When's the last time you turned on a faucet in your house and didn't get any water? Fortunately this almost never happens, thanks to your local water tower!
Water towers take advantage of the force of gravity to provide pressure for the water they contain. Every vertical foot adds point-four-three pounds per square inch to the water pressure. Towns usually keep their water pressure between fifty and one-hundred pounds per square inch, so a simple equation tells them how high to build the tower. And since a typical tower contains a full day's worth of water, the force of gravity can maintain the hydrostatic pressure of the water system even when the power goes out.
In addition to providing a reliable emergency source of water, they serve an important day-to-day purpose as well. A city's water usage varies throughout the day, usually peaking in the early morning when many people are showering and washing. Water usage during this time can be four or five times higher than during other parts of the day. To maintain water pressure during peak hours of the day, a city could invest in a very powerful pump, but this would be expensive and wasteful, since its capacity would go unused for most of the day. Instead, the city draws on the water tower's supply of water during these hours of high demand, and in conjunction with the pumps, the supply is kept stable throughout the day.