Situated on west coasts between latitudes 30° and 45° is a zone subject to alternate wet and dry seasons because it is located in the transitional zone between the dry west coast tropical desert (on the equatorward side) and the wet west coast climate (on the poleward side).
In summer, when the oceanic subtropical high is most powerfully developed and farthest north, the same desert conditions that prevail permanently farther south take over control of the climate and bring a severe drought. However, the proximity of the ocean with its cool current keeps summer temperatures to a mild 60°F (16°C) average. In winter, the humid regime of middle-latitude cyclones and moist maritime polar (mP) air masses is felt in the ample precipitation.
The dry-summer subtropical climate is particularly extensive in the Mediterranean lands. Hence, the name Mediterranean climate is commonly used for this climate.
Koppen classifies the climate of the Mediterranean lands as Csa, a temperate rainy climate with dry, hot summers. Those narrow coastal belts directly bordering the cool currents of the open Atlantic, Pacific, and Indian oceans, Koppen designates as Csb, distinguished by a markedly cooler summer than the Csa climate. Thus Monterey, California, represents a Csb climate.
The annual temperature range here is 28.5°F (16°C), which is over twice that at Monterey. The summer is not rainless, but monthly averages are very low in June and July. Annual precipitation is double that of Monterey. In short, the equable influence of the cool west coast desert is replaced by a continental influence.
It is apparent that if one travels from the Mediterranean shore into North Africa, he will pass from the Mediterranean climate into the tropical steppe climate of the Sahara. Note that the temperature record resembles that of the tropical dry climates, although not so hot, but the rainfall distribution is distinctly of the same type as the Mediterranean fall, though not so copious.
The Mediterranean climate, its drought coinciding with the high temperatures of summer, incurs a large water deficiency in middle and late summer. Winter rains quickly restore the moisture deficiency and a surplus is developed by early spring.
The occurrence of a wet winter and a dry summer is unique among climate types and results in a distinctive natural vegetation of hard leafed evergreen trees and shrubs, known as sclerophyll forest. Various forms of sclerophyll woodland and scrub are also typical. Trees and shrubs must withstand the severe summer drought of two to four rainless months and intense evaporation.
Soils of the Mediterranean climate are not readily subject to simple classification. Reddish chestnut and reddish-brown soils typical of semi-arid climates are generally present. In the Mediterranean lands terra rossa, a red soil formed on limestone, occurs in various locations.
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