An oceanic climate (also called marine west coast climate and maritime climate) is the climate typically found along the west coasts at the middle latitudes of all the world's continents, and in southeastern Australia. Oceanic climates are characterized by a narrower annual range of temperatures than are encountered in other places at a comparable latitude, and do not have the extremely dry summers of Mediterranean climates.
Similar climates, at least in thermal range, are also found in tropical highlands even at considerable distance from any coastline. Generally, they fall into Köppen climate classificationCfb or Cwb. The narrow range of temperatures results not from proximity to a coastline but instead to the slight thermal range of temperatures between seasons characteristic of tropical lowlands; altitudes are high enough that such places have at least one month cooler than 18 °C and do not qualify for grouping in the true tropical climates. Unlike the norm in true oceanic climates, these moist highland tropical climates may have a marked winter drought, as in Mexico City. As with oceanic climates, winters are relatively warm and summers are comparatively cool, so the agricultural potential in both oceanic climates and moist tropical highland climates is practically identical.
Precipitation
Precipitation is both adequate and reliable at all times of the year in oceanic climates, except in certain tropical highland areas, which would have tropical savanna or steppe climates (with a dry season in winter) if not for the high altitude making them cooler (Koppen Cwb). The Pacific Northwest and south-central Chile is often considered as having an Oceanic climate, although the dry summers in parts of this area actually fit the Mediterranean climate (Koppen "Csb").
Temperature
Overall temperature characteristics vary among oceanic climates; those at the lowest latitudes are subtropical from a thermal standpoint, but more commonly a mesothermal regime prevails, with cool, but not cold, winters and warm, but not hot, summers. Summers are also generally much cooler than in areas with a humid subtropical climate. Average temperature of warmest month must be less than 22 °C, and that of the coldest month warmer than -3 °C (Although American scientists prefer 0 °C in the coldest month). Poleward of the latter is a zone of subpolar oceanic climate (Köppen Cfc), with relatively mild winters (coldest month warmer than -3) and cool summers and a summer season (average temperature at least 10 °C or 50 °F) of less than four months; examples of this climate include parts of coastal Iceland in the Northern Hemisphere and extreme southern Chile in the Southern Hemisphere (the fact that this form of climate exists in both hemispheres ruling out the use of such terms as subarctic or boreal to denote it; even though these terms are used to refer to climates characterized by short summers, they are also synonyms for "northern" and therefore inappropriate).
Additional information
The British Isles experiences a typically maritime climate, with prevailing south-westerly winds from the Atlantic Ocean. The annual average temperature range in the UK is only about 10 °C. Although the west coast of Alaska experiences a maritime climate, the absence of an equally significant warm Pacific current in the upper-mid latitudes means that these regions are generally colder in winter, with more precipitation falling as snow.
All mid-latitude oceanic climates are classified as humid. Some rainshadow climates with thermal régimes similar to those of oceanic climates but steppe-like (BSk) or even desert-like (BWk) scarcity of precipitation include lowland valleys of Washington and Oregon to the east of the Cascade Range, Patagonia in southern Argentina, and the Atacama Desert in northern Chile. Another example are coastal areas in southeast Western Australia.
The 0 °C (32 °F) isotherm (freeze line) or the -3 °C (26.6 °F) isotherms (persistent snow line) are the possible lines dividing the oceanic and the warm summer humid continental (Dfb) climates, in between which are the following regions:
Note that the latter two locations are high-altitude locations in the tropical areas that would be in savanna (Aw) or steppe (BSHw) zones except for high altitude.
Porto and San Francisco have a temperature range characteristic of an oceanic climate but so little rainfall during summer that they must be considered part of a Mediterranean climate zone, despite their milder summer temperatures.
