A fog is just a cloud at the ground. Fog formation can occur in two ways. First, the air is cooled to the dew point which leads to the formation of fog droplets. When the air temperature is the same as the dew point temperature, condensation occurs on tiny particles floating in the air. The second method of fog formation requires water to evaporate from the surface into the air, raising the dew point until condensation occurs.
Fog often dissipates with daylight. This is sometimes referred to as the fog “burning off” but that analogy is not correct. When the sun rises, the air and ground warm up. This leads to the air temperature being warmer than the dew point temperature, which causes the fog droplets to evaporate.
As the nights are getting longer in autumn, we often have clear and cloudless skies at night. The conditions help the ground, and the air near the ground, to rapidly cool during evening are clear and cloudless skies and long nights. These are the type of conditions we often have in autumn. As the air cools during the longer night the relative humidity increases, which can result in to fog formation. Windy mornings are fog free as strong winds mix the air near the ground with the drier, warmer air above.
As autumn progresses, we will see a fog during the day forming over the unfrozen lakes. This fog, called evaporation fog, forms when colder air moves over warmer water. Evaporation fog over a lake gives the appearance of steam rising out of the water and is sometimes referred to as a steam fog.
