May 1, 2001
By Thomas A. Horne
Let's say it's early evening and you're busy planning a flight for the following day. The forecast is for high pressure and clear skies all along your route and, to add to the good preflight vibrations, a walk in the night air reveals a crystal-clear, star-studded firmament devoid of a single cloud. There is a bit of a chill in the air, though, and you hurry back to the house to pack up your charts, put fresh batteries in your handheld GPS, and make your suitcase ready. You'll launch at oh-dark-thirty in the morning and be at your destination by noon.
Oh, really? From the hints I've already dropped, you might be able to guess what happens by the time oh-dark-thirty rolls around. A shallow layer of ground fog forms. What caused this to happen? Are some ground fogs denser or deeper than others? How long will it take for the fog to burn off? Is it smart to take off anyway — even if you're an experienced instrument pilot? The answers to these and other questions lie with an understanding of fog basics. Here are four key ground fog "triggers" that should make you wise to its ways:
Beware clear nights. Clear nights mean two big fog factors can exist. In the first place, clear weather implies — in many cases — high pressure. It's a high's descending air mass that creates cloud-free conditions because as air descends it warms, and thus dries any potential cloud-producing moisture. Take a clear sky and match it up with falling temperatures, and you've got a great recipe for radiational cooling. Any daytime heating stored up in the ground wicks readily away by the wee hours of the morning, sent into the open sky in much the same way that a car's radiator allows hot coolant temperatures to disperse in the ambient air. As temperatures fall during the night (actually, surface temperatures reach their lowest values right around dawn, after a whole night's worth of cooling) they can reach the dew point. Let temperatures drop within a few degrees of an air mass's dew point — the temperature to which the air must be cooled in order to reach the water-vapor saturation point — and you've got a big, big chance of fog.
The surface inversion factor. There's a general rule holding that the Earth's nighttime "skin," or surface temperature, is about 5 degrees Fahrenheit cooler than temperatures just five feet (the height of a weather instrument shelter) above it. This implies another fog-enhancing factor: surface inversions. An inversion is a condition where temperatures increase with altitude. High pressure and clear nights conspire to create inversions because the high's descending, warming air mass meets colder air near the Earth's surface. Just where they meet is a big factor in ground fog dynamics. The inversion level may be only a few feet above the ground, in which case someone walking on a ramp may have zero visibility, but someone standing on a stepladder — or in a control tower — may be able to see the top of the fog/inversion layer. Wherever it tops out, it's important to remember that the cooled air below the inversion level is stable — meaning that the warmer air above it helps cap the ground fog.
The water factor. Airports located near rivers, lakes, and coasts are more susceptible to early morning fogs. That's because there are plentiful, nearby sources of water vapor. This extra water vapor helps make the fog denser and more persistent. When you think of all the airports located near rivers and coasts — and there are a lot of them, because their shorelines offer flat terrain, perfectly suited for runways — you realize just how much fog can affect takeoffs and landings over a far-ranging area.
Draining valleys. Cold, dense air flowing down valleys can also help create and sustain fogs in riverbeds. Like radiation fogs, valley fogs are most common in the fall through spring months, when nighttime temperatures are at their lowest and the nights are longest.
When will it burn off? This depends entirely upon how soon the sun can warm the ground. As soon as surface temperatures rise, the temperature-dew point spread widens, and the fog will begin to lift and disperse. If there are clear skies above the fog layer, the sun should do this job by 10 or 11 a.m., when it's high in the sky and temperatures begin rising in earnest. But if there are cloud layers aloft, then solar heating is impaired, and it may be hours — or days, if the clouds are associated with a slow-moving low or frontal system — before enough heat gets through.
Many pilots believe that fog burns off from the top down, that the sun's heat somehow eats away the top of the fog layer and slowly works its way down. Not so. Fog burns off from below, and surface heating does the job. Obviously, if the fog is deep and dense it will take longer to burn off. Want a crude rule of thumb? If ground fog doesn't "brighten" by late morning, don't expect it to lift any time soon. A check of radar imagery or local METARs will probably confirm the presence of sun-blocking broken or overcast higher clouds.
Should you take off? If you're flying under FAR Part 91, that decision rests entirely with you, the pilot in command. Assuming you're instrument-rated, current, and proficient, you could take off in zero-zero conditions and still be perfectly legal. Legal, but stupid! What if there was a problem or emergency immediately after takeoff? How could you navigate back to the runway for a prompt landing? Answer: You couldn't. You'd be grinding along in the soup — on top, perhaps, if you climbed that high — nerves jangled, and with no way to make a legal instrument approach (let alone a visual approach) to a nearby airport. If you had smoke in the cockpit, a fire, a loss of oil pressure, or other dire situation you'd be in deep trouble indeed. And you just left the nearest airport, one that you presumably were familiar with.
A smart takeoff decision hinges on ceiling and visibility. Can't see across the airport? Better not go. Fog lifting? Can you see blue sky when you look straight up? Visibility improving? Then it may be worth a shot.
Landing in ground fog. Arriving late at night or early in the morning? Then make a point of checking in early and often with any ATIS (automatic terminal information service), AWOS (automated weather observation systems), ASOS (automated surface observing system) frequencies, or where applicable, check in with approach control and tower personnel for the latest weather changes. Listen up when it comes to the temperature-dew point spread, visibility, and mention of any indefinite ceilings (ceilings obscured by surface-based phenomena, such as ground fog) or low vertical visibilities (the visibilities upward into an obscuration). At these colder times of day, fog-warning signs can come on quickly. If the temperature is on top of the dew point and reports indicate lowering vertical visibilities, then get ready to divert to an alternate airport with better weather.
When will I see the runway environment? OK, let's assume that, based on a vertical visibility report of 200 feet, you decide to shoot an instrument landing system (ILS) approach to a fog-bound runway. Does this mean you'll see the runway or the approach lights at 200 feet agl? Maybe, but probably not. On the approach, what you see ahead is the slant-range visibility — the view on your flight path, ahead and slightly below you. Slant-range visibility is usually less than the vertical visibility into an obscuration because there are more water droplets in this longer-range view. Someone looking straight up from the runway may see traces of blue sky, but on approach to the runway a pilot may see nothing because of slant-range visibility restrictions.
The worst aspect of the lousy slant-range phenomenon can come when you're maneuvering in the traffic pattern in what seem like VFR conditions tinged with just a bit of fog. From your perch aloft, you can see very well — straight down to the runway. But turn from base to final in this "light" fog and you may suddenly encounter zero visibility as you enter the fog and experience the slant-range phenomenon. This kind of sudden instrument weather is most dangerous at low altitude. It's happened to me more than once: One minute you're on final, with the runway all lined up; the next, you see nothing as you enter the flare and attempt to look down the length of the runway — which has disappeared in fog. The only recourse is to go around or perform a missed approach, and divert to an airport with better conditions.
There are many other types of fog — advection, precipitation, upslope, steam — but ground fog caused by radiational cooling can be one of the most dangerous. It can be more localized than the other types of fog, and therefore more difficult to predict. It can form rapidly, ruin the beginning or end of a flight, and trap the unwary. Most deceptive of all is that it so often occurs when otherwise picture-perfect VFR weather is forecast.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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