the Greater Toronto Area, a area stretching from Niagara Falls to Peterborough. The green belt is designed to curb urban sprawl and to preserve valuable natural areas and farmland surrounding the city. However, some types of development including detached single residential, quarries and commercial facilities continue to get approved, exerting pressure and population growth on the Greenbelt. Toronto is among the latest in a line of cities that have implemented growth boundaries of some kind as a method of restricting urban growth, including Ottawa; Portland, Oregon; Frankfurt; Melbourne; Seoul and London. Climate Toronto's continental climate is moderated by Lake Ontario; its climate is among the mildest in Canada east of the Rocky Mountains, given Toronto's southerly latitude within the country. Downtown Toronto sits in a pocket of the humid continental climate (Köppen climate classification Dfa) zone found at the southwestern end of Lake Ontario covering the southern part of the city — including downtown (but excluding the Toronto Islands), where the annual average temperature exceeds . Some of Toronto, outside of the pre-1998 city limits, some suburban areas, and the Toronto Islands, fall within the Dfb climate regimen. This difference partly results from the urban heat island effect. Toronto is located in hardiness zone 7a, with decreasing hardiness further away from the downtown core (5b in the suburbs). There is a high degree of variability from year to year and sometimes over a period of days, particularly during the winter months. Lake Ontario's water temperature varies due to upwelling of colder water or warmer pools of surface water creating very localized thermal contrast; the deeper waters of the lake, far from the shore, remain at a near-constant water temperature of , the effect of which is either cooling or warming (in winter). This creates generally warmer nights through the colder season than would otherwise occur. When offshore winds blow in summer, they warm as they near the lakeshore in the evening; conversely, the cooling effect by the lake is most pronounced on spring afternoons, which can affect Toronto even more than other cities on the Great Lakes due its exposure to onshore winds from the east to southeast, on some days, the temperatures can be as much as cooler than areas far removed from Lake Ontario, an effect that wanes by summer when the dominant windflow becomes more southwesterly and the lake surface temperature warms. Springs and autumns are shorter