WX BRIEF

Flying in fog

An insidious danger is posed by common atmospheric conditions. What to do when forward visibility is gone.




Extremely thick radiation fog fills the river valleys of West Virginia early on an August morning. Mixing of the fog layer is evident in the cumulus-style undulations in the fog tops. Any valley airport in this region would have had near zero visibility until at least an hour or two after sunrise.

In the tropics, this may only be a few degrees, while in arid climates it may be 30 or 40 degrees. In regions that experience a distinct rainy season, that season will also be when fog occurrence is at its peak. Con­versely, distinct dry seasons are also times when fog is largely absent.

When deciding whether or not your airport may be affected by fog, it is important to pay close attention to the temperature and dew point in the area (especially at the airport). If they are close, fog is a possibility that will be made more likely if the skies are cloud-free.

At airports near rivers, lakes or swamps, pay attention also to the larger-scale movement of air. Look at the temperature and dew point upwind of the location. A cold, humid air mass moving over the region could set up steam fogs.

Likewise, if your region is snow-covered, a warmer, humid air mass could generate a thick advection fog.

Lastly, look at the direction and strength of the surface winds. Moderate and strong winds inhibit fogs, but light winds can promote them, especially if they are blowing humid air onshore or upslope. Calm winds do not usually support fog. To thicken from the surface, fog requires at least a little mixing, which isn't supplied in a dead calm.

Flying fog

If fog is present or is forecast, expect that you will have to deal with it. Many airports are in low-lying or coastal areas that are prone to fog. From above, all but the thickest fogs are largely transparent.

While it is more noticeable during daytime hours due to sunlight reflecting off the fog top, the fog will also create a haziness to surface features, with muted colors and reduced depth perception. At night, lights will appear fuzzier and fainter or more distant. If things do not look crisp, assume there is some reduction in visibility.

The real problem with fog is that even a thin fog can reduce slant and horizontal visibility to near zero. The second you enter the fog layer you stand a good chance of losing any visual reference to the surface, even if you had it at a higher altitude.

If you lose sight of the runway at those altitudes, the only prudent course of action is a go-around—not waiting to see if the runway reappears at a lower altitude. Even if you make it to the ground safely, there remains the question of whether you will be able to see far enough down the runway to avoid another aircraft that is inadvertently crossing the runway at midfield.

Similarly, even if all the taxiways are shrouded in fog, a pilot on the tarmac might look up into a clear sky. It is tempting then to try a takeoff and quickly clear the 50 or 100 ft of fog at the surface, but you don't know if any other pilots are thinking the same thing, and if so, where they are.

The worst aircraft disaster on record occurred when a departing Boeing 747-200 collided with a back-taxiing 747-100 in heavy fog at TFN (Los Rodeos, Tenerife, Canary Islands, Spain) in 1977, killing 583 people.

Some aircraft are equipped to land in fog, and taking off in low visibility is not necessarily prohibited. Rather, a pilot must evaluate the potential dangers of continuing on in such low visibility so close to or on the ground. When operating in fog, you are operating the same as if you were in a cloud—because you are. You will have trouble seeing other aircraft and they will have trouble seeing you.

You also have the pot­ential to lose visual reference to runways and taxiways without much warning, and you have no guarantee that other pilots are where they (or the controllers) say they are—they may in fact be lost in the fog.

An understanding of the nature of fog, coupled with a good briefing and assessment of the weather conditions should give you a handle on whether any fog is likely to linger and thicken—or whether you can expect a clear takeoff after going back into the FBO for another cup of coffee. As always, if you do encounter fog, please file a Pirep so those coming after you will know what to expect.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.



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