The lake create a seasonal lag , and compared to the rest of Canada and inland Ontario , Sarnia have a noticeably longer warm period following summer . However , cooler temperatures tend to prevail for longer after winter . Lake Huron can also create large temperature differences within the city in spring and early summer , particularly on hot days in late May , early June . Finally , extreme temperatures , particularly low , are rarely ever seen . Daily lows less than − 10 ° C ( 14 ° F ) are seen an average of 30 days a year , and less than − 20 ° C ( − 4 ° F ) two days a year . Summers are warm to hot and usually humid . Humidex readings can be very high at times from late May to late September . In fact , Sarnia have the second greatest number of high humidex days at or above 35 ° C ( 95 ° F ) ( with 23 @.@ 16 days on average per year ) and humidex days at or above 30 ° C ( 86 ° F ) ( with 61 @.@ 20 days on average per year ) in Canada , both after Windsor , Ontario . Thunderstorms can become quite severe from April to September . Destructive weather is very rare in the area but have occurred , such as the tornado event of 1953 .