Fog is essentially a cloud with its base near ground level. It forms under two main conditions, which are increasing moisture and/or cooling of the air.

Think back to Earth Science class in school. You will remember that our atmosphere (the air mass all around us) is a mixture of several different kinds of gas. This air mass also contains tiny particles of liquid water, that are way too small to be seen. You might also remember that there is a limit on how many of these tiny water particles can be held by an air mass. This top-end limit can change a bit, depending on the temperature of the air mass. It turns out that a warm air mass can hold a larger number of these tiny water particles than a cold air mass. When scientists talk about these tiny particles of water in the air, they call it “water vapor.”

This limit on the amount of water vapor in the air leads to an important question. What happens when this limit is reached? If the moisture in the air is increased, or if the temperature of the air mass is decreased, the limit can be reached. When this occurs, the air mass has become saturated, and some of the tiny particles of water will combine to form larger droplets that are visible, and the result is clouds or fog.

There are several ways to measure this limit on the amount of water vapor. Meteorologists will use something called “dew point,” which is the temperature the air needs to be cooled to reach saturation. When the air mass is fully saturated, the relative humidity has reached a value of 100 percent. At this point, the air cannot hold more water in the gas form and saturation occurs. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation. This is called supersaturation. The higher the dew point, the more moisture there is in the atmosphere.

Fog can be a major nuisance to travelers whether it is driving, flying or boating. Forecasting fog can be difficult but predicting it can be extremely important. The proper prediction of fog can have people better prepared to avoid travel delays and help prevent accidents.

The following is a list of primary processes that produce fog. A combination of several of these processes increases the likelihood of fog.

High Moisture Content at the Surface

The higher the dew point, the less the air must cool to reach saturation or supersaturation.

Many things can increase the dew point, or moisture content, at the surface. The dew point can increase due to a recent or current rain shower, previously saturated soils and irrigation. Evapotranspiration is when there is loss of water from the soil both by evaporation from the soil surface and transpiration from the leaves growing on it. Since vegetation undergoes evapotranspiration, fog will develop over grassy and vegetated areas first.

Wind direction from a moisture source can also increase the dew point. Widespread fog is more common with warm fronts than cold fronts. Part of the reason is that cold fronts tend to bring continental air, while a south wind associated with warm fronts bring warm maritime air.

Warm air also has a higher capacity to contain and evaporate water vapor than cold air. Because of this, fog associated with maritime air tends to be thicker than fogs that form at very low temperatures.

An upslope flow can also increase the dew point. Upslope flow is winds that are directed towards a mountain or slope. Depending on how steep the slope is, air can become trapped at the base of the mountain or very slowly rise up the mountain. Upslope is a way of cooling the temperatures while also raising the dew point.

Clear Skies Overnight

Clear skies allow the surface temperature to cool off at a higher rate compared to cloudy skies. This can cause temperatures to get closer to the dew point, which would then lead to saturation of the air. However, if it is extremely dry regardless of temperature drop or the difference in temperature and dew point is too large, fog will be not as likely. If there is limited vegetation, it will also be harder for fog to form.

Light Wind

Fog requires a mixing action by the wind. Without wind, dew will appear instead of fog. However, high wind winds can cause too much mixing of the air at the surface and higher into the atmosphere. Since air higher in the atmosphere is usually drier, strong winds tends to dry the air and prevents fog from forming.

If the surface is near saturation, a light wind will allow for some mixing of the layer of air near the surface while also remaining near saturation.

The next time any of these processes occurs, be on the lookout for the formation of fog.

Source: NWS, NOAA
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Image by Christa Regina from Pixabay

What to Look For When Forecasting Fog