thunder, sound produced along a path of a lightning flash, caused by the rapid heating and expansion of the adjacent air. Rolling thunder occurs either as a result of the time difference between sounds from the far and near end of a flash, or when mountains, layers of air, or other obstructions cause reverberations. Since sound travels about 1 mi in 5 sec, the distance between a lightning flash and an observer may be determined by counting the seconds between the flash and the thunder. Thunder as far distant as 10 to 15 mi (15 to 25 km) from an observer is usually not heard, even though lightning is often seen. See thunderstorm.

Thunder Bay, city (1991 pop. 113,946), SW Ont., Canada, on Thunder Bay inlet of Lake Superior. The city was created in 1970 by the amalgamation of the twin cities of Fort William and Port Arthur and two adjoining townships. It is one of Canada's major ports, shipping wheat, lumber, coal, and iron ore. The city has shipyards, grain elevators, lumber and pulp and paper mills, breweries, and an oil refinery. Manufactures include structural steel, buses, trucks, aircraft, and chemical products. Port Arthur, originally a military post, was founded in the late 19th cent. Fort William was built by the North West Company in 1801 to serve as its western headquarters. It was the site of a fur-trading post built in 1679 and of Fort Kaministikwia, built by the French in 1717 and later abandoned. Kakabeka Falls, nearby, is a source of water power. Lakehead Univ. is located there.

City (pop., 2001: 109,016), west-central Ontario, Canada. It is located on the northwestern shore of Lake Superior. Its first settlement was a French fur-trading post circa 1678. In the 1870s and '80s silver strikes and the arrival of the Canadian Pacific Railway brought prosperity to the twin towns of Port Arthur and Fort William that had grown up there. Their rivalry was resolved with the unification of their harbour facilities in 1906; the towns merged in 1970 and created the city of Thunder Bay. It is one of Canada's busiest ports, with grain storage and transshipment depots; other industries include shipbuilding.

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