Mediterranean climate

Continental Portugal has a clear contrast between the cool to warm (Csb) and hot (Csa) summers.

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 just "Mediterranean". Under the Köppen climate system, the first letter indicates the climate group (in this case temperate climates). Temperate climates or "C" zones 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 as a month within the high-sun months of April to September, in the case of the Northern Hemisphere and October to March, in the case of the Southern Hemisphere, and it also must contain exactly or less than one-third that of the wettest winter month. Some, however, use a 40 mm (1.6 in) level.[4][5] 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). There is a "c" with 3 or less months' average temperature above 10 °C (50 °F), but this climate is rare and is very isolated.

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.[6] Additional highland areas in the subtropics also meet Cs requirements, though they, too, are not normally associated with Mediterranean climates. The same goes for 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. Due to a lack of western-facing areas, Asia is the only continent without this climate.

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). Normally, climates that have eight or more months with a mean temperature over 10 °C (50 °F), are located in the low and subtropical latitude portions of the temperate zone (latitudes 25 to 35 north and south), and have mean temperatures around 7 °C (45 °F) in the coldest months, and warmer than 22 °C (72 °F) in the warmest months. In the Trewartha climate classification system, the cooler summer Csb zones in the Köppen system become Do or temperate oceanic climate.

Under the Holdridge life zones classification, the Mediterranean climates can be either temperate or subtropical climates. They are frequently found within the Warm Temperate region as defined by Leslie Holdridge with a mean annual biotemperature between 12 °C (54 °F) and the frost line or critical temperature line, 16 to 18 °C (61 to 64 °F) (depending on locations in the world[7] but often "simplified" as 17 °C (63 °F) (= 2^{(log₂12+0;5)} ≈ 16.97 °C (62.55 °F))[8]). Biotemperature is based on the growing season length and temperature. It is measured as the mean of all temperatures, with all temperatures below freezing and above 30 °C (86 °F) adjusted to 0 °C,[9] as plants are normally dormant at these temperatures. The frost line separates the warm temperate region from the subtropical region. It represents the dividing line between two major physiological groups of evolved plants. On the warmer side of the line, the majority of the plants are sensitive in low temperatures. They can be killed back by frosts as they have not evolved to withstand periods of cold. On the colder temperate side of the line, the total flora is adapted to survive periods of variable lengths of low temperatures, whether as seeds in the case of the annuals or as perennial plants which can withstand the cold. Only the warmest Mediterranean climates with a biotemperature between 16 °C (61 °F) to 18 °C (64 °F) and 24 °C (75 °F) are classified as subtropical climates in the Holdridge classification.

[Chile] has six, months of winter, no more, and in them, except when there is a quarter moon when it rains one or two days, all the other days have such beautiful suns...

— Pedro de Valdivia to Charles V, Holy Roman Emperor

During summer, regions of the 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 solar heating during the day from sunlight and rapid cooling at night.

In winter, the subtropical ridge migrates towards the equator and leaves the area, 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 4 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 due to higher evapotranspiration. 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 places such as the Eastern Mediterranean, or 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 basic pattern of the Mediterranean climates.

Aegean Sea of Ölüdeniz, Turkey

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, Los Angeles experiences mild to warm temperatures in the winter, with frost and snowfall almost unknown, whereas Tashkent has cold winters with annual frosts and snowfall seen in the winter; or, to consider summer, Seville experiences rather high temperatures in that season. 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 unlike dry-summer humid continental climates (although the daily diurnal range of temperature during the summer is large due to dry and clear conditions, except along the immediate coastlines). Temperatures during winter only occasionally fall below the freezing point and snow is generally seldom seen. Summer temperatures can be cool to very hot, depending on the distance from a large body of water, elevation, and latitude, among other factors. Strong winds from inland desert regions can sometimes boost summer temperatures up, quickly increasing the risk of wildfires. Notable exceptions to the usual proximity from bodies of water, thus featuring extremely high summer temperatures and cooler winters, include south-eastern Turkey and northern Iraq (Urfa, Erbil), surrounded by hot deserts to the south and mountains to the north. Those places routinely experience summer daily means of over 30 °C (86 °F), while receiving enough rainfall in winter not to fall into arid or semi-arid classifications.

As in every climatologic domain, the highland locations of the Mediterranean domain can present cooler temperatures in the summer and winter than the lowland areas, temperatures which can sometimes prohibit the growth of typical cold-sensitive Mediterranean plants. Some Spanish authors opt in to use the term Continental Mediterranean Climate for some regions with lower temperatures in winter than the coastal areas[10] (direct translation from Clima Mediterráneo Continentalizado), but most climate classifications (including Köppen's Cs zones) show no distinction as long as winter temperature means stay above freezing.

Additionally, the temperature and rainfall pattern for a Csa or even a Csb climate can exist as a microclimate in some high-altitude locations adjacent to a rare tropical As (tropical savanna climate with dry summers, typically in a rainshadow region, as in Hawaii). These have a favourable climate, with mild wet winters and fairly warm, dry summers.

The Ionian Sea, view from the island Lefkada, Greece

Makarska in Dalmatia, Croatia

The coastal Mediterranean region of Costa Brava, Catalonia, Spain

The Mediterranean forests, woodlands, and scrub biome is closely associated with Mediterranean climate zones, as are unique freshwater communities, though vegetation native to the Mediterranean climate zone can also be found in the approximate nearby climate zones, which usually tend to be the humid subtropical, oceanic and/or semi-arid zones, depending on the region and location. 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 components (e.g. competition and predation) controls become increasingly important as the flood 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.[11] Aquatic organisms in these regions show distinct long-term patterns in their structure and function,[3] and are also highly sensitive to the recent effects of climate change.[1][2]

  Hot-summer mediterranean climate (Csa)

This subtype of the Mediterranean climate (Csa) is the most common form of the Mediterranean climate, therefore it is also known as a "typical Mediterranean climate". As stated earlier, regions with this form of a Mediterranean climate experience average monthly temperatures in excess of 22.0 °C (71.6 °F) during its warmest month and an average in the coldest month between 18 and −3 °C (64 and 27 °F) or, in some applications, between 18 and 0 °C (64 and 32 °F). Also, at least four months must average above 10 °C (50 °F) to avoid becoming a cold mediterranean subtype. Regions with this form of the Mediterranean climate typically experience hot, sometimes very hot and dry summers and mild, wet winters. In a number of instances, summers here can closely resemble summers seen in arid and semi-arid climates and be close to the thresholds for them. However, high temperatures during summers are generally not quite as high as those in arid or semiarid climates due to the presence of a large body of water nearby. All areas with this subtype have wet mild winters. However, some areas with a hot Mediterranean subtype can actually experience very chilly winters, with occasional snowfall.

Csa climates are mainly found around the Mediterranean Sea, southwestern Australia, southwestern South Africa, sections of Central Asia, northern sections of Iran and Iraq, the interior of northern California west of the Sierra Nevada, along the Wasatch Front in Utah, and inland areas of southern Oregon west of the Cascade Mountains. Southern California's coasts also experience hot summers due to the shielding effect of the Channel Islands. However, unshielded areas of that coastline can have warm-summer Mediterranean climates with hot-summer areas just a few kilometres inland.

  Warm-summer mediterranean climate (Csb)

Occasionally also termed the "Cool-summer Mediterranean climate", this subtype of the Mediterranean climate (Csb) is less common and experience warm (but not hot) and dry summers, with no average monthly temperatures above 22 °C (72 °F) during its warmest month and as usual an average in the coldest month between 18 and −3 °C (64 and 27 °F) or, in some applications, between 18 and 0 °C (64 and 32 °F).

Also, at least four months must average above 10 °C (50 °F) once again.

Cool ocean currents, upwelling and higher latitudes are often the reason for this cooler type of Mediterranean climate. This is why it rarely occurs on the Mediterranean Sea shores, as it is a warm sea and is subtropical, from 3 to 6 °C above the theoric value according to Jean Demangeot.[15]

The other main reason for this cooler type is the altitude. For instance, Menton on the French coast has a Csa climate while Castellar, Alpes-Maritimes, the adjacent town just north of Menton, with an altitude between 100 and 1,382 m (328 and 4,534 ft), has a Csb climate instead.[16]

Winters are rainy and can be mild to chilly. In some instances, snow can fall on these areas.

Precipitation occurs often in the colder seasons, but there are a number of clear sunny days even during the wetter seasons.

Csb climates are found in northwestern Iberian Peninsula (namely Galicia and the Norte region and west coast of Portugal), in coastal California, in the Pacific Northwest (namely western Washington, western Oregon and southern portions of Vancouver Island in British Columbia),[17][18][19][20][21] in central Chile, in parts of southern Australia and in sections of southwestern South Africa. A few locations close to the south coast of England such as Weymouth and Portland just scrape into this climate classification due to very low rainfall in July. A trend towards slightly drier summers during the 1971-2000 climate average period, meant that this classification previously extended slightly further to include a few other weather stations in southern England, such as Bognor Regis and Teignmouth.[22] Rarer instances of this climate can be found in relatively small and isolated high altitude areas of the Andes in Northern Ecuador, Colombia, and Western Venezuela.

Distribution of the relatively rare cold-summer Mediterranean climate (Köppen type Csc) in Washington, Oregon and California

The cold-summer subtype of the Mediterranean climate (Csc) is rare and predominately found at scattered high-altitude locations along the west coasts of North and South America having a similar climate. This type is characterized by cool, dry summers, with less than four months with a mean temperature at or above 10 °C (50 °F), as well as with cool, wet winters, with no winter month having a mean temperature below 0 °C (32 °F) (or −3 °C [27 °F]), depending on the isotherm used). Regions with this climate are influenced by the dry-summer trend (though briefly) that extends considerably poleward along the west coast of the Americas, as well as the moderating influences of high altitude and relative proximity to the Pacific Ocean.

In North America, areas with Csc climate can be found in the Olympic, Cascade, Klamath, and Sierra Nevada ranges in Washington, Oregon and California. These locations are found at high altitude nearby lower altitude regions characterized by a warm-summer Mediterranean climate (Csb) or hot-summer Mediterranean climate (Csa). A rare instance of this climate occurs in the tropics, on Haleakalā Summit in Hawaii.

In South America, Csc regions can be found along the Andes in Chile and Argentina. The town of Balmaceda, Chile is one of the few towns confirmed to have this climate.

Small areas with a Csc climate can be found at high elevations in Corsica.[11]

In Norway, the small fishing village of Røst above the Arctic Circle has a climate bordering on Csc and is known as a "climatic anomaly" due to abnormally warm temperatures despite its latitude located above 67°N latitude.

Media related to Mediterranean climate at Wikimedia Commons

• Explanation of Mediterranean Climate (University of Wisconsin)