A fog machine is a machine that replicates the visual effect of fog or smoke. The fog machine is usually used in creating cinematic effects, effects for stage shows, or sometimes for Halloween or other small-scale presentations.

A fog machine may work in one of three different ways. Each method has its own benefits and drawbacks, and each has its own areas where it is the ideal method. The most common way a fog machine works in professional situations, is by heating a mixture of glycol and water to create a fog-like steam. The fog machine is comprised of a heat exchanger, a pump, and a tank. The heat exchanger is an enclosed area with an input hole and a small output nozzle.

The exchanger heats up to about 400 degrees Fahrenheit (205 Celsius), and then the pump draws the glycol-water mixture from the tank, and sprays it into the heated unit. The mixture then turns into steam, and is forced out of the nozzle on the front of the fog machine. When the steam makes contact with the relatively cool air outside, it condenses somewhat and turns into fog. This fog is still hotter than the outside air, so it rises up towards the ceiling, creating billowing clouds.

A second type of professional fog machine works in a similar manner to the more common heat-exchange variety. Instead of relying on heat, however, this type of fog machine atomizes the fluid and sends it into the air. Usually the particles created by an atomizing fog machine are much smaller than those using heat, and may not even be obviously visible to the naked eye, instead creating a sort of haze all around for light to reflect off of. This type of fog machine tends to rely more on oil-based liquids, without the high water content of heat-based fog machines.

The third, and by far cheapest and easiest form of fog machine, does not even have to be a machine, per se. This type of fog machine simply uses solid CO2 — more commonly known as dry ice — to create a fog effect. When the dry ice heats up in the ambient room temperature, it begins to create a hazy smoke. Unlike other types of fog machine, which create a fog that hangs in the air or rises, the fog created by dry ice is cold, and therefore sinks to the ground. While this may be ideal for certain situations, it is generally considered inferior to the other two types of fog machines, and dry ice is usually utilized only for home use or by those on a tight budget.