NYC Rooftop Water Towers

You see these all over the city. They look like relics of the past but New York City requires all buildings over six stories to have a rooftop water tower on it. These towers hold, like, 5,000 to 10,000 gallons of water and are necessary to prevent the need for excessively high water pressures at the lower levels. For everyday use they use the water off the top and save the bottom water for fires. When the tanks get low a switch refills them from the public water supply. Kind of like a huge toilet tank kicking on when the floaty ball gets so far down.