Fog formed when cold air passes over warmer water, creating a steamy appearance, is called steam fog.
Steam fog is also known as sea smoke or evaporation fog. Cold, dry air moving across warmer water picks up moisture through evaporation. The air immediately above the water becomes humid, but it is then cooled by the passing cold air. Once its temperature falls below the dew point, excess water vapour condenses into tiny suspended droplets.
The result resembles steam rising from a hot bath or drink, although the visible “steam” is actually liquid water droplets in fog. Because the process occurs in a shallow layer above the water, the fog may look turbulent, wispy, or divided into rising streamers. In very cold conditions, it can contribute to frost smoke or freezing fog.
Steam fog is common during cold-air outbreaks over the Great Lakes and in polar regions. It differs from advection fog, which forms when moist air moves over a colder surface, and from radiation fog, which develops as the ground cools after sunset. A related but separate feature, the steam devil, is a small vortex that draws steam fog upward.