What is the maritime climate of Canada's coasts?

Canada has two distinct maritime climate zones: the Atlantic Maritime climate of the four Atlantic provinces and the Pacific Maritime climate of British Columbia's coast. Both are characterised by reduced temperature extremes (warmer winters, cooler summers) compared to interior continental Canada, year-round precipitation, and significant fog. The thermal moderation comes from the heat capacity of the adjacent oceans, which warm and cool more slowly than the continent.

The Pacific Maritime climate is mild and wet. The British Columbia coast from Alaska to the United States border has mean January temperatures around 4 degrees Celsius and mean July temperatures around 16 degrees Celsius. Annual precipitation ranges from about 600 millimetres in Victoria (in the rain shadow of the Olympic Mountains) to more than 5,000 millimetres at the head of Knight Inlet (one of the wettest places in North America). The coast supports the largest extent of temperate coastal rainforest in the world, with old-growth Sitka spruce, western red cedar, western hemlock, and Douglas fir.

The Atlantic Maritime climate is more variable. Winter temperatures (mean January minus 4 to minus 10 degrees Celsius depending on location) are colder than the Pacific coast because the Labrador Current brings cold water south along the Atlantic shore. Summer temperatures (mean July 14 to 18 degrees Celsius) are similar to the Pacific. Annual precipitation is high (1,000 to 1,500 millimetres) and spread evenly through the year. Atlantic Canada experiences frequent storms including Atlantic hurricanes (Hurricane Juan in 2003, Fiona in September 2022) and noreaster winter storms.

Fog is a defining feature of both maritime climates. The Grand Banks off Newfoundland have about 200 fog days per year, the highest in North America, caused by the meeting of the cold Labrador Current and the warm Gulf Stream. Vancouver Island's west coast averages about 100 fog days per year, with morning fog common throughout summer. The Bay of Fundy and the Strait of Georgia have lower fog frequencies but still significantly more than continental interior Canada. The maritime climate is moderated by ocean currents (the Labrador Current, the Gulf Stream, and the Alaska Current) and prevailing westerly winds. Climate change is affecting both maritime climates, with rising sea levels threatening coastal infrastructure, more intense Atlantic hurricanes (Fiona 2022 caused about $4.4 billion in damage in Atlantic Canada), changing fish stocks, and altered fog patterns.