Mediterranean climate
From Wikipedia, the free encyclopedia.
A Mediterranean climate is one that resembles those of the lands bordering the Mediterranean Sea. These climates occur on the western coasts of continental landmasses, roughly between the latitudes of 30° and 45° north and south of the equator. Examples include California, Western Australia, Cape Province in South Africa, central Chile, and the Mediterranean Basin itself.
The northernmost Mediterranean climate may exist in a small portion of Vancouver Island, Canada, roughly in the vicinity of Victoria, British Columbia.
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Causes
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.
This summer-dry, winter-wet rainfall pattern is extremely rare, and is only found in a small number of regions on Earth. 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.
Precipitation
During summer, regions of Mediterranean climate are dominated by the subtropical high pressure, and are thus equivalent to deserts from the point of view of air mass positions, while during winter the polar front moves equatorward into regions of Mediterranean climate giving weather conditions similar to an oceanic climate. As a result, areas with this climate receive almost all their rain during the winter months, and may go anywhere from 2-5 months during the summer without having any significant precipitation.
As an example, San Francisco in California, USA, has an average of 448 mm (17.6 in.) of rain from November through April each year, but averages only 52 mm (2 in.) of rain for the rest of the year.
Temperatures east and south
All regions with Mediterranean climates have relatively mild winters, but summer temperatures are variable depending on the region. For instance, Athens, Greece experiences rather high temperatures in the summer, while San Francisco, California has cool, mild summers due to its proximity to the open Pacific Ocean. Because all regions with a Mediterranean climate are in close 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 range from mild to very warm, depending on distance from the open ocean, elevation, and latitude. Even in the warmest locations with a Mediterranean-type climate, however, temperatures don't reach the high levels of desert regions.
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 40 °C or 100 °F), although winters are rainy enough to allow lusher vegetation than is typical in deserts. Unlike the coastal climates that are designated Csb in the Köppen climate classification - characteristic of places with cooler summers - the hotter, typically inland areas have the Csa classification that indicates a hot summer. Areas that experience the typical Mediterranean pattern of cool, rainy winters and very dry summers, but which experience milder average summer temperatures include, Porto, in Portugal and San Francisco, in California.
Areas of high altitude adjacent to locations with Mediterranean climates may have the cold winters that are characteristic of a continental climate; under Köppen's scheme such places might earn the designation Dsa, Dsb,Dsf or even Dsc.
