1. The desert and steppe regions of the Earth make up about 30 percent of the Earth’s land surface. They cover about 42 million square kilometers of the world.
2. The primary cause of subtropical deserts is the prevailing global distribution of air pressure and winds. Coinciding with the dry regions in the lower latitudes is the subtropical highs which is a zone of high air pressure. These pressure systems are characterized by subsiding air currents that compress and warm the air having conditions which are the opposite for cloud forming and precipitation. The primary cause of middle-latitude deserts is their positions in the deep interior of large land masses far removed from the oceans and the presence of high mountains across the path of prevailing winds act to separate the area from water-bearing air masses. The deserts are located on the leeward sides of the mountains (rainshadow deserts).
3. Middle-latitude deserts are most common in the Northern Hemisphere.
4. The amount of precipitation that is used to determine whether a place has a dry climate or a humid climate are a variable figure because a dry climate is when the evapotranspiration is greater than the precipitation, not just having a low precipitation.
5. Four common misconceptions about deserts are that wind erosion is the most important erosional agent sculpturing deserts, deserts are hot, lifeless, and sand covered. Running water is most responsible for the erosional work in deserts, deserts can be cold and contain life, and most deserts are not sand covered.
6. Rock weathering is reduced in deserts because of the lack of moisture and the scarcity of organic acids from decaying plants.
7. As a permanent stream such as the Nile River crosses a desert discharge decreases. This compares to a humid region by the discharge increases due to tributaries and ground water.
8. The most important erosional agent in deserts is running water.
9. Wind transports sand by deflation or the lifting and removal of loose materials. Sand can be carried one meter above the surface.
10. Wind erosion is relatively more important in arid regions than in humid regions because in humid regions moisture binds particles together and vegetation anchors the soil so that wind erosion is negligible.
11. The factor that limits the depth of blowouts is vegetation cover.
12. Sand dunes migrate by the continued sand accumulation coupled with periodic slides down the slip face resulting in the slow migration in the direction of air movement.
13. Three factors that influence the form and size f a sand dune are wind direction and velocity, availability of sand, and the amount of vegetation present.
14. a) Dunes whose tips point into the wind are parabolic dunes. b) Long sand ridges oriented at right angles t the wind are transverse dunes. c) Dunes that often form along coasts where strong winds create a blowout are parabolic dunes. d) Solitary dunes whose tips point downwind are barchan dunes. e) Long sand ridges that are oriented more or less parallel to the prevailing winds are longitudinal dunes. f) An isolated dune consisting of three or four sharp-crested ridges diverging from a central high point are star dunes. g) Scalloped rows of sand oriented at right angles to the wind are barchanoid dunes.
15. Although sand dunes are the best known wind deposits, accumulations of loess are very significant in some parts of the world. Loess is windblown silt. Such deposits are found in western and northern China. Loess gives the Yellow River its name. The source of China’s loess is the extensive desert basins of central Asia. Also, in the United States loess is found in South Dakota, Nebraska, Iowa, Missouri, Illinois, and the Columbian Plateau. Two primary sources of loess are deserts and glacial outwash deposits.
16. Sea level is not a significant factor influencing erosion in desert regions because the deserts do not contain many rivers that actually reach the sea level.
17. The stages of a mountainous desert in the United States are found in the Basin and Range region. During and following uplift of the mountains, running water begins carving the elevated mass and depositing large quantities of debris in the basin. During the early stage, relief is the greatest and since erosion lowers the mountains and sediments fill in the basin, elevation decreases. During the middle stage, runoff from the sporadic rains spread over the gentler slopes at the base of the mountain quickly losing velocity. Its load is dumped forming an alluvial fan at the mouth of a canyon. Playa lakes are formed by the rare occasion of abundant rainfall causing streams t flow across alluvial fans to the center of the basin forming a shallow lake. During the late stage, the mountain front is worn back and a sloping bedrock platform called a pediment is formed. The mountains are reduced to a few large bedrock knobs projecting above the surrounding pediment and sediment-filled basins called inselbergs are formed.
18. The term that refers to the process by which desert like conditions expand into areas that were previously productive is desertificatioon. This is not strictly a natural process because both are dual natural processes and rapid alternation by human activities causes desertification. Some human processes include overgrazing of livestock herds.