Lethbridge is a city in the province of Alberta, Canada, and the largest city in southern Alberta. It is Alberta's fourth-largest city by population after Calgary, Edmonton and Red Deer, and the third-largest by area after Calgary and Edmonton. The nearby Canadian Rockies contribute to the city's cool summers, mild winters, and windy climate. Lethbridge lies southeast of Calgary on the Oldman River. Lethbridge is the commercial, financial, transportation and industrial centre of southern Alberta. The city's economy developed from drift mining for coal in the late 19th century and agriculture in the early 20th century. Half of the workforce is employed in the health, education, retail and hospitality sectors, and the top five employers are government-based.