Southern and eastern Ontario, southern Quebec, and New Brunswick could get frost as temperatures take a dive in the overnight hours. Ground temperatures around the freezing mark, clear skies, and light winds are the perfect conditions for frost.
Environment Canada, which issued the frost warnings, says it will be particularly cold in eastern Ontario, where temperatures could fall to -2°C. And southwestern Ontario will be just below freezing for a couple of hours before sunrise.
Areas under frost warnings in Ontario include Milton, Newmarket, Pickering, Waterloo, Barrie, Owen Sound, Kingston, Ottawa, and Cornwall. In Quebec, frost warnings cover Vaudreuil, Lanaudiere and Lachute. New Brunswick's frost warnings include Bathurst, Fredericton, Grand Falls, Miramichi, Woodstock and Carleton County.
Wondering why some regions are under frost warnings and others are not? It has to do with geography, explains Brian Dillon, a meteorologist at The Weather Network. Ottawa has a frost warning, but Montreal, Toronto and Mississauga do not.
Ottawa is in a valley and cool air sinks. But Montreal is protected by the St. Lawrence River, which acts as a barrier. The water temperature is warmer than the air temperature.
Toronto and Mississauga are not under frost warnings, either, because of their location on Lake Ontario and the “urban heat island effect” -- the warmth radiating from buildings.
Places like Midland, Orillia, Peterborough and Petawawa have already had killing frost in recent weeks. Frost warnings are no longer issued for the season in those locations.