On Sunday morning just after sunrise, some thick fog developed over the Ohio river. Much of central Kentucky was dealing with fog that morning but it wasn't until later that it formed over the river.
Dew point is a measure of moisture in the air. The higher the dew point, the more moisture in the air. Fog and clouds form when the dew point and the temperature meet. This can happen in two ways. Either you cool the air to the dew point level, or you add moisture to the air.
The usual way we have fog is by the air cooling to the level of the dew point. The river fog is a little different.
The air temperature was 50 degrees while the dew point was 48 degrees. The river temperature is much warmer at 65 degrees.
As that colder air moves in over the river, the warm river air adds moisture to the cooler air. The dew point was already close to the air temperature so it didn't take much added moisture to get fog.
Once the dew point came up to the level of the temperature. The fog developed. After sunrise, the temperatures quickly started to climb and the fog cleared up.