Why does steam rise from manholes in the U.S.?

Dear Straight Dope: Growing up in England, I would often watch U.S. TV shows, Starsky and Hutch for example, where I noticed that steam rose from the streets during car chases. I have traveled extensively around Europe and Africa, yet never saw any steam. Imagine my delight when I took my first trip to the U.S., Denver in fact, and saw steam, not just during car chases, but all the time! What do people put down there? Please, tell the world! (We don’t have a straight dope over here, not even a crooked one.) John Brooks, Buckinghamshire, England

Steam power is as much a part of Manhattan as subways and Times Square. The first steam generation plant began operating in 1882 — six months before the first electric service. From this plant, steam traveled through a half-mile main to distribute heat and power to the heart of the downtown Manhattan business district – today’s financial district along Wall Street. Increased public acceptance and popularity of the steam system came in 1888 when a severe blizzard nearly shut down the city. Buildings with their own boilers had difficulty obtaining sufficient coal and wood to keep the building heated. However, the New York steam system remained in service. Today, steam power has grown to play a major role in the life of the city. In fact, the Con Edison system has become the largest steam district in the United States — larger than the next four U.S. systems combined. More than 100 miles of mains and service pipes make up the Con Edison steam system. The pipes range in size from one to thirty inches in diameter, and deliver this clean, efficient energy source to about 2,000 customers from the Battery to 96th Street. In fact, Con Edison steam is so clean and pure that it meets all FDA standards. Thus, there are no adverse health effects associated with steam use. Steam can be used for everything from heating and cooling to sterilizing and food processing.