The weather has a huge impact on human settlement patterns. The amount of precipitation an area receives greatly impacts the size of the population that can be supported. Compare Figures 4.6.1 and 4.6.2 above. Look at the population density map. Find the areas that have a population of less than four people per square mile. Now compare that information to the precipitation map. Do the areas with a low population density occur in areas of high precipitation or low precipitation? Now find the areas that have over one hundred people per square mile. Compare these areas to the precipitation map. Do most areas with a high population density occur in areas of low precipitation or high precipitation? What conclusions can you draw by comparing the two maps?

You may have noticed that some areas of high population density occur in dry climates. For example, compare the population of Southern California and the precipitation of Southern California. As you can see, the precipitation level is very low and the population density is very high. What do you think this can be attributed to? Remember in the last section of this lesson how we examined climate types. Do you remember what climate type is found in Southern California? It is the Mediterranean climate. This means that the temperature in Southern California does not change much—the winters are never very cold. This might explain why a lot of people want to live in California, but what about water? How does such a dry area get enough water to support such a large population? The answer lies in the mountain ranges. The mountains are very important to the dry midlatitude climates because they work as water storage areas. During the winter months, the mountains collect snow. When spring comes, the snow slowly begins to melt. Throughout the summer the snow melts, providing water to the areas below. In order to make the most of the snowfall and collect as much water as possible, elaborate reservoir systems have been constructed in the western mountains of the United States. Reservoirs, or man-made lakes, collect mountain snow as it melts; the water is then stored in the mountain lakes until it is needed during the hot summer months.