With autumn comes the return of the foggy season

How are the various fog phenomena reported or forecast in METAR or TAF? And what was that again about the different types of fog? What are the chances of it clearing up? And when can we expect dense fog, and when can we expect high fog that is still suitable for VFR? We will address the issue of fog using a few examples so that no questions remain unanswered the next time fog rolls in.

Visibility restrictions in METAR and TAF

By definition, fog is when the meteorological visibility (Met Vis) is less than 1000 meters – in METAR, this is indicated by FG. However, there are also fog phenomena where METAR indicates higher visibility. These special cases are (details below):

VCFG (vicinity fog), Met Vis >1000 meters

MIFG (shallow fog), Met Vis 1000–9999 meters

BCFG (patches of fog), Met Vis < 1000–9999 meters

PRFG (Partial Fog), Met Vis< 1000 – 9999 meters

FZFG (Freezing Fog), < 1000 meters Vis

However, haze is also an important issue in autumn. This is reported as BR when the Met Vis is 1000-5000 meters due to a moist ground layer. The term comes from the French word brume and is also called mist in aviation jargon. Moist haze usually occurs in the morning when fog dissipates or in the evening when fog forms. BR can also occur throughout the day in stable high-pressure conditions in autumn. With the upper limit of the haze in the afternoon hours, the fog upper limit for the following day can be easily estimated in persistent weather conditions. BR is therefore considered a good indicator of the humidity and thus the fog susceptibility of the ground layer. So if you read BR in the METAR, you should also consult the TAF for the possible time of fog formation.

METAR example: VRB01KT 4200 BR NSC 14/10 Q1016 NOSIG

VCFG

If there is fog outside the airport area but within a 16-kilometer radius of the airport, this is indicated by Vicinity Fog VCFG. VCFG means that fog is also possible directly on the airfield. For TAF and METAR TREND (2-hour forecast at the end of the METAR for LSZH and LSGG), note the corresponding time stamp.

Example METAR: VRB01KT 9999 VCFG FEW070 11/11 Q1016 BECMG PRFG

MIFG

MIFG (MI stands for the French word mince) is ground or grass fog that is no higher than approx. 2 meters. In aviation terminology, it is also called shallow fog. With this type of fog, the Met Vis can be more than 1000 meters, as the Met Vis is determined at 2 meters above ground level. MIFG occurs mainly in early autumn after clear nights. As a rule, meadow fog is harmless and usually disappears 30 minutes after sunrise. However, MIFG can also be a sign of further fog development, especially if it occurs during the night: study TAF and METAR TREND.

Example METAR: VRB01KT 4500 BCFG NSC 08/07 Q1016 BECMG 1200 PRFG

PRFG

With PRFG (Partial Fog), parts of the airport are in fog. PRFG is usually reported in combination with a sector visibility. PRFG can occur when fog is forming or dissipating and is an indication of a change in the situation: pay attention to TAF and METAR TREND.

Example METAR: VRB01KT 7000 0800N PRFG FEW070 Q1016 NOSIG

FG

FG is reported when the entire airport is covered in fog. If temperatures are below freezing, FZFG is reported. FG can persist for longer depending on the fog situation. The TAF forecasts whether and when a change can be expected. Normally, the cloud base is no longer visible in fog. Instead of the cloud base, the vertical visibility VV is reported. At the VV level, the pilot can see the runway lighting. Because this is a vertical distance, the unit is the same as for the cloud base in Hekto FT.

The METAR and TAF designations can be found in a handy format in the MeteoSwiss leaflet “Aviation weather information in Switzerland.” The brochure can also be downloaded electronically at:

https://www.meteoschweiz.admin.ch/service-und-publikationen/publikationen/verschiedenes/2023/flugwetterinformationen-in-der-schweiz.html

Image source: MeteoSwiss

Safety first, every flight, every time.