Ask Ellen: Why does fog form over water on cold mornings?

Ask Ellen

GRAND RAPIDS, Mich. (WOOD) — On some quiet cold mornings you can see it; areas of fog rising specifically over lakes, rivers and streams.

So why does this happen? It has to do with how fog forms!

This specific type of fog usually forms in the fall when cold air begins rolling back into a forecast. The air drops in temperature but the rivers, lakes and streams are still relatively warm.

If there is very little wind, the cold air will settle over both the land and the water. The cold air close to the water’s surface will get both warmer and wetter. The cold air over the land will likely undergo some interactions as well, but it won’t have as big of an impact as the warm body of water.

Air that is warmer, or more humid than its surrounding environment, is lighter. This warm air begins to rise from the surface of the water and streams into the cold air still in place over the area. When this happens — poof! — it creates steam fog!

Remember, fog is just a cloud that is on the ground. There are several different ways fog can form but all are a combination of cool air interacting with warm, moist air near the surface.

Dense fog days often happen when warm air moves in over a cold, snow covered ground. Or on a summer night when temperatures drop and moisture is trapped at the surface. As temperatures continue to drop this season, have your eyes peeled for steam fog in your area, and send us a picture if you spot any near you. They usually make for a beautiful scene.

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