METEOROLOGIST JEFF HABY
Fog is a major nuisance to travelers. Whether driving or flying, fog results in travel delays and in some cases
cancellations. Forecasting fog can be difficult, but its proper prediction is extremely important. The proper
prediction of fog can have people better prepared to avoid delays and being late for work. The best preparation
is to leave early for work or school. Leaving early will help avoid the larger traffic jams that can result in
urban areas on those foggy mornings.
The following is a list of the primary processes that produce fog. A combination of several of these factors
increases the likelihood of fog:
1. Saturated air at surface
Fog forms once evaporation into the air results in supersaturation. If the dewpoint depression is small after
sunset, clear skies will cause radiational cooling of the air quickly toward saturation. The dewpoint can
increase due to a rain shower, previously saturated soils and irrigation. Since vegetation evapotranspirates moisture,
fog first forms over grassy and vegetated areas. Fog is common in situations where a daytime shower saturates the
soil, vegetation and boundary layer and then skies clear in the evening into the night hours.
2. Overnight clear skies
Clear skies allow the surface temperature to cool off at a higher rate. If dewpoints are high or the dewpoint
depression is low, saturation of the air will occur over night. Fog is not as likely if the ground is bone dry,
vegetation is sparse or the dewpoint depression is too large.
3. Wet soils and rain dampened vegetation
Wet soils and dampened vegetation continuously evaporates water vapor. This allows the temperature and the dewpoint
to converge more rapidly than would normally be the case. An afternoon shower can cause the dewpoint depression
to be near zero in the evening. If skies then clear and wind is light, fog is very likely.
4. Light wind
If the surface is near saturation, a light wind will allow for the layer of air near the surface to remain near
saturation. High wind speeds cause a mixing of air at the surface and higher into the atmosphere. Since air
higher in the atmosphere is generally drier, high wind dries the air and prevents fog from forming.
5. Slight warm air advection from maritime polar/ tropical air
Warm air advection causes rising air. Even a slight warm air advection can cause just enough uplift to make fog
more likely, especially if the warm air advection is from a moisture source such as the Gulf of Mexico, Great
lakes or Gulf Stream. Polar air and continental air tend to have larger dewpoint depressions than maritime air.
Fog that occurs in polar and continental air is primarily due to saturation from above or large
6. High dewpoint
Warm air 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.
7. Light drizzle and precipitation saturating PBL from above
Warm moist air overrunning a shallow
airmass can saturate the shallow air from above and eventually to the surface.
The contact cooling between the two air masses causes clouds and fog since the moisture in the warm air is beyond
the carrying capacity of moisture relative to the cooler air. Precipitation evaporating into the air can saturate
8. Wind direction from a moisture source
Since moisture is a key component for fog, advection from a moisture source is much more favorable than advection
from a dry source. Widespread fog is
more common with warm fronts than cold fronts. Part of the reason is that
cold fronts tend to bring continental air (central and eastern US) while a south wind
brings maritime warm air.
9. Upslope flow
Rising air lowers the dewpoint depression.