Brentwood borders on the Mediterranean (Csa) and Semi-arid climates (Bsh). Like most of East Contra Costa County, Brentwood lies in the rain shadow of Mt. Diablo and receives less rainfall than many of its neighbors. The wet season is generally October through April, though there may be a day or two of light rainfall in June or September. Summer heat is often moderated by the Delta breeze, especially in the late afternoon, causing temperatures to cool rapidly.
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