Flying in fog

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

By Karsten Shein
Comm-Inst, Climate Scientist

Deere & Company Citation Sovereign takes off from MLI (Moline IL) in light fog. Fog is a very common flight hazard at many airports and accounts for numerous accidents each year due to spatial disorientation and collisions with obscured obstacles.

As the pilot of the King Air passed over the airport, he could clearly make out the runway and all its markings. He could even see his boss's car waiting on the tarmac. Since he was running a bit late, he cancelled his IFR clearance and opted for a visual approach.

With the windsock hanging limp in this normally windy valley airport, the pilot figured this would be an easy landing. He'd made it dozens of times since the CEO didn't like to drive all the way into the city when he had the small strip so close to his home.

All went smoothly until well into his final approach, when suddenly at 200 ft AGL the fast-approaching runway disappeared in a shroud of white. In his momentary disorientation, the pilot drifted off the centerline.

He started to apply power for a go-around, but just then saw pavement out his left window. Figuring he was still lined up, the pilot attempted a landing, but now only his left wheel was over the narrow runway, while his right main settled into the soft earth and collapsed. Chalk up another aviation accident to a pilot who didn't account for the hazard of fog.

While not as foreboding as a thunderstorm or icing, fog continues to be a factor in aviation accidents. In fact, in most years the 3 are about equal in the number of accidents they contribute to.

Unlike thunderstorms or icing, whose dangers are fairly obvious, fog can lurk undetected until a pilot has almost no time to mount an adequate escape. Often invisible from above, a low fog can lure an aviator to a low and slow landing configuration before obscuring all visual references.

Unfortunately, as this article went to press, a Fairchild Metro and 6 passengers became the latest victims of fog when the aircraft crashed at ORK (Cork, Ireland) after a 3rd landing attempt in heavy fog.

What fog is

Fog is defined by international convention as a concentration of water droplets in contact with the surface that reduce horizontal visibility to under 1 km (0.62 sm). In the US, the National Weather Service also defines fog as a restriction to visibility—less than 6 miles and a temperature/dew point difference of less than 5°F. If the visibility drops below 1/4 mile, the fog is labeled dense.

The same reduction in visibility could be applied to all clouds, and fog has long been referred to as nothing more than a cloud in contact with the surface. Indeed, fog and clouds are identical in their physical properties and both are made up of water droplets.

They also form based on similar methods. However, proximity to the surface plays a much more significant role in fog formation, and therefore there are several ways in which fog can form. Although fogs are classified by their manner of formation, there are in reality only 2 primary sets of conditions that generate fog, and 1 basic principle.

The basic principle is the same as what creates clouds. In a nutshell, water vapor in the atmosphere is continually being evaporated and condensed. When there is more condensation taking place than evaporation, the net result is water in its liquid form, and as the mole­cules cluster around aerosols, they eventually form cloud droplets.

Cloud droplets themselves have physical properties that promote their growth and reduce the constituent molecules' ability to reevaporate into the air. This fundamental concept is at the core of fog formation, and is also why clouds and fog can exist even though the ambient relative humidity may be less than 100%.

The 2 primary mechanisms for fog formation are the keys to creating the initial imbalance that allows more water vapor to condense than evaporate. The first way is that the air can be cooled.

Fog rolls over Los Angeles CA on a June morning. Like many coastal cities, Los Angeles is in a basin surrounded by mountains. Cool air drains into the basin at night, and by morning may have chilled the air enough to generate fog.

The amount of water vapor the air can hold is largely a function of its temperature. Warm air can hold more moisture than cold air. We can see this by looking at relative humidity through the day. Even though the actual amount of water in the air may not change in 24 hrs, during the day the relative humidity may be only 30 or 40%, but once the sun goes down and the air cools, it jumps to 70 or 80%.

If warm air that has been allowed to take up moisture via evaporation is cooled, the air loses some of the energy that it needed to keep the water in its gaseous state, resulting in an increase in condensation. If the cooling is sufficient, the continued condensation will result in cloud droplets.

A second method for generating fog is by adding water vapor. This may be done by evaporation and then light air movement mixing that evaporated moisture into relatively drier air. Fog (and clouds) may form either by cooling, mixing or, most commonly, a combination of the two. The type of fog that results is categorized by its dominant formative mechanism.

One of the most commonly observ­ed fogs is one that forms primarily by cooling. Radiation fog forms as the surface cools beneath a clear night sky. The surface doesn't have a great ability to hold heat and it quickly loses it if there is no solar radiation to replace it. In turn, the cold surface draws energy out of the overlying air, cooling it.

Thick, early morning fog blankets Houston TX. Together with its proximity to the warm Gulf of Mexico, high humidity and cold air outbreaks from the north often conspire to dramatically reduce visibility at HOU (Hobby, Houston TX).

When the air near the surface approaches the dew point, a thin fog may form if winds are light enough—generally less than 5 kts—to prevent any condensed moisture being blown away to higher altitudes.

As the surface air cools, it also becomes denser, meaning it tends to flow downslope and collect in valleys and hollows. Airports at the bases of valleys or in low-lying areas must often contend with radiation or "valley" fog.


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