The climate of Ontario varies from seasonally and in different locations. It is affected by three air sources: cold, dry and arctic air from the north (dominant factor during the winter months, and for a longer part of the year in far northern Ontario); Pacific polar air crossing in from the western Canadian Prairies/US Northern Plains and warm, moist air from the Gulf of Mexico and the Atlantic Ocean. The effects of these major air masses on temperature and precipitation depend mainly on latitude, proximity to major bodies of water and to a small extent, terrain relief. In general, most of Ontario’s climate is classified as humid continental.Ontario has three main climatic regions.
The surrounding Great Lakes greatly influence the climatic region of southern Ontario. During the fall and winter months, heat stored from the lakes is released, moderating the climate near the shores of the lakes. This makes some parts of southern Ontario have milder winters than mid-continental areas at lower latitudes. Parts of Southwestern Ontario (generally south of a line from Sarnia-Toronto) has a moderate humid continental climate (Köppen climate classificationDfa), similar to that of the inland Mid-Atlantic states and the Great Lakes portion of the Midwestern United States. The region has warm to hot, humid summers and cold winters. Annual precipitation ranges from 750–1,000 mm (30–39 in) and is well distributed throughout the year. Most of this region lies in the lee of the Great Lakes, making for abundant snow in some areas. In December 2010, the snowbelt set a new record when it was hit by more than a metre of snow within 48 hours. The next climatic region is Central and Eastern Ontario which has a moderate humid continental climate (Köppen Dfb). This region has warm and sometimes hot summers with colder, longer winters, ample snowfall (even in regions not directly in the snowbelts) and annual precipitation similar to the rest of Southern Ontario.
In the northeastern parts of Ontario, extending far as south as Kirkland Lake, the cold waters of Hudson Bay depress summer temperatures, making it cooler than other locations in Canada or Ontario at similar latitudes. The same is true on the northern shore of Lake Superior, stored colder water left over from the winter does not warm sufficiently, cooling the over-riding hot humid air from the south, sometimes this creates large areas of fog. Along the eastern shores of Lake Superior and Lake Huron winter temperatures are slightly moderated but come with frequent heavy lake-effect snow squalls that increase seasonal snowfall totals upwards of 3 m (10 ft) in some places. These regions have higher annual precipitation in some case over 100 cm (39 in). The northernmost parts of Ontario — primarily north of 50°N — have a subarctic climate(Köppen Dfc) with long, severely cold winters and short, cool to warm summers with dramatic temperature changes possible in all seasons. With no major mountain ranges blocking sinking Arctic air masses, temperatures of −40 °C (−40 °F) are not uncommon; snowfall remains on the ground for sometimes over half the year. Snowfall accumulation can be high in some areas. Precipitation is generally less than 70 cm (28 in) and peaks in the summer months in the form of showers or thunderstorms.
Severe thunderstorms peak in summer. London, situated in Southern (Southwestern) Ontario, has the most lightning strikes per year in Canada, averaging 34 days of thunderstorm activity per year. In a typical year, Ontario averages 11 confirmed tornado touchdowns. However over the last 4 years, it has had upwards of 20 tornado touchdowns per year, with the highest frequency occurring in the Windsor-Essex – Chatham Kent area, though few are very destructive (the majority between F0 to F2 on the Fujita scale). Ontario had a record 29 tornadoes in both 2006 and 2009. Tropical depression remnants occasionally bring heavy rains and winds in the south, but are rarely deadly. A notable exception was Hurricane Hazel which struck Southern Ontario centred on Toronto, in October 1954.
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