This article will be appearing in the next edition of Canadian Aviator
Tis the Season…Fog Season
(Foggy IFR, Foggy Flying, The East Coast Veil, Obscurity , Obscure Fog, Advection Fog)
Most pilots know there are six types of fog: advection, radiation, upslope, frontal, steam and ice fog. Canada’s East Coast will be entering fog season annoying pilots from early April until well into the summer due to advection fog, but this type can loom anytime of the year if the winds blow in from the ocean.
If you are flying out East during fog season, expect numerous challenges. Airports like St. John’s, N.L (YYT), Halifax, N.S (YHZ), Sydney, N.S (YQY), Yarmouth, N.S (YQI), St John, N.B (YSJ) is where thick fog lurks. Growing up in Halifax, I thought it normal living in a continual thick blanket of fog this time of year. Not until I moved to Ontario did I realize the abundance of VFR.
Advection Fog (also called sea fog) forms when warm moist air advects over a relatively cool surface (water or land). Advection is the movement of air horizontally not to be confused with convection, which is the movement of air vertically. Where the warm waters of the Gulf Stream collide with the cold Labrador Current it’s conducive to forming some of the foggiest places on the planet. Advection fog is not as dependent on wind speed as it is on wind direction. Many claim this fog can lift to a stratus layer under strong surface winds, and indeed this does happen. However, when the cooling is extreme, like over the very chilly waters off Canada's eastern shoreline, think thick fog! You will often find gale-force winds and be unable to see beyond a few feet. The world’s foggiest place is found off the Grand Banks of Newfoundland, where the Gulf Stream and the Labrador Current butt heads. Your career may see you flying supply helicopters to oil rigs located in those waters. And where are the choppers based? At the foggiest airport in Canada, St John’s, of course! In a past article, I mentioned St John’s, a.k.a Torbay, is the windiest, cloudiest, second rainiest, foggiest city with the most freezing precipitation in the country. It goes without saying it has Canada’s lowest VFR ranking, about 65% in the summer lowering to just 62% in the winter.
As mentioned, wind direction is a very important factor in the formation of advection fog. For example, advection fog is almost a definite at the Halifax International when the winds are persistent from 070 degrees (true) to 240 degrees. This low lying IFR menace does not form in Halifax when west-to-northerly winds blow.
But don’t think advection fog is strictly an East coast thing. The west coast of Canada and the U.S also gets mired in it. Cold waters hug British Columbia and the States of Washington, Oregon and California and all the way south to the tip of the Baja Peninsula. While Vancouver sees some advection fog, it’s the west coast of Vancouver Island that is the real home to this chilled air mass. Sometimes you’ll see advection fog form over Lake Ontario and move inland to Toronto. Advection fog can be enhanced when a relatively moist air mass overrides a snow-covered terrain. The solid snow may sublimate directly into water vapour, adding to the low-level moisture. This process is conducive to very dense fog.
It takes most of the winter to cool the waters off the East Coast, so “fog season” plagues the Atlantic Provinces for months. The year when I moved back to Halifax as a commuting pilot, the airport authority decided to shut down their two ILS approaches for runway work in July. They had been informed there was minimal chance of fog forming at that time of year. Wrong! I had to book off my first pairing that summer. I couldn't get to Toronto to work my scheduled flight to Paris, France because there were no airplanes! Numerous flights were cancelled.
Advection fog can move well inland and be enhanced by hilly terrain (YHZ is about 15 miles inland on a high point). This type of fog may retreat to the coast during the day as the sun burns it off, but will quickly return as the evening cools. However, a cloud deck may impede the sun from burning off the fog, in which case it will remain until a wind shift causes it to break up. This fog can persist for days and wilt a person’s spirit. If it thickens, drizzle may form, which can cover an extensive area. But no matter its diverse guise, fog is every aviator’s challenge.
My attempt at poetry…
It prances in diverse guises
It marks its misty presence as it ascends a hill little by little
As a warm wind moves over chilling waters it will form an immense white blanket
It can stay for days and wilt a spirit
Or come and go at the onset of dawn
It may accompany a gale obscuring a pilot’s line of sight to mere feet
It can ally with warm raindrops inducing low visibility
Or play havoc in bitterly cold Arctic air
It can be a sign of seasonal change as it lunges from warm water
No matter its origin... it will challenge any aviator…
Pilots should always be looking at the spread between the temperature and dew point. If the spread is 2°C or less, anticipate FOG!
Fog (FG) is when visibility lowers to less than 5/8 of a statute mile whereas mist (BR) is 5/8 of a mile or more.
BR for mist is derived from the French word Brume.
When flying on the fog-infested East Coast, the answer a pilot usually gives as to when he saw the lights on an ILS approach, is a curt “minimums!” The truth may have had been stretched a bit as to when they actually saw the lights. (ahem)
Urban myth: It’s claimed the Halifax airport location was chosen because it was in a region of reduced fog, but when the trees were cut down to build the airport, fog materialized. My take is any airport near the Atlantic coast, especially one built on the highest terrain in the area, will be conducive to fog. Rather than burning off the fog from the heat of the trees, the clearing allowed the low clouds to reach the surface! St. John’s, Newfoundland suffers from the same plight, as it too is perched upon a hill.