Choropleth maps use color in order to illustrate the variable the map is designed to describe. In the case of classed choropleth maps, the colors are dependent on each other as values in a range. This is useful for explaining certain natural phenomena such as floods, pH levels in soils, and possible pollution in a series of local areas. The map displayed shows the annual average precipitation across the United States from the years 1961 to 1990. The numbers all relate to each other because more rainfall equates to a higher number, and thus, a different color altogether. The map shows that the northwest United States gets the most rain while the southwest gets very little in comparison.
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