash floods. However, any surface water soon sinks into the sand or is evaporated.

Although major rivers, such as the Nile and Colorado, carry sufficient water from their mountain sources to reach the sea, most desert river courses remain dry throughout the year, filling only occasionally when torrential downpours cause flash floods.

Under extreme day–night temperatures, rock minerals expand and contract. This process may also be aided by occasional wetting from rainfall or groundwater. As time goes by, the rock disintegrates into particles of sand or gravel. (90% of the Sahara is made up of rocky or stony areas, with only 10% covered by the more familiar sand – derived from the breakdown of sandstone rock.)

Figure 3.29 Earth's water cycle. Liquid water on the surface is warmed and evaporates, turning to water vapor. When this gas rises, it cools and condenses, turning back into cloud droplets of liquid water. These droplets collide, growing in size until they are heavy enough to fall towards the surface in the form of precipitation (rain, sleet, snow, etc.). Rain seeps into the ground or flows over the surface, feeding streams and rivers which flow downhill into lakes or the ocean. Melting snow and ice also feed the surface drainage systems.

(NASA‐GSFC)

With no surface moisture and no vegetation to slow the wind or bind the loose material together, wind becomes the main agent of erosion and transportation. The smallest dust particles (less than 0.2 mm) are carried in suspension in the air, while larger sand grains more commonly bounce across the surface, displacing other particles – a process known as saltation.

These grains, often traveling at high speed, act like sandpaper when they collide with bare rock surfaces. Wind erosion shapes a wide variety of strange rock formations, such as rock pedestals which have been eroded at their base to leave narrow necks, and yardangs – elongated ridges up to 8 m high, separated by troughs where less resistant material has been removed.

The loose material is always shifting location, though the finest dust travels the furthest. Sometimes the grains are removed by the wind, leaving deflation hollows. If these fill with temporary ponds or lakes, which then evaporate, layers of salt may be left behind. The most famous example is the Bonneville salt flats in Utah, USA, the remains of an ancient lake that formed during the last ice age and subsequently evaporated. The salts on the surface contain potassium, magnesium, lithium, and sodium chloride (common table salt).

Moving sand piles up when it meets an obstacle, forming dunes. These may take different shapes, depending on the variability of the wind. Barchans are crescent‐shaped dunes with one steep side and one more gently sloping side. They form when the wind blows from one dominant direction. The sand moves up the gentler slope until it reaches the crest, then falls down the lee side. This process also drives the entire dune forward. Barchans can be up to 30 m high and 400 m long.

Longitudinal or linear dunes lie parallel to the prevailing wind (Figure 3.31). They form symmetrical ridges up to 100 m high, 600 m wide, and 80 km long. They generally form on the lee side of an obstacle where sand is abundant and the wind is constant and strong. At least some of these are derived from barchans which have been modified by cross winds. Transverse dunes lie at right angles to the prevailing wind direction. They have a gently sloping windward side and a steeply sloping leeward side.

Figure 3.30 This false color Landsat image shows the Grand Canyon of northern Arizona which has been created by the Colorado river after millions of years of erosion. In this westward‐looking image, the river cuts through the salmon‐colored rock of the Colorado Plateau. The Little Colorado river enters from the east (bottom) across the Painted Desert. Nearby uplands are covered in green vegetation. The fairly rapid downward erosion by the river, combined with the lack of rainfall in this desert region, has preserved the canyon's remarkably steep sides.

(NASA‐GSFC)

Figure 3.31 Linear dunes of the Namib Sand Sea imaged by an astronaut aboard the International Space Station. The highest dunes show smaller linear dunes along their crests. Linear dunes are generally aligned parallel to the formative wind – in this case, strong winds from the south. This simple pattern is disrupted near the Tsondab river valley (top left), which acts as a funnel for winds from the east. These less frequent but strong winter winds are channeled down the valley and usually carry large amounts of sand. These easterly winds significantly deflect all the linear dunes near the valley so that they point downwind (center). Further inland (right), the north‐pointing and west‐pointing patterns appear superimposed, making a rectangular pattern.

(NASA‐JSC)

Figure 3.32 A satellite image of a dust storm sweeping from the Algerian desert over the Canary Islands. Such storms may carry dust and aerosols all the way across the Atlantic Ocean to the Caribbean and Florida, blocking sunlight and affecting surface temperatures.

(SeaWiFS Project, NASA‐GSFC, and ORBIMAGE)

Huge amounts of loose material can be removed from deserts or dry regions where there is little vegetation. Extensive deposits of fertile, windblown sediment are found in a broad zone from western Europe to China. This material, known as loess, was moved during the cold, dry periods of the recent ice age. On a smaller scale, dust from the Mongolian desert often blankets the Chinese city of Beijing, and Saharan dust frequently moves across Europe, with occasional excursions as far as North America (Figure 3.32).