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Steam fog: why ponds seem to smoke on cold autumn mornings

October to November

On a sharply cold, calm morning a pond or slow river that is still holding autumn warmth appears to smoke, with wisps rising and curling off the surface. The National Weather Service calls this steam fog: cold air moves over warmer water, the air picks up moisture until its humidity reaches 100 percent, and the fog forms right at the surface.

Why this time of year

Water loses heat much more slowly than air does, so in late autumn a pond can still be well above the temperature of the air sitting on top of it after a clear night. That contrast is the trigger rather than the date, which is why a mild week produces none of this and the morning after the first hard cold snap produces a lot of it.

How to notice it

  1. Go to a pond, a slow river or a reservoir at sunrise on the first calm, cold morning after a clear night.
  2. Look for wisps rising off the WATER itself, thickest over open, calm surface. The NWS describes the appearance as wisps of smoke rising off the surface of the water.
  3. Compare it with any fog lying over the fields nearby. That is radiation fog and it forms the opposite way round, at night under clear skies with calm winds, as the ground radiates away the heat it absorbed during the day.
  4. Check the same water again after a mild spell. It should be gone until the next sharp cold morning, which is the proof that this runs on the air-to-water contrast rather than on the calendar.

Try this with kids: The pond looks like a hot drink steaming on a cold day. Which do you think is warmer here, the water or the air above it?

steam fog, also called evaporation fog, is fog that forms when cold air picks up moisture from warmer water underneath it

Safety

  • The ground and the rocks at a water's edge are often frosty and slick on exactly these mornings. Keep back from the edge.

Sources

  • weather.govNWS Louisville fog tutorial, the load-bearing source. Verbatim: steam fog forms when cold air moves over warm water; when the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100 percent and fog forms; this type of fog takes on the appearance of wisps of smoke rising off the surface of the water. The same page defines radiation fog as forming at night under clear skies with calm winds when heat absorbed by the earth's surface during the day is radiated into space, which is the contrast drawn in the third notice step.
  • forecast.weather.govNWS glossary. Steam fog: fog formed when water vapor is added to air which is much colder than the source of the vapor, and it may be formed when very cold air drifts across relatively warm water. Evaporation-mixing fog: fog that forms when the evaporation of water raises the dew point of the adjacent air. Note this is a query returning the whole F section rather than a stable deep link to either term.

Checked against these sources on September 13, 2026.

In the guide for October, November

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