Ontario's long American border is formed almost entirely by lakes and rivers, starting in Lake of the Woods and continuing to the Saint Lawrence River near Cornwall; it passes through the four Great Lakes Ontario shares with bordering states, namely Lakes Superior, Huron (which includes Georgian Bay), Erie, and Ontario (for which the province is named; the name Ontario itself is a corruption of the Iroquois word Onitariio, meaning "beautiful lake", or Kanadario, variously translated as "beautiful water"). There are approximately 250,000 lakes and over 100,000 kilometres (62,000 mi) of rivers in the province.
Almost 94% of the population is concentrated within Southern Ontario, where the population was over 12,100,000 in the 2006 census. The Golden Horseshoe is the most populous part of Southern Ontario with a population of 8,102,163.
Ontario is the most populous province in Canada. Southern Ontario is one of the most dense regions in the country. The north is vast and sparse compared to the south. Ottawa (the nation's capital) is located in Ontario bordering Quebec. Located within the Golden Horseshoe, Toronto is the capital of Ontario, the financial centre of Canada, and the country's most populous city.
Ontario, owing to its size has diverse geology that varies in structure, age, and lithology. About 61% of the province is covered by the Canadian Shield, mostly with Precambrian rock. These rocks contain large mineral deposits that are vital to the economy of northern Ontario. The shield can further be divided into three sections. The northwestern parts of the Shield in Ontario, located roughly north and west of Sudbury is known as the Superior Province. The Superior Province is the largest of the three regions of the Canadian Shield portion founded in Ontario, covering about 70% of the Shield portion in Ontario. This region is more than 2.5 billion years old and is composed of felsic intrusive rocks. In the northernmost parts of the Superior Province, the geology of the region is dominated by granite and gneiss rocks. The central region of the Shield, known as the Grenville Province, located south of Sudbury is 1.0 to 1.6 billons yeard old and is dominated by sedimentary rocks showing evidence of being subjected to metamorphism. It makes up about 20% of the Canadian Shield in Ontario. These rocks were metamorphosed between 990 million years ago and 1.08 billion years ago. The third region, known as the Southern province which is a narrow region from Sault Ste. Marie to Kirkland Lake is made of rocks dating 1.8 to 2.4 billion years ago. The Hudson Bay lowlands, located north of the Canadian Shield is mainly made of sedimentary rocks from the Silurian Period although some parts date from the Ordovician and Devonian periods. This area covers 25% of the province. Most of bedrock in the Hudson Bay lowlands are composed of limestone and carbonate-dominated sedimentary rock.
An extensive amount of land to the south and west shores of James Bay and Hudson Bay are low and swampy. The height of the land in North-east and North-west; generally north of Lake Superior is the Canadian Shield where most of Ontario's highest points are found.
Further south, many hilltops of the Algonquin, Haliburton and Madawaska Highlands, which are also part of the shield that covers much of the north, surpass altitudes of 500m (1640').
The highest areas in the southern portion of the province are found in Dufferin, Grey and the western side of Simcoe counties, where the elevation ranges from 430m (1,400') to 540m (1,750'). Much of the higher land sits atop the Niagara Escarpment in a generally flat area known as the Dundalk Highlands. Just to the south, in Wellington and Waterloo County, general elevations are from 300m (1,000') to 400m (1,300'). A striking topographical feature of the Niagara Escarpment are its limestone cliff faces, in general between 80m (250') to 100 (330') above the surrounding land, extending from the Niagara peninsula northwest to the Bruce Peninsula.
The flattest areas of the province outside of the lowlands of the far north are found in southwestern and eastern Ontario.
Niagara Falls, one of Ontario's most noted tourist destination and a source for hydroelectricity
Ontario is known for the large number of lakes and rivers it contains. About one-third of the world's fresh water can be found in Ontario. Ontario is also known for being the only province in Canada that touches the Great Lakes. Ontario touches four of the Great Lakes: Huron, Lake Ontario (the province is named after the lake), Erie and Superior.
More recently, Ontario's vast rivers and lakes have made possible hydroelectric power, mills and the more forms of industrialization. Most of Ontario is fed by rainfall, and in most parts snow is relied on. Precipitation is most common in the southern and central parts of Ontario where variations between winter and summer or spring and fall are not especially great; but winter and spring are less aqueous than in northern and northwestern Ontario.
Köppen climate classification Dfa: Southwestern Ontario, the cities of Windsor, London and the southern/western section of the Golden Horseshoe region including Hamilton, Niagara, Oakville and the city of Toronto, have a moderate humid continental climate, similar to that of the inland Mid-Atlantic and the lower Great Lakes portion of the Midwestern United States. The region has warm, humid summers and cold, usually moist winters. Extreme heat and cold usually occur for short periods. It is considered a temperate climate when compared with the remainder of continental Canada, excluding coastal areas. In the fall and winter, temperatures are moderated by the delayed cooling of the Great Lakes, this effect reversed in spring and summer when afternoon warming is tempered. The lakes' moderating effects allow for a longer growing season than areas at similar latitudes in the continent's interior, some areas exceed 200 frost-free days and have an annual mean temperature of 10°C (50°F). Both spring and fall generally consist of mild days and cool nights but are prone to drastic temperature changes over a short timespan. Annual precipitation ranges from 75–110 cm (30–43 in) and is well distributed throughout the year with a usual summer peak. Upland areas in this region have cooler conditions, generally more precipitation (especially snowfall) putting them into the Dfb climate scheme. Most of this region lies in the lee of the Great Lakes making for abundant snow in some areas (London, Goderich for example) over 2 m (79") while some other areas are not in the direct snowbelt and receive closer to an average of 1 m (39") of snow per year.
One recent storm in Lucan between December 1–3, 2010 dropped 170 cm (67") of snow during a 72-hour storm.
Köppen climate classification Dfb: The second climatic zone covers the northern half of Southern Ontario, including the northern and more elevated parts of the Golden Horseshoe, Central and Eastern Ontario (includes Ottawa). Also included is the southern reaches of Northern Ontario, including the cities of Sudbury and North Bay, which have a more severe humid continental climate. This region has warm and sometimes hot summers (although shorter in length than Southwestern Ontario) with cold, longer winters with roughly equal annual precipitation as the south. Along the eastern shores of Lake Superior and Lake Huron (Georgian Bay), frequent heavy lake-effect snow squalls increase seasonal snowfall totals upwards of 3 m (120 in). Such conditions and the absence of long stretches of brutal cold make for excellent winter recreation.
Köppen climate classification Dfc: The northernmost parts of Ontario—primarily north of 50°N and with no major cities in the area—have a subarctic climate (Köppen) with long, severely cold winters and short, cool to warm summers with dramatic temperature changes possible in all seasons. In summer, hot weather occasionally reaches even the northernmost parts of Ontario for brief periods, although humidity is generally lower than in the south. With no major mountain ranges blocking sinking Arctic air masses, temperatures of −40 °C (−40.0 °F) are not uncommon. The snow stays on the ground much longer in here than other regions of Ontario; snow cover is usually present to some extent between October and May.
Severe and non-severe thunderstorms peak in frequency from June through August but occur at any time during the warmer season. Thunderstorms form from daytime convective heating and frontal activity, in the south, lake breeze convergences also intensify storms. Another severe type of thunderstorm is known as a Mesocyclonic Convective Complex or Derecho, which is a larger cluster-type thunderstorm mass with a more or less circular shape, often with a pronounced bow shape at its front or leading edge. During periods of hot weather in summer, they often develop in the afternoon west of the Great Lakes but strike Southern and Central Ontario at night with great forward motion, bringing severe straight-line winds over wide areas resulting in damage to forests, power interruption and infrastructure damage. The areas with the highest severe weather frequency in the province are Southwestern (Windsor,Chatham, Stratford corridor) and Central Ontario (Simcoe County including the city of Barrie, Lake Simcoe and the Kawartha Lakes region), both areas often get amplified storms resulting from the Lake Breeze Front convergence. London has the most lightning strikes per year in Canada, averaging 34 days of thunderstorm activity per year. In typical year, Ontario averages 15 confirmed tornado touchdowns, they are rarely destructive (the vast majority are classified as F0 or F1 on the Fujita scale). In Northern Ontario, some tornadoes go undetected by ground spotters because of the sparse population and remote landscape; they are often discovered after the fact by aircraft pilots, where aerial observations of damaged forest confirm occurrences. All of Northern Ontario north of a line from Lake Nipigon to Timmins has no weather radar coverage by Environment Canada making it difficult to detect tornadoes in far northern Ontario when they occur. Tropical depression remnants can cause copious rains and winds in the south, but are rarely deadly. Notable exceptions were Hurricane Hazel in October 1954.