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Fog and visibility


Seasonal cooling often produces restrictions to surface visibility.

By Karsten Shein
Comm-Inst. Climate Scientist

Morning radiation fog covers LGU (Logan UT). Although approaching aircraft might see the runway from above, they would likely lose sight of it as they enter the fog. Collisions can occur if an aircraft or vehicle is not where it is supposed to be, or hasn’t communicated its position.

Even though ATIS indicated 1/4 mile of visibility in fog, the pilots could see the runway lights through what was a thick but shallow early morning fog. “Want to give it a go?” the PIC asked his copilot. “Why not.

We can always miss if we lose visual.” replied the right seater. Calling final approach on common traffic advisory frequency (CTAF), they passed the outer marker on a bit steeper glidepath to maintain sight of the runway lights as long as possible.

Just before they descended into the fog, they spotted a red flashing light on the runway ahead. The crew aborted the landing immediately, pushing the throttles forward and climbing over a small piston aircraft exiting the runway on a crossing taxiway. Apparently, the piston aircraft pilot had not switched his radios to CTAF, so he did not hear the bizjet’s communications.

This is just one example of the many dangers that can be hidden in the fog. Since the dawn of aviation, fog has been one of the deadlier weather phenomena faced by pilots. Fog was a factor in what remains the deadliest aviation disaster in history, when 2 Boeing 747s collided in heavy fog at TFN (Tenerife, Spain), claiming 583 lives.

And as recently as this past July, a business aircraft crashed short of the runway during a second, early morning RNAV approach attempt at F70 (French Valley, Murrieta CA), killing 6 aboard. The AWOS observation at the time reported a 300 ft overcast with 1/4 mile visibility in fog – below the approach minimums.

Fog has often been described as a cloud in contact with the ground, which is not incorrect. Like clouds at higher altitudes, fog is condensed water droplets in large quantities that reduce visibility by scattering light. A cloud can become fog if it moves into rising terrain that places it in contact with the ground, or fog can form as clear surface air is cooled, moisturized, or both. The mechanisms that produce these processes help us divide fog into a few different types.

Cooling fog

How high do these mountains rise? Clouds that come into contact with rising terrain, or air that cools and condenses its water vapor as it is lifted by the terrain can obscure higher ground. Pilots should ensure that they follow their artificial horizon and maintain at least MEA or the appropriate approach glide path to avoid controlled flight into terrain (CFIT).

Radiation fog (often called valley fog) is a cooling fog that forms primarily after a humid night under clear skies. At night, the land or water loses heat, which in turn lowers the temperature of the overlying air until it reaches its dew point.

Since the amount of water vapor that air can hold depends on temperature, further cooling requires the air to condense the water vapor it can no longer hold, producing a thin and shallow fog (if winds are less than ~3 kts). The longer the air cools and the higher the dewpoint, the thicker the fog becomes – particularly under a nocturnal temperature inversion.

Fortunately, most radiation fog is only a few hundred feet thick and usually dissipates within a few hours after the sun rises. In these situations, surface warmth evaporates the base of the fog first, giving rise to the saying “the fog is lifting,” meaning visibility at the surface improves quickest.

Advection fog is a cooling and moisturizing fog, in which moist air is advected to a location from nearby and settles over a colder surface. One example is the famous San Francisco fog, where humid air from the Pacific Ocean flows over the cold waters of San Francisco Bay.

Similar fog is common in many coastal environments where a marine layer forms. Marine layers are air masses in which warm air rests over a colder water surface and creates a temperature inversion as it cools and humidifies from the underlying water. Onshore winds can move the fog inland, sometimes blanketing airports many miles away.

Unlike radiation fog, advection fog is deeper and thicker, often reducing visibility to zero over several hundred feet of altitude. Also, depending on the prevailing weather patterns, it can remain in place for several days.

Other fogs

Upslope fog is a cooling fog that forms as humid air is forced to lift and cool by the rising terrain it encounters. Advection fogs often become upslope fogs as they flow into coastal mountain ranges.

These fogs are particularly dangerous because not only do they obscure rising terrain, but they often slope with the terrain, and in the absence of a visual reference of the true horizon, unsuspecting pilots may level their path with the fog’s false horizon, ignoring minimum en-route altitudes (MEAs) and their instruments – putting them in a dangerous attitude. Upslope fogs don’t only occur in mountains.

They can cover broad areas and are often the cause of shallow widespread fog in the US central plains as humid air from the Gulf of Mexico flows northward.

Other fogs are usually thinner, although some can cause additional issues for pilots. These include frontal fog, which forms as frontal rain saturates the surface layer; steam fog, from cold air moving over warm water; freezing fog, which forms when supercooled liquid fog forms in subfreezing air and freezes to exposed surfaces; and ice fog, which happens in exceedingly cold conditions where the fog forms as ice crystals.

While all these fogs reduce horizontal visibility, freezing fog can accrete on aircraft and must be removed before takeoff. It also creates slick conditions on taxiways, runways, and tarmacs. Ice fog is highly reflective, and using taxi or landing lights can eliminate forward vision.

Operating in fog

Power lines atop a mountain ridge. Fog not only reduces horizontal visibility but also impairs depth perception, making it challenging to avoid an obstacle that may suddenly appear. Pilots aloft should ensure they remain above the MEA and taxiing aircraft should not move faster than they can safely stop if a collision is imminent.

Naturally, operating in any reduced visibility situation demands enhanced vigilance from flight crews. Many fog-related accidents have occurred because an aircraft or ground vehicle took a wrong turn or misjudged a position or distance. In reduced visibility, a hazard might not materialize until the last moment, leaving little time to react.

Many larger business aircraft are now equipped with enhanced/low visibility vision systems to help pilots see at night, but these systems quickly lose their effectiveness as fog grows thicker. Light gathering devices can’t help when the fog droplets have scattered most light away from the system, and high humidity and dense fog can diffuse infrared heat signatures completely.

Instead, airborne pilots approaching a fogged in airport should ensure that their transponder and traffic collision avoidance system (TCAS) – if equipped – are operating, and that they are in regular communication with ATC and other pilots who may be in the area.

While it is okay under Part 91 to shoot an approach that is below minimums, it is not okay under Part 121 or 135. When fog is the cause of reported low visibility, pilots should expect that even though they may clearly see the airport from aloft, they will lose forward visibility as they descend. If that happens at or below the minimum descent altitude/height (MDA/MDH), they must miss the approach.

On the ground, pilots should exercise extreme caution. Even moderate fog can reduce depth perception and make it easy to misread airport markings. Always stop before crossing a foggy runway and query CTAF or confirm crossing with ATC. Don’t assume an approaching or departing aircraft knows you are there or cleared to cross.

There is little regulation regarding the use of aircraft lighting in fog. Pilots should use common sense and limit lighting in fog to lights that won’t further reduce your or other pilots’ ability to see and be seen. Pilots should also refresh themselves on the airport layout and the meanings of lighting systems used at the airport, as unlighted markings may be missed easily.

Particularly at non-towered airports, pilots are encouraged report their position on CTAF frequently, even if they are not aware of other operating aircraft or ground vehicles. This will ensure that anyone listening will have an idea of where you are and avoid potential collisions.

However, one cannot be sure that all the pilots taxiing or on approach are as familiar with the airport as you may be. If you are not sure where another vehicle is, do not be afraid to ask. If you are not sure where you are, the best option is to stop immediately (unless you know you are on an active runway) and ask for assistance. Smartphone map apps or GPS avionics may also help pinpoint your position within a few meters.

Of course, the best course of action is to avoiding operating in fog. Fog is predictable and should appear in TAFs and weather briefings. At some airports, radiation fog is a frequent occurrence from a few hours before to a few hours after dawn. Avoiding takeoffs and landings at those times should keep you out of the soup. Even during longer-lasting fog, such as coastal advection fog, wind shifts can improve visibility for a period, allowing safer aircraft movement.

It is always advisable (and required on IFR flight plans) to have an alternate destination airport. Pilots may consider alternates at which forecast fog is less likely, or where precision approaches and ATC ground control are available. Regardless of what weather conditions you encounter, don’t forget to file a pirep to improve weather guidance for your fellow aviators.

SheinKarsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.