1. Deserts are found in subtropical areas because as a mass of warm air rises over the equator and flows towards the poles, it converges with other air masses moving towards the poles. This convergence of upper level air forces air to sink at about 30 degrees north and south of the equator. Descending air compresses, heats-up, and causes evaporation.
Deserts occur in Nevada because the Sierra Nevada and Cascade Ranges act as a rain shadow. Air moving over these mountains descends, compresses, heats and causes evaporation as it enters Nevada.
Mongolia is so far from the ocean, storms will likely have dropped their moisture before reaching there.
Coastal Peru and Chile is dry because there is a cold ocean current offshore. Winds will generally warm as they blow from the cold current to the warm land. Thus precipitation is unlikely.
2. In humid environments the trend is toward chemical weathering, well developed soils, rounded topography, and perennial streams with base level at sea level. In arid environments, the trend is toward mechanical weathering, thin, poorly developed soils, angular topography and intermittent streams with local base levels rising as basins fill in with sediments.
3. The most important agent of erosion in deserts is water.
4. Water is highly effective in eroding deserts because thin desert soils on steep slopes are poorly anchored down by sparse vegetation. These soils quickly saturate during thunderstorms - becoming mudflows.
5. The Colorado Plateau is underlain by near-horizontal sedimentary rocks ("layer-cake geology").
When streams down cut through the plateau they leave behind progressively smaller, flat-topped, elevated platforms (plateaus, mesas, buttes) as the stream canyons widen. In the arid climate, rounding of the landscape does not occur.
6. The Basin and Range is characterized by a system of near parallel normal faults.
Streams flowing out of the ranges encounter an abrupt flattening of gradient as they cross over these faults onto the near level basin on the down thrown side of the fault. The gradient change slows the streams down, causing them to drop most of their load where the basin and the range meet. This deposition forms alluvial fans which may coalesce to form a bajada. Finer particles and the streams’ dissolved load will generally precipitate in the middles of basins as playas.
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