How do they make fog in movies?

The kinds of fog that we see in movies are different from natural fog.

Natural fog forms when cold air makes water vapor in the air condense into tiny droplets. The fog can stay for a long time because small air currents keep lifting the droplets upward and mixing them around. Finally, the temperature rises and changes the droplets back into water vapor, which is invisible.

People have found several ways to make fog, usually by using chemicals that are denser than air. These fogs tend to sink to the ground. The simplest way to make fog is to drop dry ice (frozen carbon dioxide) into hot water. As the water warms the dry ice, carbon dioxide gas bubbles off the ice and flows up and over the sides of the container. The gas itself is invisible, but it is cold enough to make the water vapor in the air condense into droplets. The water droplets form only in air that mixes with the cold, carbon dioxide gas. That gas is dense, so the fog stays near the ground.

Never touch dry ice without the help of an adult. It’s cold enough to cause frostbite.