What Is a Fog Bank?

Fog

Fog forms when water droplets in the atmosphere clump together and create what is essentially a cloud at ground level. This is caused when the temperature and the dew point of the air are almost equal. The dew point is a meteorological term for the point at which water vapor in air condenses to form fog. Usually the air is cooled at constant pressure to reach the dew point. A fog bank forms when warm air passes over cold water.

People living in coastal areas, such as southwest Florida with its warm, humid climate, are likely to see fog banks fairly frequently in the winter months. The reason for this is that the sea temperatures are colder than the moist, warm air traveling over it. When air blown from a warmer region where water temperatures are also higher, arrives over a coastline where the sea is perhaps 10 degrees lower, the cold water starts to reduce the temperature of the air just above it. This action takes the air down to the dew point and forms a bank of sea fog. Wind direction and speed can also affect its formation.

A fog bank is unlikely to form if the winds are significantly warmer than the sea temperature and are drier. This is because a greater difference in temperature between the sea and the air makes it less likely that the air will cool to the dew point, a key factor in creating fog. Also, if the wind comes from less humid areas it contains less water vapor, and fog is less likely to form. Another wind factor that makes fog less likely is if the winds are traveling at speeds of 10 to 15 miles per hour, because the air is churned up and the water droplets can't clump together.

Fog banks don't often travel far inland. The reason for this is that as the fog bank comes ashore, the land is warmer than the sea and this warms up the air in the fog bank and dissipates it. Meteorologists warn that although the fog bank may have cleared around the coast, it can still pose dangers for boats further out at sea. Other factors that keep a fog bank away from beaches are cold fronts bringing drier, cooler air, or winds blowing from land to sea that keep fog off the coast.