The decrease of temperature leads to the liquefaction of water vapor.

Dry ice is solid carbon dioxide, which is characterized by changing directly from solid to gas at room temperature. This process is called sublimation. Dry ice comes into contact with the surrounding air and begins to sublimate, absorbing a lot of heat. This process makes the temperature of the surrounding air drop rapidly, which is lower than the sublimation temperature of dry ice. As the temperature drops, the water vapor in the air begins to liquefy and form tiny water droplets. Water droplets are suspended in the air to form fog, which looks like dry ice smoking.