Fog is a cloud on the ground. It is a situation in which the air is humid enough that cloud particles of moisture condense out of the air. The most likely time for fog to develop is in the overnight hours. This is because at this time the air is generally cooling off and the temperature is thus dropping closer to the dewpoint. Since the coolest temperatures of the day generally occur around sunrise, it is not too surprising that this time also experiences the most fog typically. Once fog develops, it will persist as long as moisture can continue to condense out of the air. Once the condensation process is slowed by rising temperature or other factors, the fog will begin to dissipate.

It is in the morning hours that temperatures generally warm at the fastest rate. When comparing the 6 am to 11 am time frame to the 11 am to 4 pm time frame, generally much greater warming will occur in the 6 am to 11 am time frame. This is because on many nights a shallow layer of cool air will pool at the surface. Long wave ground emission contributes to cooling the air. This effect is most noticeable when the winds are fairly light since less mixing will occur with air higher aloft. Once the sun starts shining on the earth’s surface, the shallow layer of cool air will quickly mix out with warmer and often drier air aloft. Also, warming takes place from the sun warming the ground surface which in turn warms the surface layer of air.

It is the early morning sunlight hours that fog tends to dissipate. Under certain meteorological circumstances, fog can persists all day long and can develop at times besides the overnight hours. In general though, fog develops overnight and dissipates (mixes out) in the early morning sunlight hours. When the air warms, the temperature will increase above the dewpoint. Generally when conditions are saturated in the air at the earth’s surface, the temperature will equal the dewpoint (Relative Humidity 100%). Once the air warms, typically the temperature will increase above the dewpoint value. This will cause the tiny cloud (fog) droplets to evaporate. Also, as the sun warm’s the surface, convective thermals will start mixing out the air (think of mixing as like water mixing when it is boiling by being warmed by the burner below). These two processes (mixing out of surface air with drier air aloft and the temperature warming) cause fog to decrease in density and then eventually dissipate altogether. On a foggy morning, this process will often play out thus it is a meteorological treat to watch!