How Ground Fog Forms

Anyone who drives a car or other vehicle at night or during the early morning has experienced the phenomenon of what is called “ground fog” – especially during cooler nights in the fall.

Ground fog hugs the ground and does not reach up to the lower cloud level. This is different than the other main type of fog called “advection fog”. Advection fog occurs when warm air passes over a colder area such as water. This is common along the Atlantic and Pacific coastlines.

Any discussion of ground fog (also called radiation fog) must begin with an explanation of what’s called the “dew point”. The dew point is the temperature at which the air becomes 100% saturated with water. When this happens, the air condenses into water droplets – the visual effect being fog. The lower the dew point temperature, the drier the air. Higher dew points generate high humidity. The bigger the difference between the air temperature and the dew point temperature, the more comfortable the weather conditions are. And if the air temperature gets within a degree of the dew point, fog can form.

But first, the ground has to be warm. Then on calm, clear nights if the temperature is cooler, once the dew point is reached we can have ground fog.

The air temperature itself does not affect the dew point. It has more to do with the humidity. Certain parts of the country of course, tend to have higher humidity levels than other parts. The higher the overall humidity, the higher the dew point.

A very popular place for ground fog to form is in valleys because colder air tends to sink, while warmer air rises. Because the colder air sinks, the fog will tend to gather in these lower lying areas.

There are two things besides the dew point that can affect the forming of ground fog: wind and clouds. The wind can bring in drier air from higher altitudes hindering the formation of ground fog. And the presence of clouds has a blanket effect keeping heat from rising.

Once the sun comes up and the temperature rises, the sun “burns off” the fog by warming the upper portion of the cloud layer. The denser the fog, the more time it takes to get rid of it, because the fog burns off starting from the top of the cloud layer.

As far as the average traveler is concerned, trying to predict when it’s going to happen could be an exercise in futility. The best bet is to consult the weather reports.