The Rocky Mountains run parallel to the western coast of the United States. While parts of the California coast are lush and tropical, the area just east of the mountain range is a dry desert. What BEST explains the drastic climate difference between these two adjacent areas?

The mountains cause rainfall.

Explanation:

The fact that the mountains run parallel to the coast is the key in the difference in climate between the two areas. Since they are parallel to the sea, they block the wind and moisture coming from the sea towards the land, the mountains cause this air to rise and when it hits the cold air at higher altitudes, it results in rain. This leads to the lush and tropical areas whereas without the parallel mountains to the coast, the amount of rain the area receives decreases, resulting in a desert like environment.

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