Mediterranean climate

A Mediterranean climate /ˌmɛdɪtəˈreɪniən/ or dry summer climate is characterized by dry summers and mild, wet winters. The climate receives its name from the Mediterranean Basin, where this climate type is most common. Mediterranean climate zones are typically located along the western sides of continents, between roughly 30 and 45 degrees north and south of the equator. The main cause of Mediterranean, or dry summer climate, is the subtropical ridge which extends northwards during the summer and migrates south during the winter due to increasing north-south temperature differences. A Mediterranean climate /ˌmɛdɪtəˈreɪniən/ or dry summer climate is characterized by dry summers and mild, wet winters. The climate receives its name from the Mediterranean Basin, where this climate type is most common. Mediterranean climate zones are typically located along the western sides of continents, between roughly 30 and 45 degrees north and south of the equator. The main cause of Mediterranean, or dry summer climate, is the subtropical ridge which extends northwards during the summer and migrates south during the winter due to increasing north-south temperature differences. The resulting vegetation of Mediterranean climates are the garrigue or maquis in the Mediterranean Basin, the chaparral in California, the fynbos in South Africa, the mallee in Australia, and the matorral in Chile. Areas with this climate are where the so-called 'Mediterranean trinity' of agricultural products have traditionally developed: wheat, vine and olive. Most large, historic cities of the Mediterranean basin lie within Mediterranean climatic zones, including Algiers, Athens, Beirut, İzmir, Jerusalem, Marseille, Naples, Rome, Tunis, and Valencia. Major cities with Mediterranean climates outside of the Mediterranean basin include Adelaide, Cape Town, Casablanca, Dushanbe, Los Angeles, Lisbon, Perth, San Francisco, Santiago and Victoria. Under the Köppen climate classification, 'hot dry-summer' climates (classified as Csa) and 'cool dry-summer' climates (classified as Csb) are often referred to as 'Mediterranean'. Under the Köppen climate system, the first letter indicates the climate group (in this case temperate climates). Temperate climates or 'C' zones have an average temperature above 0 °C (32 °F) (or −3 °C (27 °F)), but below 18 °C (64 °F), in their coolest months. The second letter indicates the precipitation pattern ('s' represents dry summers). Köppen has defined a dry summer month as a month with less than 30 mm (1.2 in) of precipitation and with less than one-third that of the wettest winter month. Some, however, use a 40 mm (1.6 in) level. The third letter indicates the degree of summer heat: 'a' represents an average temperature in the warmest month above 22 °C (72 °F), while 'b' indicates the average temperature in the warmest month below 22 °C (72 °F). Under the Köppen classification, dry-summer climates (Csa, Csb) usually occur on the western sides of continents. Csb zones in the Köppen system include areas normally not associated with Mediterranean climates but with Oceanic climates, such as much of the Pacific Northwest, much of southern Chile, parts of west-central Argentina, and parts of New Zealand. Additional highland areas in the subtropics also meet Cs requirements, though they, too, are not normally associated with Mediterranean climates, as do a number of oceanic islands such as Madeira, the Juan Fernández Islands, the western part of the Canary Islands, and the eastern part of the Azores. Under Trewartha's modified Köppen climate classification, the two major requirements for a Cs climate are revised. Under Trewartha's system, at least eight months must have average temperatures of 10 °C (50 °F) or higher (subtropical), and the average annual precipitation must not exceed 900 mm (35 in). Thus, under this system, many Csb zones in the Köppen system become Do (temperate oceanic), and the rare Csc zones become Eo (subpolar oceanic), with only the classic dry-summer to warm winter, low annual rainfall locations included in the Mediterranean type climate. During summer, regions of Mediterranean climate are strongly influenced by the subtropical ridge which keeps atmospheric conditions very dry with minimal cloud coverage. In some areas, such as coastal California, the cold current has a stabilizing effect on the surrounding air, further reducing the chances for rain, but often causing thick layers of marine fog that usually evaporates by mid-day. Similar to desert climates, in many Mediterranean climates there is a strong diurnal character to daily temperatures in the warm summer months due to strong heating during the day from sunlight and rapid cooling at night. In winter, the subtropical ridge migrates towards the equator, making rainfall much more likely. As a result, areas with this climate receive almost all of their precipitation during their winter and spring seasons, and may go anywhere from 3 to 6 months during the summer and early fall without having any significant precipitation. In the lower latitudes, precipitation usually decreases in both the winter and summer. Toward the polar latitudes, total moisture usually increases, for instance, the Mediterranean climate in Southern Europe has more rain. The rainfall also tends to be more evenly distributed throughout the year in Southern Europe, while in the Eastern Mediterranean, and in Southern California, the summer is nearly or completely dry. In places where evapotranspiration is higher, steppe climates tend to prevail, but still follow the weather pattern of the Mediterranean climate. The majority of the regions with Mediterranean climates have relatively mild winters and very warm summers. However winter and summer temperatures can vary greatly between different regions with a Mediterranean climate. For instance, in the case of winters, Barcelona and Los Angeles experience mild temperatures in the winter, with frost and snowfall almost unknown, whereas Tashkent has colder winters with annual frosts and snowfall. Or to consider summer, Athens experiences rather high temperatures in that season (48 °C (118 °F) has been measured in nearby Eleusis). In contrast, San Francisco has cool summers with daily highs around 21 °C (70 °F) due to the continuous upwelling of cold subsurface waters along the coast.Because most 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 fall below the freezing point and snow is generally seldom seen. In the summer, the temperatures range from mild to very hot, depending on distance from a large body of water, elevation, and latitude. Even in the warmest locations with a Mediterranean-type climate, however, temperatures usually do not reach the highest readings found in adjacent desert regions because of cooling from water bodies, although strong winds from inland desert regions can sometimes boost summer temperatures, quickly increasing the risk of wildfires.