Geography of Toronto

Milliken, a residential Scarborough neighbourhood, during an ice storm, on 15 April 2018.

Despite being cold, extended snow-free periods occur in most winter seasons and precipitation can fall as rain with temperatures sometimes climbing above 10 °C (50 °F). Average winter snowfall is 121.5 cm (47.83 in) at the weather station in Downtown Toronto[4] and 108.5 cm (42.72 in) at Toronto Pearson International Airport.[5]

The average January maximum / minimum is between −1 and −7 °C (30 and 19 °F) in the city.[4] There are usually a few colder periods where temperatures remain below −10 °C (14 °F) and less frequently below −20 °C (−4 °F) at night (especially in the northern suburbs), with wind chills making it feel like −30 °C (−22 °F).[6]

Due to its position on the northwest shore of Lake Ontario, Toronto is not a direct target of heavy, wind−whipped lake-effect snow squalls that hit other Great Lake cities on the south/east shorelines of the lakes, in areas where prevailing winds amplify lake effect. Despite this, there are usually two or more heavy snowfalls each winter which deposit at least 15 cm (5.91 in) accumulation, usually from powerful winter storms known as Alberta clippers, "Colorado Lows" or panhandle hooks that pick up moisture en route to the Great Lakes. These storms can produce strong easterly driven winds that fetch additional moisture from Lake Ontario. They frequently come with a volatile mix of snow, ice pellets, freezing rain and sometimes just ordinary rain, all of which can disrupt transportation, and in severe cases, interrupt power supply. A sustained freezing rain event occurred on December 22, 2013, plunging 30 percent of the city into darkness, some until after Christmas Day.[7]

Toronto snowploughs clearing the highway of snow during a winter storm.

Such storms can also produce large snowfall amounts, higher totals found in areas closer to Lake Ontario, sometimes falling over a series of days or weeks creating havoc. On January 13, 1999, after a series of snowstorms, then-Toronto mayor Mel Lastman called in the Canadian Armed Forces to assist with snow removal and clearing streets. Within twelve days, the downtown Toronto weather station at the University of Toronto (Trinity College near Queen's Park) recorded an average season's worth of 118.4 cm (46.61 in) of snow, much of it lake effect from Lake Ontario and a monthly record for January, but fell short of the snowiest month overall March 1870, with 158.5 cm (62.40 in), of which 89 cm (35.04 in) fell over a five-day span. February 2008 set a record a snowfall record for the month with 76.8 cm (30.24 in) falling at the airport. The winter of 2007–08 brought accumulated seasonal snowfall totals of 209.7 cm (82.56 in) downtown and 194.0 cm (76.4 in) at the airport. The heavy winter snows, in combination with record rains during June–July of that year made 2008 the wettest year on the climate record with over 1,070 mm (42.13 in) of total precipitation. The largest single 24-hour and daily snowfall records occurred on December 11–12, 1944, when 57.2 cm (22.5 in) accumulated, with 48.3 cm (19.0 in) on December 11 alone. More recently on January 16–17, 2022, a blizzard dropped up to 55 cm (21.7 in) (in sections of Scarborough) of accumulated snow in the city, crippling public transportation, with most of it falling on the morning of January 17. The official storm total at Pearson Airport was 34.4 cm (13.5 in)

On the opposite extreme, the winter of 2011–12 had the lowest seasonal snowfall total with only 41.8 cm (16.46 in).[8] March 2012 was the warmest March on record.[9] The least snowfall in a calendar year was 2006, with only 32.4 cm (12.76 in). The El Niño influenced winter of 2009–10 had 52.4 cm (20.63 in), March 2010 recorded no measurable snow, the first such occurrence in any March since 1946, and was followed by the warmest April ever on record.[10]

Winter weather conditions in Toronto can vary greatly from one year to another and is significantly affected by global weather patterns such as El Niño–Southern Oscillation (ENSO). El Niño winters on the weak to moderate side are generally colder or below average in terms of temperatures, with stronger ones being mild and dry, with near to decreased snowfall (such as the winters of 2009–2010 and 2015–2016 were drier than normal), while La Niña winters tend have more precipitation, with more snowfall (the aforementioned winters of 1999 and 2008 were La Niña), but are usually near to above average in terms of temperatures. ENSO Neutral years are more unpredictable, such as 2013-14 which was the coldest, longest winter in recent memory.

Boating around the Toronto Islands. Excursions to the islands are a popular summer activity in the city.

Maximum temperatures typically range from 23 to 31 °C (73 to 88 °F) with moderate to high humidity, proximity to Lake Ontario and the other lakes contribute to summer humidity but far away sources like the Gulf of Mexico also factor in. The subtropical jet can move well north of the area in the midsummer influenced largely by the strength and position of the Bermuda-Azores High and ridging over the North American continent, in some summers, it is a dominating influence. Lake effect cooling varies greatly depending on location within the city and strength and direction of localized winds, variation in water temperature and currents due to recent weather, the most pronounced effect on the Toronto Islands and also on the immediate lakeshore such as in The Beaches, and Scarborough Bluffs, hence the term "cooler by the lake". This is less a factor in the west and northwest parts of the city. Temperatures over 32 °C (90 °F) occur on average seven days each year (three consecutive days of such temperatures is defined as a heat wave, which occur in most summers but maximums rarely exceed 38 °C (100 °F), which last occurred on July 21, 2011). However, coupled with high humidity, the humidex value can rise well above 40 °C (104 °F) during these heat events, creating great discomfort. Nighttime temperatures generally hover close to 20 °C (68 °F) in the city but during hotter spells remain closer to 25 °C (77 °F). Summer heat episodes are usually broken by cooler, drier periods that can be brief.

Thunderstorms are also a regular occurrence and can pop up quickly, especially west and north of the city in areas more prone to the "lake breeze front" or "lake breeze thunderstorms" phenomenon, in which intense, sharply defined squall lines develop quickly on summer afternoons inland, amplified by localized variation in wind patterns between the Great Lakes.[11] These storms sometimes, but not always move into the city causing localized flooding resulting from downpours with high rainfall amounts, intense lightning but less often severe winds knocking down trees and powerlines.

In addition to snowstorms, ice storms, windstorms, heavy rainfall events associated with tropical storms or very severe thunderstorms, tornadoes are rare but do occur, particularly in the northern and western suburbs. That area sits on the northeastern edge of "tornado alley" where tornadoes occur with the most frequency per area in Canada. Downtown Toronto, on the other hand, generally avoids tornadic storms resulting from lake breeze inflows which assist in breaking up formation of tornadoes. Tornado warnings have been posted for the city on a few occasions in the early 21st century, however, no touchdowns have been confirmed within city limits since a weak tornado hit Scarborough in 1998. A pair of dangerous F2 tornados did touchdown in neighbouring Vaughan during a tornado outbreak on August 20, 2009. A powerful derecho affected the city on May 21, 2022.

Toronto is susceptible to flash flooding due to its various ravines and valleys that can easily flood, as well as generally poor drainage due to its large urban area. Flooding can result from remnants of tropical storms, heavy rainfall combined with snowmelt, or summertime thunderstorms stalled out along lake breeze boundaries. Low-lying areas, such as the lower reaches of the Don River valley, are the most likely to flood. This often leads to the shutdown of the Don Valley Parkway and GO Transit's Richmond Hill line. A second mouth is being built for the Don River to mitigate flooding, creating Villiers Island.

The Weston Golf and Country Club was left submerged after the Humber River overflowed its banks during Hurricane Hazel in 1954.

The tropical storm remnant of Hurricane Hazel caused 81 deaths in October 1954 due to flooding that swept homes along river banks into Lake Ontario. A sudden downburst during a strong thunderstorm was believed to have played a contributing factor in the Air France Flight 358 crash on August 2, 2005. Just a few weeks later on August 19, part of a tornado outbreak in Southern Ontario, record-breaking intense rainfall, the worst since Hazel, deluged north-central sections of the city within a couple of hours and destroying a section of Finch Avenue West in North York resulting in record insurance claims. Some rain gauges recorded 175 mm (7 in) of rain, over 100 mm (4 in) in just one hour. Another large rainstorm with intense, record rainfall amounts struck a wide swath of the city during the afternoon rush hour on July 8, 2013 flooding city streets, subway tunnels, basements and knocking out power for over two million residents, stranding commuters, some having to be rescued from a submerged train. A daily rainfall record of 126.4 mm (4.98 in) was set at Pearson Airport, most falling in an hour and a half. Flood insurance claims are likely to exceed the 2005 storm due to a wider area affected.[12] In the springs of 2017 and 2019, heavy rain combined with snowmelt led to the water levels of Lake Ontario to rise to record levels, flooding parts of the Toronto Islands and closing it to the public.

The most severe heat wave in Toronto occurred in 1936, during the 1936 North American heat wave, when downtown temperatures in Toronto exceeded 40 °C (104 °F) on three consecutive days (July 8 – 10), with nighttime temperatures not falling below 25 °C (77 °F).[13][14] The city was ill-equipped at that time to handle such a prolonged extreme heat wave, and heat stroke claimed 225 lives in the city,[15] not counting indirect deaths from causes such as drowning.[16] The hottest month recorded, however, was July 1921, when the average maximum temperature downtown was 31.2 °C (88.2 °F), with a monthly mean temperature of 25.5 °C (77.9 °F), this was equalled in July 2020. At Toronto Pearson International Airport, the highest monthly maximum average was 31.2 °C (88.2 °F) in July 1955,[17] however the highest mean of 25.0 °C (77.0 °F) was set in July 2020, records there began in 1937.[18] The highest humidex (combined feel of temperature and humidity) reached 50.3 °C (122.5 °F) at the airport during the heat wave of July 1995, downtown it reached 51.2 °C (124.2 °F) on July 21, 2011 during the 2011 North American heat wave.

The coldest minimum temperature of −33 °C (−27 °F) was recorded on January 10, 1859. The coldest temperature recorded at the airport was −31.3 °C (−24.3 °F) on January 4, 1981, and the coldest windchill recorded was −44.7 °C (−48.5 °F) on the same day.[5][6] The coldest month overall was −12.6 °C (9.3 °F) at the airport in February 2015 and at downtown in February 1875.[19] Other notable cold waves occurred in January 1994 and February 1934. Winter cold snaps pose a danger as they often come with high winds, leaving the city's homeless population very prone to frostbite and hypothermia.

Based on public records provided by Environment and Climate Change Canada, the average annual temperature has increased 1.5 °C (2.7 °F) at Pearson Airport over a period of 30 years. If compared with the thirty-year normals from 1971 to 2000, more of this increase occurred at night: the average minimum temperature has been 1.9 °C (3.4 °F) higher. Average precipitation during the same period was close to the average of the previous period, snowfall totals down only marginally with slightly higher rainfall. In order from the top: December, September, July and January have seen the highest average temperature increases. Part of this warming is likely attributed to increased urban growth surrounding the airport.

An older study conducted in the 1990s analysed the heat island effect comparing data from selected regional stations, including both Downtown Toronto and Pearson Airport, enough for a large area of the old city limits, and parts of Etobicoke to have a different Köppen climate classification from the surrounding area.[20]