San Diego–Tijuana straddles a Mediterranean climate and Semi-arid climate area. The Mediterranean climate is characterized by generally warm, dry summers on the near coastal regions while with a slight temperature increase westward, and relatively cool, mild, wet winters; this is the climate that dominates northern San Diego–Tijuana. In southern San Diego–Tijuana, the semi-arid climate is observed, though characteristics of the Dry-Summer Subtropical Mediterranean climate are present, with most of the annual precipitation falling in the winter. The Peninsular Ranges assist in containing moisture to the coastal areas and create a rain shadow to the east as they are west-facing mountains.