The Greater Toronto Area (GTA) extends beyond the city boundaries and includes the regional municipalities of Halton, Peel, York and Durham. The GTA is part of a larger, natural ecosystem known as the Greater Toronto Bioregion. This ecosystem is bounded by Lake Ontario, the Niagara Escarpment, and the Oak Ridges Moraine, and includes many watersheds that drain into Lake Ontario. It is also located at the northern extent of the Carolinian forest zone.

Climate

Toronto's climate is moderated by Lake Ontario; its climate is among the mildest and least snowy in Canada east of the Rocky Mountain range. That said, the climate has great annual and year to year variability, particularly during the winter months. It would be considered a cold climate versus most world cities, except in summer.

There can be siginficant regional variations in temperatures and even conditions in the Greater Toronto area similar to those in large coastal cities, due to such factors as local topography, lake shoreline proximity, urban land density, isolated severe weather, etc. For example, on a sunny spring day it might be only 8°C (46°F) at the Scarborough Bluffs with a cool onshore wind but at the same time in Richmond Hill (some 25km to the north) and away from the cool lake waters, it might be 17°C (63°F) with little breeze. The Airport conditions might be partly cloudy and 13°C (55°F). Conversely, in mid-winter might be a mild 1°C at the Bluffs with clouds while Richmond Hill is experiencing heavy snow squalls with a -4°C (26°F) temperature and a -15°C (5°F) windchill.

In general, mild periods do occur throughout the winter (temperatures in the 5-10 °C range (40-50°F) or even higher are not uncommon) triggering regular melting. Added to that the urban heat island effect, there are snow free periods even in mid-winter. The average January maximum is -1 °C (30 °F). There are usually (but not every winter) two or three bitter cold snaps, where maximum temperatures only reach into the -10°C to -15°C range.

Due to its location on the northwest shore of Lake Ontario Toronto is not so prone to heavy, wind-whipped lake effect snow squalls experienced in nearby American cities such as Rochester, Buffalo and Syracuse, NY or elsewhere in Southern Ontario, e.g. Barrie and London. All these cities are located to the south or east of The Lakes making them more vulnerable to predominate winds creating lake effect snow. Despite this, there is usually two or more heavy snowfalls each winter with at least 20 cm (8 in.) accumulation. They can be accompanied by strong east or north-east winds fetching additional moisture from Lake Ontario. Average winter snowfall is 133 cm (52 in).

Summer maximum temperatures typically range from 25–32 °C (77–90 °F) with moderate to high humidity. Temperatures higher than 32 °C (90 °F) occur and occasionally approach 38 °C (100 °F). Such intense heat episodes usually last no more than a few days but can be uncomfortable as they often arrive with high humidity and smog. In recent years, air pollution is increasing, mostly from vehicular exhaust and transported air pollution from heavy industry in the Midwest United States and Southern Ontario. There were a record 52 days with "smog warnings" over the summer of 2005, far exceeding the previous annual record of 28 days in 2001. Sunshine is abundant through summer, but severe thunderstorms are 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 amplified by localized wind patterns between the Great Lakes. 1. These storms sometimes move into the city. In August 2005 there were two such examples of these type of storms creating havoc, the first occurred on August 2, 2005 and is thought to have been a contributing factor in an Air France Airbus A340 crash landing into a ravine that afternoon. The second happened on the afternoon of August 19, 2005 in what has been described as a "once in a thousand year" event, up to 183 mm (over 7 in.) of rain fell in parts of the northern end of the city in under a couple of hours. Numerous roadways and bridges were washed out and insurance claims from backed up sewers and flood damage exceeded $500 million - the worst flood in Toronto in 51 years. One major thoroughfare, Finch Avenue West, was completely washed out by Black Creek creating a huge 7 m (23 foot) deep hole. Re-construction of the roadway is still underway as of January 2006.

Springs and autumns feature varied, changeable weather with alternating periods of dry, sunny weather and rain. Nights are generally cool, but frosts are rare in the city. Snow can fall in early spring or late fall but usually melts quickly or even before making contact with the ground. Also it is at these times of year where great temperature contrasts (up to 30 °C) can occur in a short time frame. Along the Lake Ontario shoreline, spring days tend to be much cooler than further inland, whereas from late summer to early winter, nights are warmer the closer you are to the lake.

The highest temperatures in Toronto at the city weather station was 41 °C (105 °F) recorded on 3 consecutive days from July 7-July 9, 1936. The coldest -33 °C (-25 °F) was recorded on January 10, 1859. Annual average precipitation is 83.4 cm (32.8 in).