A cloud tank allows film-makers to create and manipulate clouds; with this technique, a tank is filled up with salt water, and a piece of plastic is placed on to the surface. Then fresh water is poured on top; the plastic is then removed and paint is injected into the water, creating a cloud effect. Though there are alternatives to this technique (such as condensed milk, which can used as a substitute for paint, or rubbing alcohol can be used as one of the liquids.) The clouds can be controlled using other materials; for example, a funnel can be used to make the paint give off a 'tornado' effect.
In films, the 'clouds' from the tank are composited onto scenes. An example of a movie that uses this effect is Close Encounters of the Third Kind. The clouds start to appear at 1:03.
Scott Squires, our guest speaker from a few weeks ago worked to help create the cloud tanks in this film.
A water tank allows film-makers to create scenes using large bodies of water within the confines of a studio, and can vary depending on the scale of the film being created. For The Perfect Storm, a studio floor was torn open to make way for a tank, as well as a large wind machine, plus a blue screen surrounding the entire stage. Surrounding the 2,000 gallon tank are dump tanks, which catch water that goes over the edge of the large tank. The image above is the boat on actual water; ILM used the blue screen to make the body of water stretch into the distance.
For James Cameron's Titanic, the illusion of the ship being out at sea was created by having the water tank strategically built along the coastline, so that there is an actual physical horizon to work with while filming. There were also tanks with ship interiors built inside them, designed to be flooded with water once the ship hits the iceberg. The tanks are also angled to give us the illusion of the ship being tilted when it sinks. Below is a sped up video of the Titanic set being built, including the famous water tank:
An example of a company that specializes in water tanks for film is The Mediterranean Film Studio located in Malta. Currently, there are three water tanks built specially for filming in all of the United States. This one was recently built in North Carolina, measuring 60 by 60 feet.