The Types of Fog

Causes and effects of humidity


What kind of fog are you in? When I was a weather forecaster in the United States Air Force in the 60s, two of my main concerns were

Would fog form tonight? And, if so,

When would it dissipate?

This article will discuss the

Definition of fog,

Various types of fog, and

Problems weather forecasters have in prognosticating the formation and dissipation of fog.

Fog is clouds at or near the surface. Clouds form when humidity reaches 100 % or when free air cools to the dew point temperature.

In the 60s, we referred to the two main types of fog as “warm fog” and “cold fog.” Now they are referred to as “advection fog” and “radiation fog.” Two other types of fog have crept into normal meteorological terminology. These are “upslope fog” and “evaporation fog.”

Advection Fog results form the movement (advection) of warm, moist air over a colder land mass. During the winter this is common when snow covers much of the Midwest. The snow cools the bottom portion of the moist air mass resulting in condensation. This type of fog can be widespread and hazardous to sir travel.

Radiation Fog is caused by cooling close to the earth’s surface. On a clear night heat from the earth’s surface radiates into space. If the temperature at the surface drops to the dew point, radiation fog can form. This type of fog is usually thin and is often referred to as ground fog.

Upslope Fog is common near the Rockies especially around Denver. When the wind is out of the east, the air rises in elevation as it approaches the mountains. This can cool the air to its dew point resulting in widespread fog.

Evaporation Fog, or mixing fog, forms when sufficient water vapor is added to the air by evaporation, and the moist air mixed with cooler, drier air. The two most common types of evaporation fog are steam fog and frontal fog. Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air, the moist air cools until its humidity reaches 100%, and fog forms. Frontal fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. When enough rain has evaporated into the layer of cool surface air, the humidity of this air reaches 100 % and fog forms.

The definitions of the various types seem simple enough. Just get the air to reach 100% relative humidity and fog forms. Since fog is a major nuisance to travelers, whether by driving or flying, its proper prediction is extremely important.

To make a proper prediction, the weather forecaster must consider

The synoptic situation (the position of highs and lows and various frontal boundaries),
Climatology of the station,
Air-mass stability,
Amount of cooling expected,
Strength of the wind,
Dew point-temperature spread, and
Trajectory of the air over favorable types of underlying surfaces.

There are many factors to each of these. Thank God for computers. In the 60s, we had to plot all this information on charts before analyzing it.

On those foggy late nights when you are wending your way home, or early mornings when you are commuting to work, or on those days you find your airplane grounded in Seattle, have sympathy for the local weather forecaster, and ask yourself, “What type of fog am I in?Write your article here