Another way that the lithosphere affects climate is its impact on precipitation levels. The shape of the lithosphere has a big influence on which areas receive precipitation. Look at Figure 4.3.2; the map shows precipitation levels in the US. Use the legend to find the areas that receive the most rain. On the east side of the country, notice how far inland the precipitation levels remain relatively high. Now look at the western side of the country. The northwest coast receives a significant amount of rain, but the pattern does not extend very far inland. What do you think explains this? One large factor is the mountainous nature of the western US.

Deserts are often caused by the orographic effect, which is the cooling effect that happens when air is forced to rise so that it can go over a mountain. As the air rises, the water vapor condenses and precipitation occurs. This means all of the water gets dumped on one side of the mountain, creating a humid environment, and by the time the air reaches the other side of the mountain it no longer has any water vapor left in it. This creates a desert.

Look at Figure 4.3.2 again. Notice how there is a dividing line down the continent where the dry desert air meets the humid air coming up from the Gulf of Mexico. The terrain across the central plains slopes downward from the Rocky Mountains towards the east. This slope makes a nice corridor for the dry air coming off the mountains to flow east. To the south, the Gulf of Mexico produces moist air that flows north. When the dry and moist air meet, a boundary line forms. It is along this line that a perfect environment sometimes exists for the creation of tornadoes. The area is often called Tornado Alley. There are more tornados in the US than anywhere else in the world because it has the right combination of terrain, water, and wind currents.