An oceanic climate (also known as marine, west coast and maritime) is the Köppen classification of the climate typical of west coasts in higher middle latitudes of continents, and generally features cool summers (relative to their latitude) and cool but not cold winters, with a relatively narrow annual temperature range and few extremes of temperature, with the exception for transitional areas to continental, subarctic and highland climates. Oceanic climates are defined as having a monthly mean temperature below 22 °C (72 °F) in the warmest month, and above 0 °C (32 °F) in the coldest month.
It typically lacks a dry season, as precipitation is more evenly dispersed throughout the year. It is the predominant climate type across much of Western Europe, the Pacific Northwest region of the United States and Canada, portions of central Mexico, southwestern South America, southeastern Australia including Tasmania, New Zealand as well as isolated locations elsewhere. Oceanic climates generally have cool summers and cool (but not cold) winters. They are characterised by a narrower annual range of temperatures than are encountered in other places at a comparable latitude, and generally do not have the extremely dry summers of Mediterranean climates or the hot summers of humid subtropical. Oceanic climates are most dominant in Europe, where they spread much farther inland than in other continents.
Oceanic climates can have much storm activity as they are located in the belt of the stormy westerlies. Many oceanic climates have frequent cloudy or overcast conditions due to the near constant storms and lows tracking over or near them. The annual range of temperatures is smaller than typical climates at these latitudes due to the constant stable marine air masses that pass through oceanic climates, which lack both very warm and very cool fronts.
Precipitation is both adequate and reliable throughout the year in oceanic climates. Extended months of rain and cloudy conditions are common in oceanic climates. Seattle is an example of this. Between October and May, Seattle experiences high rainfall and is mostly or partly cloudy six out of every seven days.
In most areas with an oceanic climate, precipitation comes in the form of rain for the majority of the year. However, some areas with this climate see some snowfall annually during winter. Outside of Australia and parts of New Zealand, most areas with an oceanic climate experience at least one snowstorm per year. In the poleward locations of the oceanic climate zone ("subpolar oceanic climates," described in greater detail below), snowfall is more frequent and commonplace.
Overall temperature characteristics of the oceanic climates feature cool temperatures and infrequent extremes of temperature. In the Köppen climate classification, Oceanic climates have a mean temperature of 0 °C (32 °F) or higher in the coldest month, compared to continental climates where the coldest month has a mean temperature of below 0 °C (32 °F). Summers are cool, with the warmest month having a mean temperature below 22 °C (72 °F). Poleward of the latter is a zone of the aforementioned subpolar oceanic climate (Köppen Cfc), with long but relatively mild (for their latitude) winters and cool and short summers (average temperatures of at least 10 °C (50 °F) for one to three months). Examples of this climate include parts of coastal Iceland in the Northern Hemisphere and extreme southern Chile and Argentina in the Southern Hemisphere (examples include Ushuaia and Punta Arenas).
Oceanic climates are not necessarily always found in coastal locations on the aforementioned parallels; however, in most cases oceanic climates parallel higher middle latitude oceans. The polar jet stream, which moves in a west to east direction across the middle latitudes, advancing low pressure systems, storms, and fronts. In coastal areas of the higher middle latitudes (45–60° latitude), the prevailing onshore flow creates the basic structure of most oceanic climates. Oceanic climates are a product and reflection of the ocean adjacent to them. In the fall, winter, and early spring, when the polar jet stream is most active, the frequent passing of marine weather systems creates the frequent fog, cloudy skies, and light drizzle often associated with oceanic climates. In summer, high pressure often pushes the prevailing westerlies north of many oceanic climates, often creating a drier summer climate (for example in the Pacific Northwest of the United States and Canada).
The North Atlantic Gulf Stream, a tropical oceanic current that passes north of the Caribbean and up the East Coast of the United States to North Carolina, then heads east-northeast to the Azores, is thought to greatly modify the climate of Northwest Europe. As a result of the Gulf Stream, west-coast areas located in high latitudes like Ireland, the UK, and Norway have much milder winters (for their latitude) than would otherwise be the case. The lowland attributes of western Europe also help drive marine air masses into continental areas, enabling cities such as Dresden, Prague, and Vienna to have maritime climates in spite of being located well inland from the ocean.
The only noteworthy area of Maritime Climate at or near sea-level within Africa is in South Africa from Mossel Bay on the Western Cape coast to Plettenberg Bay, with additional pockets of this climate inland of the Eastern Cape and KwaZulu-Natal coast. It is usually warm most of the year with no pronounced rainy season, but slightly more rain in autumn and spring. The Tristan da Cunha archipelago in the South Atlantic also has an oceanic climate.
The only significant areas where this climate is found at or near sea level in Asia is on the Black Sea coast in northern Turkey, in small pockets along or near the Caspian Sea in Azerbaijan and in small pockets along or near the Tsugaru Strait in northern Japan.
The oceanic climate is prevalent in the more southerly locations of Oceania. A mild maritime climate is in existence in New Zealand. It occurs in a few areas of Australia, namely in the southeast, although average high temperatures during summers there tend to be higher and the summers drier than is typical of subtropical highland climates, with summer maxima sometimes exceeding 40 °C (104 °F), southeastern New South Wales (starting from the Illawarra region) and the southern parts of Victoria, the island of Tasmania, also exhibit a mild maritime climate. It can also be found along the western areas of the south coast of Western Australia, parts with steppe-like (BSk) or even desert-like (BWk) scarcity of precipitation.
The subtropical highland variety of the oceanic climate exists in elevated portions of the world that are within either the tropics or subtropics, though it is typically found in mountainous locations in some tropical countries. Despite the latitude, the higher altitudes of these regions mean that the climate tends to share characteristics with oceanic climates, though it also tends to experience noticeably drier weather during the lower-sun "winter" season. In locations outside the tropics, other than the drying trend in the winter, subtropical highland climates tend to be essentially identical to an oceanic climate, with mild summers and noticeably cooler winters, plus, in some instances, some snowfall. In the tropics, a subtropical highland climate tends to feature spring-like weather year-round. Temperatures there remain relatively constant throughout the year and snowfall is seldom seen.
Areas with this climate feature monthly averages below 22 °C (72 °F) but above −3 °C (27 °F) (or 0 °C (32 °F) using American standards). At least one month's average temperature is below 18 °C (64 °F). Without the elevation, many of these regions would likely feature either tropical or humid subtropical climates. These regions usually carry a Cwb or Cfb designation, though very small areas in Yunnan, Sichuan and parts of Argentina and Bolivia have summers sufficiently short to be Cwc with fewer than four months over 10 °C (50 °F).Copacabana, Bolivia, is one the few confirmed towns that features this rare variation of the subtropical highland climate.
Areas with subpolar oceanic climates feature an oceanic climate but are usually located closer to polar regions. As a result of their location, these regions tend to be on the cool end of oceanic climates. Snowfall tends to be more common here than in other oceanic climates. Subpolar oceanic climates are less prone to temperature extremes than subarctic climates or continental climates, featuring milder winters than these climates. Subpolar oceanic climates feature only one to three months of average monthly temperatures that are at least 10 °C (50 °F). As with oceanic climates, none of its average monthly temperatures fall below -3.0 °C (26.6 °F) or 0 °C depending on the isotherm used. Typically, these areas in the warmest month experience daytime maximum temperatures below 17 °C (63 °F), while the coldest month features highs near or slightly above freezing and lows just below freezing. It typically carries a Cfc designation.
This variant of an oceanic climate is found in parts of coastal Iceland, the Faroe Islands, small sections of the Scottish Highlands, the Scottish archipelago of Shetland, northwestern coastal areas of Norway such as Lofoten and reaching to 70°N on some islands, uplands near the coast of southwestern Norway, the Aleutian Islands of Alaska and northern parts of the Alaskan Panhandle, the far south of Chile and Argentina, and a few highland areas of Tasmania, and the Australian and Southern Alps. This type of climate is even found in the very remote parts of the Papuan Highlands in Indonesia. The classification used for this regime is Cfc. In the most marine of those areas affected by this regime, temperatures above 20 °C (68 °F) are extreme weather events, even in the midst of summer. Temperatures above 30 °C (86 °F) have been recorded on rare occasions in some areas of this climate, and in winter temperatures down to −20 °C (−4 °F) have seldom been recorded in some areas.