Köppen climate classification
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.