glg101r2_week7_lab_reading1

glg101r2_week7_lab_reading1 - Deserts 265 15 Deserts Topics...

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Deserts 265 #1 Air over an ocean acquires moisture through evaporation. Moves inland, replacing rising land air. #2 Warm land adds heat to air. #4 Rising air is cooled (because of decreasing atmospheric pressure). Moisture condenses, forming clouds and rain. #3b Cold air is more dense, so it sinks. #3a Warm air is less dense, so it rises. 15 Deserts Topics A. What are the principles that apply to the atmosphere as concerns the transfer of moisture? B. What are Hadley and Ferrel cells? How do they explain the occurrence of subtropical deserts? C. What are the explanations for deserts ascribed to rain shadows and cold-water coasts? What are two examples of how animals use principles of atmosphere dynamics? D. What is the practical definition of aridity? What is the definition of ‘evaporation index,’ and what EI value characterizes the Great American Desert? What are some of the erosional and depositional landforms that characterize deserts? What are four kinds of sand dunes, and how does each kind record the direction of wind that shaped it? E. What geologic history is recorded by features evident on the Lakeside, Nebraska quadrangle? F. What is the mechanism for the development of desert minerals? What are some of the elements and minerals that form in desert environments? How are salt domes formed? Why does salt survive as Earth’s surface in places such as Iran? A. The what and where of deserts—but first, a few principles. Figure 15.1 This simple model illustrates principles #1 through #4. The locations of deserts are largely controlled by atmospheric circulation. Why? Because the agent that moves moisture from its principal source—the world ocean—onto the land, is wind. The capacity of air to obtain moisture from the ocean through evaporation , along with the effectiveness of air to deposit that moisture on land through condensation , involves the following four principles, which are illustrated in Figure 15.1. #1: Air obtains moisture from the ocean through evaporation of sea water. #2: Land absorbs solar energy more readily than does sea water, so air over a landmass is warmed to a greater degree than that over an adjacent ocean. #3: Warm air is less dense than cold air, so warm air tends to rise, and cold tends to sink. #4: Warm air has a greater moisture- holding capacity than does cold air, so as warm air is cooled it tends to lose moisture (as rain or snow). Q 15.1 Where moisture-laden air moves from sea to land, which is almost everywhere the case, (a) it is heated by the warmer land, (b) made less dense, and (c) caused to rise. At higher elevations moisture within the air condenses, producing clouds and rain. Review: In one sentence, why the condensation?
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266 Deserts B. Global Subtropical deserts. From the principles discussed and illus- trated on the previous page, it follows that rising air promotes rainfall, and descending air promotes dryness. That simple principle pretty much explains the geography of deserts.
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