Water tower
A water tower or elevated water tank is a very large tank constructed for the purpose of holding a supply of water at a height sufficient to pressurize a water supply system. Many water towers were constructed during the industrial revolution and some of these are now considered architectural landmarks and monuments and may not be demolished. Some are converted to apartments or exclusive penthouses.
In certain areas, such as New York City, smaller water towers are constructed for individual buildings.

# Construction
A typical water tower is constructed of steel, reinforced or prestressed concrete or bricks. It is usually spherical or cylindrical and is approximately 50 feet (16 metres) in diameter. It typically has a height of approximately 120 feet (40 metres).

# Purpose
The users of the water supply (a town, factory, or just a building) need to have water pressure to maintain the safety of the water supply.  If a water supply is not pressurized sufficiently, several things can happen:
- Water may not reach the upper floors of a building;
- Water may not spray from a tap with sufficient flow
- Without a water tower, parts of gravity flow water supply systems in hilly areas may be subject to negative pressures (see siphon). Negative pressure in the system may cause shallow groundwater to be sucked into a leaky water supply system, polluting it with microorganisms, dirt, sand, fertilizers, and any other toxic contaminants that may be in the groundwater;
- The water towers supply water even during power cuts because they use gravity to pressurize the water pipes in people's homes, although it cannot supply water for a long time without electricity because electricity is needed to pump water up into the tower.
- A further purpose is that a water tower serves as a reservoir to help with water needs during peak usage times.  The water level in the tower typically falls during the peak usage hours of the day, and then pumps fill it back up during the night.  This process also keeps the water from freezing in cold weather, since it is constantly being used and refilled.

# Operation
The height of the tower provides the hydrostatic pressure for the water supply system, and it may be supplemented with a pump. The volume of the reservoir and diameter of the piping provide and sustain flow rate. However, relying on a pump to provide pressure is expensive; to keep up with varying demand, the pump would have to provide a constantly varying output pressure (and thus need an expensive control system) and it would have to be sized sufficiently to give the same pressure at high flow rates.
Very high flow rates are needed when fighting fires. With a water tower present, pumps can be sized for average demand, not peak demand; the tower can provide water pressure during the day and the pumps can refill the water tower at night when demand is very low.

# Decoration
Water towers can be surrounded by ornate coverings including fancy brickwork, a large ivy-covered trellis or it can be simply painted. Some city water towers had the name of the city painted in large letters on the roof, as a navigational aid to aviators. Sometimes the decoration can be humorous, as Granger, Iowa has two water towers, labeled HOT and COLD. The The House in the Clouds in Thorpeness was built to resemble a house in order to disguise the eyesore, whilst the lower floors were used for accommodation. When the town was connected to the mains water supply, the tank was dismantled and converted to additional living space.
Sapp Bros. truck stops uses a water tower with a handle and spout -- looking like a coffee pot -- as the company logo. Many of their facilities have thus-decorated actual water towers (presumably non-functional) on-site.
The first and original "Mushroom" -- Svampen in Swedish -- was built in Orebro in Sweden in the early 1950s and later copies were built around the world including Saudi-Arabia and Kuwait.

# Uses today
Water towers are very common in India, where the electricity supply is erratic in most places.  Water tanks are used atop houses and multi-story houses to store water from erratic supplies.
In many countries, water towers have been taken out of the water supply system and replaced by pumps alone. Water towers are often regarded to be the monuments of civil engineering. Some are rejuvenated and converted to serve modern purposes. A good example of the latter is Wieza Cisnien in Wroclaw, Poland.

# Railway use
All railways making use of steam locomotives require a means of replenishing the locomotive's water tank. This is most commonly achieved by means of a water tower feeding one or more water cranes, usually located at stations and locomotive sheds.

# Further uses
Some water towers are also used as observation towers. There are even water towers with restaurants, such as the Goldbergturm in Sindelfingen, Germany.  It is also common to use water towers as the location of transmission mechanisms in the UHF range with small power, for instance for closed rural broadcasting service, portable radio, or cellular telephone service.

# New York City
In the 1800s, New York City required that all buildings higher than 6 stories be equipped with a rooftop water tower.  This was necessary to prevent the need for excessively high pressures at lower elevations, which could burst pipes.  In modern times, the towers have become fashionable in some circles.  As of 2006, the neighborhood of Tribeca requires water towers on all buildings, whether or not they are being used. Two companies in New York build water towers, both of which are family businesses in operation since the 1800s. 
The original water tower builders were barrel makers who expanded their craft to meet a modern need as buildings in the city grew taller in height. Even today, no sealant is used to hold the water in. Tank walls are held together with cables but leak through every gap when first filled. As the wood swells, the gaps close and become impermeable.
The rooftop tanks store 5,000 to 10,000 gallons of water until it is needed in the building below. The upper portion of water is skimmed off the top for everyday use while the water in the bottom of the tank is held in reserve to fight fire. When the water drops below a certain level, a pump is triggered and the tank is refilled.

# Famous towers
Famous water towers in the USA include:
- Chicago Water Tower
- Volunteer Park Water Tower, Capitol Hill, Seattle, Washington
- Warner Bros. Studios Water Tower, Burbank, California (In the animated TV series Animaniacs, it was used to incarcerate the characters Yakko, Wakko, and Dot.
- Peachoid next to I-85 on the edge of Gaffney, South Carolina
- Earful Tower at Disney-MGM Studios
- Florence Y'all Water Tower in Florence, Kentucky
- Union Water Sphere in Union Township, New Jersey
Famous water towers in the UK include:
- Dock Tower in Grimsby
- 'House in the Clouds' in Thorpeness, Suffolk
- Jumbo in Colchester, Essex
Famous water towers in Sweden include:
- Svampen in Orebro