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  • Fog Forecasting
  • Good Outline/Rules of Thumb
  • Radiation Fog Ingredients
  • Forecast Advection Fog



    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

    Fog requires a mixing action by wind; without wind, dew will appear instead of fog. 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 radiational cooling.

    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 the atmosphere.

    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.


    Fog is formed when air at or near the earth's surface becomes saturated by any of the three processes - cooling, addition of moisture, or mixing with another air parcel. Generally, fog forms in a stable air mass environment. Fog does not generally form with an unstable atmosphere because vertical mixing results in convective or layered cloudiness.

    The cooling process may involve one or more of the following:
        A.  Nocturnal radiational cooling of the earth's surface, which results in cooling the lowest
              layer of air near the surface.
        B.  Advection of air over a colder surface.
        C.  Adiabatic cooling due to lifting an air parcel either by force lifting or orographic lifting.
        D.  Evaporative cooling; precipitation falling through drier air.

    The addition of moisture to an air mass may be brought about by one or more of the following:
        A.  Advection of moisture
        B.  Evaporation from a wet surface.
        C.  Evaporation from falling precipitation.
        D.  Turbulent mixing of moisture in the lower portion of the atmosphere.

    Fog classifications:
        1.   Radiational
              A.   Ground fog
              B.   Continental high inversion fog
        2.   Advection (sea or lake fog)
        3.   Combination
              A.   Upslope fog
              B.   Advection-radiation fog
              C.   Frontal fog

    General rules of thumb for fog forecasting:
       (1). If the mixing ratio increases with height in the boundary layer, consider forecasting
              fog. This assumes clear skies and no dry air advection
       (2). If the mixing ratio decreases with height in the boundary layer, forecast dew or frost
              (assuming clear skies and no moisture advection).
       (3). If rain ends late in the day, followed by clearing and little wind, consider forecasting



    The prime time ingredients for radiation fog are saturated soil, light wind, initially clear skies, and a low afternoon dewpoint depression. The more factors that are present, the more likely the fog will be. Saturated soils continuously evaporate moisture into the air, insuring the dewpoint depression (difference between temperature and dewpoint) will remain low. Light wind reduces the amount of mixing of air in the PBL. If winds are light, moisture evaporating from the surface will remain near the surface and not mix with drier air aloft. If wind is calm, expect fog to be very close to the ground or non-existent. In a calm situation with a low dewpoint depression and moist soils, expect a thick dew or frost instead of radiation fog.

    Clear skies allow the maximum amount of longwave radiation to leave the earth. The absence of clouds will prevent any of the radiation from being trapped between the cloud and the ground. The more temperatures cool, the quicker the temperature will reach the dewpoint. A low dewpoint depression can occur by adding moisture to the air while at the same time cooling the air. The best way to rapidly decrease the dewpoint depression is for it to rain. An afternoon rain increases the likelihood of overnight fog dramatically. The afternoon rain saturates the soil and reduces the afternoon dewpoint depression. If skies begin to clear at night and winds are from 5 and 10 miles per hour after rain occurred the preceding day, fog is extremely likely. Other processes can produce fog such as upslope flow and contact cooling.

    Radiation fog will dissipate from daytime heating. This is due to surface air warming by conduction and becoming unsaturated. Radiation fog usually dissipates by mid-morning.



    Advection fog is fog produced when air that is warmer and more moist than the ground surface moves over the ground surface. The term advection means a horizontal movement of air. Unlike radiation fog, advection fog can occur even when it is windy. Also unlike radiation fog, advection fog can occur when the skies aloft are initially cloudy.

    The setup for advection fog will often include an advection pattern bringing in warmer and more moist air from the south. The set-up for the ground surface will be a snow covered ground or a saturated ground that has been chilled by cold temperatures before the winds shift back from a southerly type direction.

    Since the ground surface is very cold it will influence the temperature of the air adjacent to the ground surface. This air will be chilled more than it otherwise would be due to the very cold surface ground temperature. If there is snow or moisture on the ground then the air will be cold and moist. When winds shift to the south it will bring in warmer air. This warmer air will be cooled due to the influence of the cold land surface. As air cools the temperature drops closer to the dewpoint. If the mixing of the warmer air with the colder air produces a relative humidity of 100% then fog can form.

    Considering air that is saturated, as the temperature increases the amount of moisture in the air increases at an increasing rate. Warm air that is near saturation will saturate quickly when it is mixed with cold saturated air. This is because the amount of moisture in the air from mixing the air is greater than the amount of moisture needed for saturation at the temperature of the mixed air. The air, instead of supersaturating, will produce condensation in the form of fog.

    For example, suppose air that has a temperature and dewpoint of 0 C is mixed with air that has a temperature and dewpoint of 20 C. The temperature of mixing this air if it is mixed in equal proportions is 10 C. A saturation vapor pressure at 10 C is 12.3 mb. The air with a temperature of 0 C has a saturation vapor pressure of 6.1 mb and the air with a temperature of 20 C has a saturation vapor pressure of 23.4 mb. When this air is mixed the new saturation vapor pressure is (6.1 + 23.4) /2 = 14.8 mb. Since 14.8 is the amount of moisture in the air after mixing and mixed air at 10 C only needs 12.3 mb to be saturated, the air is supersaturated. Instead of the Relative Humidity increasing above 100% though, condensation and fog will form.

    Advection fog will likely dissipate when the colder surface warms or when increasing turbulent mixing (i.e., winds increasing above 15 knots) lifts fog into a low cloud deck. NOTE: When surface winds cease or change direction, fog often continues, but simply stops moving.