 The complex of refining and chemical companies is called Chemical Valley and located south of downtown Sarnia . The city has the highest level of <unk> air pollution of any Canadian city because of its reliance on the petrochemical industry . About 60 percent of the particulate matter , however , comes from the neighboring United States . Lake Huron is cooler than the air in summer and warmer than the air in winter ; therefore , it <unk> Sarnia 's humid continental climate , which makes temperature extremes of hot and cold very rare . In the winter , Sarnia experiences lake @-@ effect snow because Arctic air blows across the warmer waters of Lake Huron and <unk> to form snow squalls once over land . 
