Advection Fog

Operations Duty Managers

Reductions in visibility can impact operations any time it deteriorates below 2SM. True impacts at an airport depend on the capabilities on the airfield specific to each runway, including the available instrumentation and lighting capabilities. Not all runways have the same capabilities, making it possible that the best runway for foggy conditions is not available because the prevailing wind is forcing operations on a different configuration. For example, in CYUL it is only 06L that has the best capabilities to operate in foggy conditions (CAT II operations). No other runway at the airport is certified to that same level, meaning that foggy conditions requiring CAT II or III (worse) while on a 24L/R or single runway 06R operation will lead to reductions in flow far more drastic than delays already felt if 06L was available. Of note specifically to forecasters: this specific caveat can change the landing/alternate limits based on which runway is active (among other factors specific to aircraft type/crew certification).

• Fog is responsible for some of the most high impact weather events at airports. The TAF is closely monitored to determine duration of the fog and a conservative approach is used in anticipating its lifting as fog can be difficult to forecast.
• Surrounding airports are closely monitored to give a better sense of when fog will move in/out. Upstream observations are also closely monitored to see how low conditions drop and if there is any variability, and are compared to TAFs when possible to see how well it has been forecast.
• Discussions with forecasters are key here, along with a strong watch of current conditions, assessment of satellite imagery when possible, and communication to customers.
• When/if fog occurs during higher traffic periods at a major airport, the questions become how long it will last and how low conditions will get. Longer lasting events when demand exceeds capacity are likely to need targeted traffic management initiatives.
• Operations become particularly difficult when visibilities drop below 1/2SM and when they drop to a 1/4SM or less, become even more challenging with arrival rates with the possibility of TMIs depending on throughput.
• The TAF and Forecaster Notes usually provide the first indication of fog. 

Because of the difficult nature of forecasting fog, additional weather forecast products can be referenced to determine its extent and severity. Wind direction off Lake Ontario with a close temp/dew point spread can be a key indicator along with conditions at Billy Bishop. Surrounding stations on a METAR tool can also be good indicators when coupled with webcams (Burlington, Billy Bishop, or Brampton).

• Mostly NTMU depends on the TAF and detailed discussions with the forecaster regarding fog. Fog causes greatly reduced operations when the visibility is reduced to or below approach limitations, greatly reducing the arrival rate and also reducing the ability of aircraft to move on the field.
• Fog is always seen with a great deal of uncertainty because it can be difficult to predict its formation and dissipation.
• In CYYZ, fog can reduce the arrival rate to zero or 10 arrival rate in the worst fog conditions. With fog usually occurring during a spring or fall morning for 4 or 5 hours, it can disrupt the schedule for the entire day, making it difficult for the airlines to get back on schedule.
• It is difficult to plan the recovery as the fog dissipation is so difficult to accurately predict and often happens quite rapidly leaving many unused arrival spaces on its initial dissipation.

ATC (Major Tower)

Fog impacts at major towers are dependent on multiple factors specific to aviation. These factors include airport equipment, reduced/low-visibility plans, runway lighting and certification, combined with airline-specific capabilities and crew training. The same visibility and ceiling may have two completely different resultant impacts at two different airports, depending on all the factors listed above. Fog events require some basic preparation.

Examples below are for CYUL:

• The tower advises the Airport Duty Manager (ADM) to secure the sensitive area of the ILS CAT II approach (an approach that allows some of the lowest landing limits) and about 15 minutes later, we are confirmed that everyone is ready. We often prepare for the worst case (foggy conditions) but oftentimes, the fog doesn't come to pass. The presence of water near the airport (CYUL) often leads to unexpected fog. For example, a layer of fog 75 feet high can advect from the St. Lawrence to cover the entire airport. The tower can be clear of fog with 15NM visibility but the aircraft below can have difficulty moving.
• Fog banks that are only partially over a runway can still greatly impact operations, so as soon as there is mention of fog, preparations are made. As an example: one morning there was fog at the airport, from 1/3 down runway 06L/06R. Ahead of this were VFR, clear conditions - however in the fog conditions were in the CAT II range. This setup still required additional spacing between aircraft and restrictions on both departures and arrivals.

ATC (Regional Tower)

Fog impacts at regional towers are dependent on multiple factors specific to aviation. These factors include airport equipment, reduced/low-visibility plans, runway lighting and certification, combined with airline-specific capabilities and crew training. The same visibility and ceiling may have two completely different resultant impacts at two different airports, depending on all the factors listed above.

At CYAM (Sault-Ste-Marie, ON) fog is major in the spring and fall. Reduced visibility requires extra caution moving vehicles and aircraft around the field. Because CYAM is surrounded by water on 3 sides we often get fog over half the field and not the other half. Poor weather observations and forecasting of fog can result in unplanned reductions in serviceability.

Fog, in Aviation, basically means a ground-based cloud layer which obstructs Visibility to ½ SM or less (METAR abbreviation FG).

Mist (METAR abbreviation BR) is the reporting term used to describe a ground-based cloud layer that obstructs visibility to greater than ½ SM, but less than 6 SM. A related data point here is dewpoint depression (or spread). A specialist monitors this information at a properly equipped Weather Observing Site and can in this way verify the obstruction to visibility as being mist, versus another hydro or lithometeor (drizzle/haze/smoke/dust, etc).

This is vitally important information, because the presence of mist or fog in sub-zero weather also infers icing, potentially adversely affecting all airframes and carburetor-equipped aircraft in the area.

Fog has another important impact on flight operations. Good visibility is essential to Visual Flight Rules Operations in general, and IFR Operations during the departure and arrival phases (and taxiing as well, under very low visibility). Anything which restricts visibility is going to be of interest to all pilots.

The formation of fog or mist can take several paths. The mechanism of formation can be roughly equated with the intensity and duration of the phenomenon at the observation point. Frontal or advection fog tends to be thicker in density and longer in duration (hours or even days), versus radiation fog, which forms “pools” in low open ground under clear skies and dissipates in minutes or hours once sunlight reaches the surface of the earth. Upslope fog can last as long as the wind and temperature/dewpoint spread support it.

FIC

These different characteristics will define how an FSS briefs a pilot of the threat – i.e. if there is some radiation fog at the destination airport giving IFR conditions at the time of departure, a Specialist may be able to tell the pilot that by the time they arrive overhead their destination, it will be CAVOK. If there is a warm front arriving over the destination, though, the duration and density of frontal fog make this a potential “showstopper” for VFR pilots and maybe even IFR flights. There are other variables which come into play, but these examples illustrate the main differences of interest to Aviation Support staff.

AAS

As mist/fog can often be affected by local effects, and minute shifts in wind and temperature, Advisory Specialists are always very attentive when BR/FG is occurring, especially when the temperature is near or below freezing.

• Potential to significantly reduce airport/ATC capacity due to low visibility and CAT II operations. Significant probability of losing approach capability due to RVRs below required minima.
• Fog and its intensity tends to be difficult to forecast- in addition to the TAF, we often rely on the tower/FSS personnel who relay reports on visibility, which we can then pass along to pilots.
• Another tool we have is the live RVR. For example in CYUL, the RVR monitor indicates 6000ft most of the time, and turns red when conditions go below 6000ft, instantly sending a flag that visibility is deteriorating. When available, pilots will often want to do the CAT II ILS (we do not have CAT III in Montreal Terminal). CAT II requires us to bring aircraft slightly further on final, and we also need a lot more spacing between each arrival (often 8-10nm between arrivals instead of the usual 3), as the aircraft has to be off the runway and clear of a critical zone before the next arrival gets into its final approach phase. Requiring more space between arrivals has a domino effect, the Terminal gets backed up, we ask for more space to the enroute, they get backed up, aircraft have to hold, etc. Since fog is hard to predict with precision, when this happens we can suddenly find ourselves with too many aircraft, but thankfully it doesn’t happen very often.

Fog has a large impact on ATC operations. When visibility is extremely low and the corresponding RVR dips below company thresholds, aircraft cannot land at airports and must hold. This is not complex on its own, but the main issue that arises with fog and low visibility are that different airlines have different minimums for flying approaches. Sometimes within a company, one aircraft can land at an airport with low visibility while an identical aircraft cannot (depending on pilot qualifications.) This makes for difficult sessions for air traffic controllers as we need to question everyone on what their minimums are and then pass them in front or clear them out of the way for aircraft who can land. The lack of consistency and the unpredictability create very complex situations coupled with rapidly changing conditions on the ground. For example, Pilot A informs ATC that they require 2600 RVR and 1/2SM visibility and Pilot B needs 1400 RVR and 1/4SM visibility. The conditions on the ground are 2800 RVR, so both aircraft can land. Conditions then deteriorate and drop to 2000 RVR. Pilot A needs to be removed from the ATC sequence so pilot B can fly an approach.

• Thick fog could prevent aircraft from land which can create a backlog of flights all the way up to high-level controllers.
• Persistent fog can trigger ground stops for arrivals and there is often a reduction in the amount of flights that can take off.
• Workload would increase in the high-level with potential holding and re-routes should the fog not dissipate rapidly.

VFR general aviation (GA) pilots can be significantly affected by fog given that it reduces visibility well below minimum legal limits, leading to delays and/or cancelled flights. Fog can be variable and unpredictable once it sets in and even if it appears to be dissipating, it can reoccur under certain conditions. It can also occur on a very small scale (example: a lake), which can have significant impacts for VFR float planes but due to how small the feature may be may not be captured in forecast products. For this reason, it is important to look at the factors contributing to the formation and predicted dissipation of fog, information that could be received from a FIC weather briefing. This is also where PIREPs become invaluable. Having information, especially in/over regions that do not have TAFs or observations, can provide pilots, forecasters, and FIC specialists with a better understanding of where this phenomenon is occurring.

TAF, GFA are often used to predict fog and to a GA pilot often flying away from larger airports, the GFA can sometimes be more helpful because it describes conditions over a larger area than the TAF might not account for, but which might still affect the airport or area you will fly in. Auto observations can help a lot, as well as webcams for these airports. In addition to predictions for fog directly, pilots should be aware of the conditions that can produce fog (Temperature/dewpoint spread, wind, temperature history, geography, etc) and to be on alert for those conditions, even when fog is not predicted.

Fog seriously impacts visibility, so little difference is made between PROB/TEMPO/FROM if using the TAF product alone, that is why a good GA pilot will consider the products and factors described above. For GA flights and flight training, fog can lead to flight cancellations (loss of income) or diversions (increase in operating costs).

Impacts also vary based on the type of fog and how well it can be forecast. Understanding the risks and the type of fog forecast (radiation fog, advection fog, frontal fog) becomes important to VFR pilots, from both a flight planning and safety perspective.