 Because carbon dioxide cannot exist as a liquid at atmospheric pressure, the dry ice sublimates and instantly produces a gas, condensing water vapour and creating a thick white fog.A fan placed at the top of the container directs the fog where it is needed, creating a rolling fog that lies low to the ground.As the submerged dry ice cools the water, the amount and duration of fog produced will be reduced, requiring "rest" periods to reheat the water.