From Academic Kids
In most places with Mediterranean climates, rainfall results almost entirely from the procession of the westerly zone of frontal storms during the low-sun season; during the summer, the subtropical high pressure zone dominates the seasonal weather, preventing rainfall by inhibiting rainstorms from forming.
While examples of places with wet winters and dry summers exist in every thermal-based climate zone on Earth from the equatorial to the polar, the greatest concentration of Mediterranean climates is encountered immediately poleward of the great dry belt of subtropical deserts but equatorward of the zone of maritime temperate or oceanic climates. They are never on the east coasts of continents.
Areas with this climate receive almost all their rain during the winter months, and may go 4-5 months during the summer without having any precipitation. This is due to the shifting of the wind belts following the apparent movement of the sun.
Because of the proximity to large bodies of water, temperatures are generally moderate with a comparatively small range of temperatures between the winter low and summer high, although the daily range of temperatures during the summer is large, except along the immediate coasts. Temperatures during winter rarely reach freezing (except in areas with a high elevation), and snow is almost unheard of. In the summer, the temperatures, while warm, don't reach the high levels of inland desert areas. The average temperature for most lowland locations having this climate is about 10°C (50°F) in winter and roughly 27°C (80°F) in summer. The northernmost Mediterranean climate is in the vicinity of Victoria, British Columbia; temperatures there average about 4°C (39°F) in winter and 16°C (61°F) in summer.
Inland locations sheltered from or distant from sea breezes can experience severe heat during the summer. Locations inside the Sacramento Valley of northern California, for example, are subject to summer temperatures characteristic of hot deserts (often around 38?C or 100?F), although winters are rainy enough to allow lush vegetation. Unlike the coastal climates that have K?n's Csb classification characteristic of places with cooler summers, the hotter, typically inland areas have the Csa classification that indicates a hot summer.
Areas of high altitude adjacent to locations with Mediterranean climates may have the cold winters that are characteristic of a continental climate; under K?n's scheme such places might earn the designation Dsa, Dsb or even Dsc.
- Explanation of Mediterranean Climate (http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/climate_systems/mediterranean.html)