Weather Why: Fog for some, sun for others this morning

Some parts of mid-Missouri woke up to a thick layer of fog this morning, while some saw clear blue skies to start the day.

Why was that?

Well, for starters, we need to have an understanding as to why fog forms… and it’s many differing types. In general when we forecast fog, we look at how air temperatures drop during the night, in relation to the observed dew point (dew point is air temperature measurement at which water vapor will be able to condense). If the actual surface air temperature falls to the dew point, fog will begin to start forming.

In the case of this morning’s fog, it tended to be focused near rivers, water sources and valleys (which tend to cool off faster at night). The extra amount of water vapor due to those water sources assists in driving the dew point closer to the falling nighttime temperatures as well.

In extreme cases, satellite can pick up on valley and river fog, which from a birdseye view can look like gray veins on the earth’s surface.

Photo courtesy of the NWS Wisconsin LaCrosse office

So why does fog tend to occur more this time of year? It all has to do with much drier air that’s in place in the upper levels of the atmosphere during fall which can lead to clear days and clear nights. On clear, wind-free nights, warm air (from day time heating) is faster to escape into the upper parts of the atmosphere, accelerated by the drier upper levels, allowing the surface to cool much faster, in turn leading to surface AIR temperatures cooling off quickly as well.

As explained, if that air temperature is able to drop to the dew point, it’s only a matter of time before water vapor in the air begins to condense and form surface fog.

It’s also why that fog is so quick to burn off once the sun comes up. The air temperature goes up, separating the connection it had with the dew point, causing the fog/water vapor to disperse and improve visibility.

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