Fog is often an underrated weather hazard. At times, it can be just as dangerous as a severe storm. Fog is most dangerous to travelers. With lowered visibility, it can be much more difficult to judge where other vehicles are. This reduces the reaction time and leads to numerous accidents and accidents where many vehicles can be involved (chain reaction accidents). The ingredients for fog formation are fairly intuitive to understand (saturated air, cooling air, wet ground) but it can still be difficult to forecast if fog will develop and how dense it will be. Often fog advisories occur as the fog event is happening but it can be difficult to forecast in advance. This writing goes over reasons for the difficulty in fog forecasting.

One reason for the difficulty in fog forecasting is that it can be easy to forget about. With all the attention that is paid to high temperature, low temperature and precipitation, fog can be a forgotten weather forecast event. Often fog has to develop before it is remembered that it is an event that is important to forecast for. Fog is not sensational like a severe storm and it is not sought after like knowing the high or low temperature. Often fog is just not a priority and not at the forefront of the mind when developing a weather forecast.

Another reason in the difficulty of forecasting fog is that the density of fog is so variable across the local forecast area. Some locations can have dense fog while others have only a light fog. Fog tends to be densest in wet vegetated area, lower elevations near streams that are cooler and wetter and where emissions from factories add condensation nuclei to the air which leads to a higher density of cloud droplet formation. Fog is a cloud on the ground. Just like the sky can have thinner and thicker clouds, fog can be variable in density. Fog will tend to be thinner where the wind speed is higher, where there is more urban surface and less wet vegetation, over warmer surfaces and sometimes higher elevation surfaces when exposed to more wind and less cold air pooling. It is important for a forecaster to recognize the “fog prone” areas within the local forecast region. These regions are especially important to know about where major highways systems move through these areas. When driving, the sudden increasing density of fog can make highway travel very dangerous.

Fog can be difficult to forecast for since slight changes in weather variables can mean the difference between dense fog and very little fog. Several of these variables are explained below. These are forecast generalizations that have exceptions but often these tendencies will be noticed. These factors work together to help determine how dense fog will be:

Wind: A higher wind speed will mix the air more and this tends to reduce fog density while a light wind tends to increase fog density.

Lifting: Any amount of low level dynamic lifting will help increase fog density. This lifting will help condense moisture in the air. For example, a slight warm air advection pattern in saturated air can produce enough lifting to help develop a dense fog. Fog can also be produced from an upslope flow of saturated air (such as air flowing up a mountain slope). Since rising air cools, if it is already saturated, the upslope flow will ensure a relative humidity of 100% which aids in fog formation.

Wet soil/ground: One of the most important ingredients for dense fog is a wet ground. A previous rainfall that soaks the soil, vegetation and ground can mean the difference between dense fog at the surface and a light fog at the surface. The wet ground and soil contributes to a constant supply of moisture that can be evaporated into the air and when this combines with overnight cooling, it can cause the relative humidity to stay at 100% which increases the likelihood of dense fog.

Overnight cooling: To aid in fog development, it helps to have clear skies at night. The clear skies aids in the ground cooling rapidly at night. This cooling helps bring the temperature to the dewpoint. With a high dewpoint from a wet ground, overnight cooling can help saturate the air and this can lead to dense fog.

Precipitation: Precipitation adds moisture to the air and saturates the ground which helps raise the relative humidity toward 100%. Fog can many times occur with a sustained rainfall. With light wind and lifting, fog will many times develop.

Cool air pooling: Since cold air is denser than warmer air, the colder air will flow into the valleys. This region also tends to have more moisture due to streams and denser vegetation. Thus, cooler air pooling into valleys can lead to dense fog in these areas.

Lake/Ocean moisture input: Lakes and ocean water add a continuous supply of moisture to the air. When other factors are in place that can cause fog to develop, often fog will be produced densest over and near these moisture sources.