Ontario is one of the thirteen provinces and territories of Canada. Located in Central Canada, it is Canada’s most populous province, with 38.3 percent of the country’s population, and is the second-largest province by total area (after Quebec). Ontario is Canada’s fourth-largest jurisdiction in total area when the territories of the Northwest Territories and Nunavut are included. It is home to the nation’s capital city, Ottawa, and the nation’s most populous city, Toronto, which is also Ontario’s provincial capital.

Ontario is bordered by the province of Manitoba to the west, Hudson Bay and James Bay to the north, and Quebec to the east and northeast, and to the south by the U.S. states of (from west to east) Minnesota, Michigan, Ohio, Pennsylvania, and New York. Almost all of Ontario’s 2,700 km (1,678 mi) border with the United States follows inland waterways: from the westerly Lake of the Woods, eastward along the major rivers and lakes of the Great Lakes/Saint Lawrence River drainage system.

These include Rainy River, Pigeon River, Lake Superior, St. Mary’s River, Lake Huron, St. Clair River, Lake St. Clair, Detroit River, Lake Erie, Niagara River, Lake Ontario and the St. Lawrence River from Kingston, to the Quebec boundary just east of Cornwall. There is only about 1 km (0.6 mi) of land border, made up of portages including Height of Land Portage on the Minnesota border.

Ontario is sometimes conceptually divided into two regions, Northern Ontario and Southern Ontario. The great majority of Ontario’s population and arable land is in the south. In contrast, the larger, northern part of Ontario is sparsely populated with cold winters and heavy forestation.

CLIMATE

Ontario’s climate varies by season and location. Three air sources affect it: cold, dry, 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.

The surrounding Great Lakes greatly influence the climatic region of southern Ontario. During the fall and winter, the release of heat stored by the lakes moderates the climate near the shores. This gives parts of southern Ontario milder winters than mid-continental areas at lower latitudes. Parts of Southwestern Ontario (generally south of a line from Sarnia–Toronto) have a moderate humid continental climate, similar to 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. 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 southern part of the province on the sand spit of Long Point, the climate is moderated by Lake Erie during the winter and summer months that it is a borderline maritime climate with mild winters compared to the rest of the province. The summers are cool compared to other parts of the province and the winters are relatively mild.

In the northeastern parts of Ontario, extending south as far as Kirkland Lake, the cold waters of Hudson Bay depress summer temperatures, making it cooler than other locations at similar latitudes. The same is true on the northern shore of Lake Superior, which cools hot, humid air from the south, leading to cooler summer temperatures.

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 to upwards of 3 m (10 ft) in some places. These regions have higher annual precipitation, in some places over 100 cm (39 in).

Cold northwesterly wind over the Great Lakes creating lake-effect snow. Lake-effect snow most frequently occurs in the snowbelt regions of the province.

The northernmost parts of Ontario – primarily north of 50°N – have a subarctic climate 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; snow remains on the ground for sometimes over half the year.

Snow 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 rain or thunderstorms.

Severe thunderstorms peak in summer. Windsor, in Southern (Southwestern) Ontario, has the most lightning strikes per year in Canada, averaging 33 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 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.