Mediterranean Climate Regions

The Mediterranean climate (Köppen Csa and Csb) is defined by a distinctive seasonal inversion rare among the world's climate types: rainfall concentrated in the mild winter half of the year and a pronounced dry season in summer, when at least three times as much precipitation falls in the wettest winter month as in the driest summer month. This pattern arises because the subtropical high-pressure belt, centered over the oceans at roughly 30 degrees latitude, migrates poleward in summer, extending its dry, subsiding air over these coasts and suppressing rainfall, then retreats equatorward in winter, allowing the mid-latitude westerlies and their associated frontal storm systems to bring rain. Csa climates have hot summers (warmest month above 22°C), typical of the Mediterranean Basin's interior and much of California's Central Valley, while Csb climates have warmer but not hot summers, as in coastal San Francisco or much of southwestern Australia, moderated by cool, upwelling-fed ocean currents just offshore.