By Robert N. Rossier, EAA 472091
This story first ran in the June 2019 issue of EAA Sport Aviation.
Depending on your location, as the seasons change, so do the chances of fog. Some locations you are notorious for early summer fog, such as the New England coast where I spend most of my time these days. Although conditions can vary from one year to the next, the cool coastal waters conspire to drop the temperature below the dew point, creating a frustrating layer of fog that seems to come and go of its own volition. Driven by air and water temperatures, humidity levels, and winds, dreary conditions of dense gray that bring flying to a halt can be tenacious.
I remember one June when the fog seemed to roll in at my home airport like clockwork every day at precisely 5 p.m. You could just about set your watch by the arrival of the thick wall of fog rolling slowly across the terrain, creeping through the trees and blanketing the runways. It was a sight to behold from the air. If you weren’t on the ground by the time this plodding gray mass engulfed the airport, you were looking for another airport at which to land, with no prospects of landing here again until sometime the next day.
The primary culprit in this scenario is a phenomenon called advection fog, which forms when moist air blows over a colder land mass. This can happen frequently where large bodies of water cool less rapidly than the surrounding land. Given a light wind, the moist air over the relatively warm water is pushed over the colder terrain, where the air cools rapidly, causing the fog to form.
Clearly, coastal New England isn’t the only geographic area prone to fog. Other meteorological and climatological conditions can cause fog to form as well. But, no matter where fog occurs, it’s a problem for pilots.
The Upslope Upset
Another type of fog we see in the summer is upslope fog, which occurs when moist, stable air is forced upward, causing it to cool adiabatically, reaching the dew point. Bluffs along the coast are a great place for this to occur as the wind pushes nearly saturated air up and over the higher terrain. From the air, it can look like a wave of white washing up over the shore and then spreading over the terrain for long distances.
Such conditions can put an airport out of commission when the wind direction and humidity are in an unfortunate alignment. It can be a beautiful sunny day over most of the area, with the unfortunately placed airport shut down in a dense fog. And all it takes is a slight shift in wind conditions for the fog to clear, or to set back in.
Radiation Fog
Another phenomenon that can surprise us at times is radiation fog, which occurs on a cloudless night when energy radiates into space, causing the ground temperature to rapidly drop. If it drops below the dew point, the overlying layer of air also cools, and fog sets in. This phenomenon is common in many regions, but it had a special impact for me when flying in the mountains of Colorado. At the time, I was based at Jeffco Airport (BJC) and would make frequent flights west into the mountains.
On this particular morning, I had planned to hop across the Continental Divide, stop at Kremmling (K20V) for some fuel, and then continue to Glenwood Springs (KGWS) via Eagle (KEGE). It was early morning when we departed Jeffco. With no reported weather for the Kremmling area, and nobody answering at the FBO, I had only the area forecast by which to plan my flight. It was a sparkling morning on the east side of the Divide, but once I crossed over the pass and turned toward Granby, I was greeted by sweeping vistas of fog-filled valleys. It was a sight to behold, albeit not the one I had been expecting. The sea of fog seemed to consume everything, including Kremmling airport. With about a third of my fuel consumed, I was just a bit concerned. But 20 minutes later, I was in radio range of Eagle and received word that the weather there was clear. Planning and communication are vital elements for a trek into the mountains, even on a gorgeous summer morning.
An Inside Job
Another form of fog you don’t read about is what I refer to as the “inside job” — fog that forms on the interior of aircraft windows. This can occur extremely unexpectedly when climbing from relatively warm and moist conditions into a much cooler layer above. It happened to me one day when climbing through about 500 feet on an otherwise beautiful morning. In an instant and without warning, every window in the aircraft took on a thick coating of fog. Still at low altitude, I was not inclined to descend. Instead, I leveled off at a safe altitude, quickly turned on the defroster, and waited for the fog to “lift.” Fortunately, there were no other aircraft nearby and I was soon flying visually again, but the unexpected transition to “instrument flight” was a bit disconcerting.
Mounting Frustration
Fog can be frustrating not just for pilots, but for anyone who travels by air. One foggy morning in what had been a particularly foggy June, our director of operations finally reached his breaking point. We had all been waiting patiently for hours, and it was probably the 200th time he had been asked the question, “So when is the fog going to lift?” or “When will we be flying?” He finally gave them the precise answer they were looking for, “The fog will lift at 11:47, and we’ll be flying by noon.” The definitive and authoritative answer seemed to placate some passengers, but in truth, there was no way to know quite when the conditions would be good enough for a VFR flight to Block Island.
Frustration continued to mount, with one anxiously waiting passenger making a keen observation. “I just saw some other planes take off. How come we’re not flying yet?” I was tempted to point out that while they did see aircraft taking off, they didn’t see any airplanes landing. Even if local conditions had begun to improve, the fog at our destination was still deep and thick. We never did fly that day.
The truth, of course, is that we don’t know exactly when the fog will dissipate. Despite the improvements in meteorology and modeling of the atmospheric conditions, we can’t seem to pin down exactly when localized conditions will shift, causing the fog to abate. If we hang around long enough and watch the trend, we might get an idea, but it certainly isn’t following any known or predictable schedule. In some instances, it can disappear and reform in a matter of minutes. As one former chief pilot used to say, “Forecasts aren’t like train schedules; there are no set arrival times.”
When we think about hazardous weather, we typically think about convective cells, squall lines, hail, microbursts, and severe turbulence. But the truth is that fog can also be a nefarious foe — one that we should be on the lookout for, and one we should treat with due respect.
Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.