Fog in aviation - more than just poor visibility
For pilots, fog means much more than a temporary restriction of visibility. It affects take-offs, landings and sometimes entire flight routes. While the term „fog“ is generally used to describe any reduction in visibility, meteorology has a precise definition: fog is present when the horizontal visibility falls below 1 000 metres falls. In the METAR weather reporting system, the code FG (fog) is used. Above this limit, meteorologists speak of Haze (BR), when visibility is between 1,000 and 5,000 metres - a typical autumn phenomenon in stable high pressure conditions.
In practical flight operations, this means that as soon as a METAR reports FG, take-off or landing according to visual flight rules (VFR) is hardly possible. This makes it all the more important to interpret the meteorological codes correctly and react to possible fog formation at an early stage.
METAR codes for fog and haze at a glance
In addition to the classic FG, there is a whole series of special abbreviations that describe different types of fog and their intensity. This information helps pilots to better assess the situation:
- VCFG (Vicinity Fog): Fog in the surrounding area - within 16 km of the aerodrome, but not directly over the runway. An indication of possible fog expansion.
- MIFG (Shallow Fog): Flat ground or meadow fog, usually only up to 2 metres high. Often occurs on clear nights and dissipates shortly after sunrise.
- BCFG (Patches of Fog): Fog banks or patches, often harbingers of a closed fog cover.
- PRFG (Partial Fog): Partial fog over parts of the aerodrome. Typical during transitional phases between fog formation and dissipation.
- FZFG (Freezing Fog): Freezing fog, dangerous due to risk of icing on wings and propellers.
A special case is the code BR (crap). It indicates damp haze with visibility of between 1,000 and 5,000 metres - often the intermediate stage between clear visibility and fog. BR often occurs in the evening when fog forms or in the morning when it dissipates. If BR persists throughout the day, this indicates a moist base layer and is a reliable indicator of possible fog formation the following night.
Sample METAR messages
Example:
VRB01KT 4200 BR NSC 14/10 Q1016 NOSIG
→ Light wind, visibility 4 200 metres, humid haze, no significant changes expected.
Or:
VRB01KT 9999 VCFG FEW070 11/11 Q1016 BECMG PRFG
→ Fog in the neighbourhood, light clouds, tendency to partial fog (BECMG = becoming).
Such details are particularly important for flight schools and private pilots in order to plan take-offs and landings in advance.
The different types of fog - from the ground to the heights
The most common type of fog in Central Europe is the Radiation fog. It forms on clear, windless nights when the ground radiates heat and the layer of air near the ground cools down. If the temperature drops to the dew point, dew forms first, followed by moist haze (BR) and finally dense fog (FG). After frosty nights, this can develop into freezing fog (FZFG).
A special case is the High fog - a closed layer of fog hovering above the ground. In METAR it is classified as OVC030 or similar when the lower limit is around 3,000 feet. High fog typically occurs on the Swiss Plateau and in southern Germany at Bisenlagen or inversions. For VFR pilots, this means that flying under high fog is often possible as long as the base is high enough.
Other types of fog are:
- Advection nebula, which occurs when warm, moist air flows over a cold surface - e.g. on coasts or lakes.
- Orographic fog, which forms on mountain slopes when moist air rises and cools. It is particularly common in the Alps and the Jura.
Fog forecast and dissipation - what pilots should look out for
How long fog persists depends on temperature, humidity, wind and solar radiation. In the autumn months, when the sun is lower in the sky, dense radiation fog can persist well into the afternoon. The decisive factor is the energy balance: the shorter the length of the day and the weaker the radiation, the thicker the fog remains.
It is therefore advisable to look at the following indicators when preparing for a flight:
- Spread between temperature and dew point: The smaller the difference, the more likely it is that fog will form.
- Air pressure change: If the pressure drops or a front moves in, the fog can be mixed - good chances of resolution.
- Wind speed: Even a light wind can „tear“ the fog apart. With a breeze, on the other hand, the fog is often „pumped“ into a stable layer and remains persistent.
- Cloud cover: Medium-high clouds with light counter-radiation prevent strong cooling and can weaken fog formation.
Conclusion: Good preparation is crucial
The fog season presents pilots with special challenges every year. Those who understand the meteorological codes and mechanisms can significantly reduce risks. It is important not only to read METAR and TAF, but also to interpret them - transitional phenomena such as MIFG, PRFG or BR in particular provide valuable information.
On the ground, it also helps to check local weather stations and webcams to track fog development in real time. The following applies to safe flight operations: always be prepared for alternatives in fog - and it is better to stay on the ground too much than to take off in unclear visibility conditions.
With the right knowledge about fog types and their development, autumn remains challenging but not unmanageable - even for visual flyers.
Source references:
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