WX BRIEF

Flying in fog

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




The thickness of radiation fog depends on how soon after sunset it forms and whether there is sufficient moisture and weak mixing. A fog that forms early has more nocturnal cooling time in which to thicken than a fog that forms closer to dawn. Similarly, if the air was holding a lot of moisture, either from being warm or humid, the fog can become quite thick.

But radiation fog will become thickest if there is a very light wind that can mix the surface fog upward, saturating progressively higher levels. Without mixing, the fog, though it may become thick, will likely be limited to a few dozen feet, whereas adequate mixing can create a fog deck several hundred feet thick.

Not all cooling is the result of loss of heat to the surface. Air can also be cooled by forcing it to rise. Humid air that is pushed up against a mountain range is an example of this. With nowhere to go but up, the air rises and cools. If it has to rise high enough, it may cool to its dew point and form a fog that shrouds the slopes, which is why this type of fog is known as upslope fog.

Advection fog is another very common type of fog, which occurs when relatively humid air flows over a cold surface. If the surface is cold enough, it will extract sufficient energy from the overlying air to chill it to its dew point and generate fog.

These advection fogs are what are normally seen in coastal regions such as San Francisco Bay. In these regions, synoptic patterns may carry humid tropical air from out at sea in over the cold coastal currents, forming a fog that can blanket the region and persist for days. Advection fogs also occur when warmer, humid air moves over a snow or ice-covered area.

Although not as intense as advection fog, cold, dry air moving over a warm, humid surface can generate a thin fog known as steam fog. This is an evaporation and mixing type of fog similar to when you exhale into cold air.

The heated humid air quickly rises and mixes into the cold air above, condensing the water vapor into what appears as steam. Sailors in high latitudes often experienced this as sea smoke, as frigid air would flow off the ice pack over warmer water.

Fronts often produce fog as well. As rain falls from the clouds above the front, some of it is evaporated by the colder air through which it is falling. This sort of fog tends not to last long, and it occurs either directly ahead of a warm front or immediately behind a cold front. It can be enhanced if the front is over a snow pack.

Ice fog cometh

Still one other type of fog is sometimes identified, although it is really a product of one of the other mechanisms just described. Ice fog is fog composed entirely of ice crystals. This happens only when the air is extremely cold, such as one might find over an arctic ice pack.

In general, the air must be (usually much) colder than 14°F (–10°C). Ice fog is often confused with freezing fog—another fog that occurs in cold temperatures. Freezing fog is made of super-cooled liquid cloud droplets. It occurs when the air is below, but within about 20°F (11°C) of, freezing.

This type of fog can be dangerous to aircraft because it can quickly create a fine rime ice on all of the aircraft's surfaces while the aircraft is on the ground. If fog (or any liquid precipitation) is present with air temperatures below freezing, applying deicing fluid is a must.

Burning off

As long as conditions are right, fog has the potential to persist indefinitely. Coastal advection fogs are the worst offenders, as steady on­shore airflow can pump an endless supply of moisture over a cold bay. Persistent thick fogs have been known to cover coastal communities for a week or more. Fortunately, fog is usually ephemeral, forming overnight as the air cools, and most fogs tend to be moisture limited, meaning they do not get too thick.

Radiation fogs are thickest just after dawn, as the rising sun has not sent enough radiation to the surface to offset the deficit. But by a half hour or so after sunrise, enough energy has heated the surface to begin to reverse the fog process. Starting at the lowest levels, the fog evaporates. As the heating continues, progressively higher levels clear out, giving the impression that the fog is lifting.

Alternatively, once the sun rises, heating of the surrounding area can also break up the fog. As some places will heat faster than others, local winds can pick up, enhancing mixing, bringing in drier air, and transporting the water droplets to unsaturated altitudes where they can evaporate quickly.

However, because some valleys and depressions in the landscape can remain sheltered from the effects of sun and wind until late in the morning, these areas often see fog persisting for several hours after sunrise. Likewise, because rivers are an ample moisture source and often found at the bases of valleys, nearby airports are likely to see longer lasting fog than the surrounding region.

Because fogs are relatively thin, forming just as the air temperature nears the dew point, it does not take a lot of wind to ensure sufficient mixing to overcome any fog development. In fact, if the wind exceeds about 5 kts, it is unlikely that a radiation fog will form. By their nature, advection and upslope fogs may still form in slightly stronger winds, assuming an adequate moisture supply, but above the 5 to 10-kt range, it is unlikely that fog will form or persist.

Climatology

Thick upslope fog blankets the mountainsides near GVA (Geneva, Swizerland). Despite the sunshine, a steady upslope wind and high humidity can combine to generate these persistent fogs that can completely obscure the terrain.

Fog forms in just about every corner of Earth, from high Arctic sea smokes to thick steam fog in tropical jungles. Nearly all mountain ranges experience some upslope fog, although those along coasts, in humid regions, or near a good moisture source are going to be more conducive to fog than the rest.

Even a gentle rise in elevation can give rise to fog. For example, a hybrid advection/radiation fog occasionally covers large parts of the US Great Plains when humid Gulf of Mexico air flows inland during winter.

Advection fog is, however, most prevalent where moisture-laden air is either streaming onshore across cold water or moving over cold land. The west coasts of the mid-latitude continents are the hotspots for advection fog.

The California redwoods and vegetation in South America's Atacama Desert survive solely because of the moisture derived from fog.

Since fog is a function of daily temperature regimes and moisture, fog is most likely in places and at times when there is abundant moisture and a daily temperature that can fluctuate enough to bring the temperature to the dew point.

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