A Mediterranean climate is one that resembles the climate of the lands in the Mediterranean Basin, which includes over half of the area with this climate type world-wide. In addition to the areas surrounding the Mediterranean Sea, this climate type prevails in much of California, in parts of Western and South Australia, in southwestern South Africa and in parts of central Chile.
The climate is characterized by warm to hot, dry summers and cool, wet winters. For example, the city of Perth, Australia, in the southern hemisphere winter months of June-August, experiences 450 mm (17.7 inches) of rainfall and an average daily minimum of 8°C (46°F). Meanwhile during the summer months of December to February the city only averages 32mm (1.3 inches).
Mediterranean climate zones are associated with the five large subtropical high pressure cells of the oceans, the Azores High, South Atlantic High, North Pacific High, South Pacific High, and Indian Ocean High. These high pressure cells shift polarward in the summer and equatorward in the winter, playing a major role in the formation of the world's tropical deserts and the zones of Mediterranean climate polarward of the deserts. For example, the Azores High is associated with the Sahara Desert and the Mediterranean Basin's climate. The South Atlantic High is similarly associated with the Namib Desert and the Mediterranean climate of the Western part of South Africa. The North Pacific High is related to the Sonoran Desert and California's climate, while the South Pacific High is related to the Atacama Desert and central Chile's climate, and the Indian Ocean High is related to the deserts of western Australia (Great Sandy Desert, Great Victoria Desert, and Gibson Desert) and the Mediterranean climate of southwest and south-central Australia.
The Mediterranean forests, woodlands, and scrub biome is closely associated with Mediterranean climate zones. Particularly distinctive of the climate are sclerophyll shrublands, called maquis in the Mediterranean Basin, chaparral in California, matorral in Chile, fynbos in South Africa, and mallee and kwongan shrublands in Australia. Aquatic communities in Mediterranean climate regions are adapted to a yearly cycle in which abiotic (environmental) controls of stream populations and community structure dominate during floods, biotic (e.g. competition and predation) controls become increasingly important as the discharge declines, and environmental controls regain dominance as environmental conditions become very harsh (i.e. hot and dry); as a result, these communities are well suited to recover from droughts, floods, and fires.
During summer, regions of Mediterranean climate (also known as Dry-Summer Subtropical for the Csa areas) are dominated by subtropical high pressure cells, with dry sinking air capping a surface marine layer of varying humidity but making rainfall impossible or unlikely but for the odd thunderstorm, while during winter the polar jet stream and associated periodic storms reach into the lower latitudes of the Mediterranean zones, bringing rain, with snow at higher elevations. As a result, areas with this climate receive almost all of their yearly rainfall during the winter season, and may go anywhere from 4-6 months during the summer without having any significant precipitation.
Toward the equatorial end, winter precipitation increases. Toward the polar end, total moisture usually increases; in Europe there is more summer rain further north while along the American west coast the winters become more intensely wet and the dry seasons shorter as one moves north.
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 (48.0 °C has been measured in nearby Eleusina), whereas San Francisco has cool, mild summers due to the upwelling of cold subsurface waters along the coast. Because all regions with a Mediterranean climate are near 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 temperature during the summer is large due to dry and clear conditions, except along the immediate coasts). Temperatures during winter only occasionally reach freezing and snow occurs only rarely at sea level, but often in surrounding mountains due to wet conditions. 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 usually don't reach the highest readings found in adjacent desert regions due to cooling from water bodies, although strong winds from inland desert regions can sometimes boost summer temperatures, quickly resulting in a much-increased forest fire risk.
Inland locations sheltered from or distant from sea breezes can experience severe heat during the summer. Locations in the Northern half of the Sacramento Valley of Northern California, for example, are subject to summer temperatures characteristic of hot desert at times because of high temperture and very low humidity(often around 40°C/104F), although winters are very rainy and foggy enough to allow lushes vegetation than is typical in deserts but the vegetation because a fire risk at times because of dry summers. The central valley of California is not always very hot because of some ocean fluence known as the "delta breeze" which cools temperture during warm summer days. At times the "delta breeze" is strong enough to bring some coastal fog to the valley which brings cooler weather and higher humidity. Unlike the coastal climates that are designated Csb in the Köppen climate classification - characteristic of places with cooler summers - the hotter, typically inland areas are classified as Csa, which indicates a hot summer. Porto, Portugal, experiences the typical Mediterranean pattern of cool, rainy winters and dry summers, but has relatively mild average summer temperatures.
On another note, locations that are slightly higher latitude or elevation and are cut off from milder ocean winds may have somewhat colder winters and more distinct seasons with occasional snow. This "temperate Mediterranean" climate is most noticeable in the Rogue and Umpqua Basins of southwestern Oregon, central Spain, southeastern France away from the immediate coastline, northern Italy, and northern Greece. In these areas some plants (such as citrus) that are commonly associated with milder Mediterranean climates will freeze to death in a severe winter and are thus not part of the regular landscape.
Areas of high altitude adjacent to locations with Mediterranean climates, such as the "Mesetas" or plateaus of central Spain, may have the cold winters that are characteristic of a continental climate (see Continental Mediterranean climate); under Köppen's scheme such places might earn the designation Dsa (at lower latitudes above Csa), Dsb (either at high elevations in the lower latitudes or at lower elevations in the mid-latitudes above Csb) or even Dsc (just below the tree line). An example of a very humid Mediterranean Snow climate Dfsc is the highest summit on Orjen, Zubacki kabao in the subadriatic Dinaric Alps in Montenegro.
The natural vegetation of Mediterranean lands has to survive long, hot periods of summer droughts and prolonging wet periods. Mediterranean vegetation includes the following:
Most natural vegetation in Mediterranean areas has long since been cleared for agriculture. In places such as the Sacramento Valley in California, irrigation has led to intensive farming but some still in rural areas.