Rising air promotes rainfall; descending air promotes dryness. The rising air causes precipitation on top of the mountain. However, the side protected from the wind in a depression makes this side a repository for cooler and dryer air. Why are nights in arid lands surprisingly cold? Hint: Think of a feature of the atmosphere that holds daytime heat during the night.

Deserts have very few clouds thanks to their lack of precipitation. Clouds trap heat during the night in most areas but not in deserts. Combined with the lack of sunlight, this causes a precipitous drop in temperature.

Notice in Figure 15.14 that some of the dunes are not perfectly symmetrical like that shown in Figure 15.13. (A) Describe this asymmetry, and (B) try to explain it. Hint: Study the caption to Figure 15.13A.

In 15.14, the rain has turned the bedrock to sand and the wind from varying directions has blown the sand into asymmetrical dues. In 15.13, the wind has come from a constant and steady direction.

15.18. Which is the steeper side of this dune -- the northwest side or the southeast side? Hint: You must answer Q15.17 and use the map on page 278 to answer this question.

Southeast.

15.19. Which kind of dune illustrated in Figure...

From which direction did the wind that fashioned the dunes of the Lakeside region blow (NE, NW, SE, or SW)?
Northwest.

Lab Summary

This lab explained why deserts occur in terms of the principles of warm vs. cold air (warm air tends to rise because it is less dense, versus cold air which sinks) and wind direction. After learning about the facts behind weather-generation, it then asked me to apply these ideas to explain various phenomena, such as the fact that deserts are so cold at night and why sand dunes form in various ways. This progression enabled me to better understand the principles of the lab.

What I found most challenging was visualizing the different ways dunes were formed. There are many counterintuitive aspects regarding how winds shape dunes and the different types and shapes of dunes were new concepts to me. Overall, I found the lab extremely interesting because I knew very little about deserts. Many questions which I had about this climate type, such as why deserts are so cold during the night, were finally answered.

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