Perhaps the most difficult forecast that aviation meteorologists are expected to produce is the possibility that either fog and/or freezing fog will occur at a particular aerodrome.
Fortunately FPI has access to very detailed 3D profiles, called “skewt diagrams”, which are not available on public websites.
Looking at the upper level conditions overhead the aerodrome greatly assists us to determine if the profile is favorable for fog formation. In many cases, we can provide advance notifications days in advance, which may help in decision making.
In the Northern hemisphere and during the fall and winter months, a low sun angle and limited daylight is a catalyst that encourages the formation of fog.
In addition, what else is necessary for radiation fog to form?
- Nearly calm surface winds
- Surface high pressure overhead or nearby
- Presence of an upper level ridge at 500 mb (FL 180)
- Very light winds aloft, generally less than 10 knots up to 700 mb (FL100)
- Strong, low level inversion which, in combination with the above items, does not allow for significant mixing to occur
- Surface temperatures at or below 0C needed for freezing fog
- Shallow layer of moisture near the surface, with dry air just above the surface, which does not allow for significant cloud formation
- Very narrow temp/dewpoint spread
- Analyzing the conditions from the day before — If the pattern does not change synoptically from day to day, then it is a good bet that similar conditions can be expected the following day.
Terrain is also an important factor to consider. Generally, locations that are lower in elevation are more susceptible to fog formation. Even an elevation change of a few hundred feet can make a significant difference.
A perfect example of this was on Dec. 28th, 2024, when EDDF (Frankfurt Main airport) was reporting visibilities from 300 to 600 meters with indefinite ceilings of 200 to 300 feet in freezing fog with light and variable winds. The elevation of EDDF is 363 feet.
At the same time, EDFH (Frankfurt Hahn airport), located approximately 51 nautical miles to the west of EDDF with an elevation of 1,649 feet, was reporting CAVOK (“ceiling and visibility are OK”) conditions during the same time.
Synoptically on Dec. 28th, 2024, almost perfect conditions were in place for dense freezing fog to form. An unusually strong upper ridge was in place from France to Poland with very light winds aloft. At the surface, very strong high pressure (1035mb) prevailed over much of Western and Central Europe. The airmass aloft was also very dry over Germany with the relative humidity from 5,000 feet to 30,000 feet at around 28 percent. Thus, opportunities for middle and upper level cloudiness were reduced.
Below are a few examples that illustrate how freezing fog appears in three dimensions.
Notice how well the high resolution EURO model outperformed the US GFS model, especially in the boundary layer. It was much too dry, and therefore the US GFS model would not indicate that freezing fog would be expected. It should also be noted how the forecast did not change significantly between the 45 and 33 hour forecast.
(EURO model:)
(US GFS model @ 45Hrs:)
(US GFS model @ 33Hrs:)
Typically, we shy away from presenting any long range forecasted images; however, this particular event was well forecasted at least five days ahead of time based on the EURO model.
Please reach out to FPI with any specific weather-related questions or concerns related to your trip planning. Also, for ongoing weather updates 3x/week by FPI’s resident aviation-specific meteorology expert, Brad Asselin, follow FPI’s LinkedIn page here. FPI’s 24/7 Meteorology/Flight Planning team is fully in-house with experience exceeding 150 years collectively, specifically within aviation. Click here for ten great reasons you can trust in FPI’s weather expertise for your trip management.
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