It is true that large mountains can create a rain shadow. Moist air originating from over the ocean is blown inland by the wind, it rises up a mountainside, cooling as it rises, causing it to condense and fall as precipitation. In California, the most obvious example of this is in the Sierra Nevada. The western side of the Sierra Nevada experiences a greater amount of rainfall than does the eastern side because much of the moisture in air that reaches the Sierra Nevada condenses before it ever makes it to the east side of the mountains. I don't know anything about the specifics of Agoura Hills rain and how it compares to rainfall in Malibu, but my guess is that there isn't a very big difference in the amounts of rainfall. The mountains between these two cities aren't very big and couldn't likely cause a rain shadow in Agoura Hills. However, more important than the size of the mountains is the direction of wind. Malibu is directly south of Agoura Hills, so if it were possible for the mountains between these two cities to cause a rain shadow over Agoura Hills, then the moisture-laden air must be moving due north from Malibu toward Agoura. I don't think this is the case. In fact, just from what I remember from articles on the recent fires in Malibu, I'm pretty sure Malibu gets some pretty strong north-to-south winds, which would indicate that rain in Agoura does not originate in Malibu, possibly negating the rain shadow theory for that area and making a comparison between the cities' rainfall is difficult.
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