Wherever you are flying, fog can sneak up on you...unless you get the jump on it first.
Fog’s effect on flight can range wildly in type, degree of severity, and duration depending on location, temperature, precipitation, time of day, and many other factors. Perhaps the presence of fog in the area will barely affect your flight. On the contrary, fog may be the reason you’ll need to be approved and equipped for CAT II operations in order to land at your destination.
Most of the time, fog can be dealt with according to its type with minimal hassle and only occasional deviations. But, as with all other aspects of aviation, complacency breeds trouble quickly and thorough preparation can mitigate a world of hazard. Know when to expect fog and how it might change your flight plan before you take off, and your encounters with fog will remain unremarkable.
Cloud to ground
Fog is a cloud--specifically, a cloud that has formed close to the ground. This occurs when the temperature and dew point are very close together, with a difference of less than 4 °F or 2 °C. This is usually a result of cold and warm air masses encountering one another, such as when warm air moves over a cold surface, or when the ground cools the warm air directly above it at night.
While almost all fog occurs in the same way, different classifications of fog can help you determine if, when, or where fog will form, allowing you to better prepare for and avoid it.
Fog over water
Fog that appears over bodies of water can come in different varieties: sea fog, lake fog, steam fog, or sea smoke. It occurs when cold air moves over warm water, and is most prevalent during the colder months. Not only does it hamper visibility, but it can also cause turbulence and icing.
Also known as mountain fog, this occurs at higher altitudes as moist, warm air is forced up slopes by wind to cooler, drier areas. Upslope fog can persist for days.
Freezing fog, or ice fog, is one of the most dangerous to aircraft operations. In freezing temperatures, excess water vapor in the air may freeze directly into ice crystals. Generally, freezing fog is specific to arctic regions, but it can be found at lower latitudes during the winter, as well. Freezing fog can contaminate an aircraft just as snow, ice, or freezing rain can.
When fog develops in one location and is blown to another by the wind, this is advection fog. Typically, advection fog will appear along coasts as fog that forms over bodies of water moves inland. Advection fog is moved by gentle sea breezes--winds higher than 15 knots will result in low stratus clouds instead.
This is the fog many of us recognize on cool mornings, collecting in lower areas or settling over flat land in a thick blanket. It develops overnight as warm air near the ground cools to the saturation point and extends higher as the air above the initial fog layer continues to cool; it typically dissipates quickly once the sun rises.
Frontal fog, true to its name, is associated with fronts. It normally forms north of a warm front where the colder air lingers under the warm air from the front. This fog can be incredibly persistent due to its tendency to linger in an area for a longer period of time and remain in conjunction with rain or snow.
Now you see, now you don’t
The most obvious hazard of fog is the reduction to visibility. If you’re still on the ground, your call is relatively simple to make: can you meet your visibility requirements, or not? If fog has reduced visibility too much, consider the type of fog before deciding whether or not to wait it out. Radiation fog in the early morning will likely dissipate quickly, while upslope fog in the mountains or a sea fog moving inland may be an indication to expect longer delays.
Fog visibility can get tricky while in flight. Depending on the type of fog, you may find yourself able to see the ground just fine when looking straight down; however, during approach and landing the horizontal visibility become near nonexistent. Know beforehand whether the area you will be flying into is a likely location for fog at the planned arrival time and forecast conditions, and file your flight plan accordingly.
Finally, don’t forget the other hazards of fog: turbulence and freezing precipitation. Most fog will not cause significant turbulence, though sea fog can and may cause trouble for pilots performing various low-altitude, overwater operations. Freezing fog will leave deposits of rime icing on aircraft which must be removed before takeoff. If encountered during flight, anti-icing systems will be required to keep ice from building up.
Fog can be trickier to avoid than other weather concerns for many reasons: it can be tough to spot from the air in certain situations, and you may not always be able to pick it up on infrared imagery. Fog may closely resemble clouds, and it will not show up on radar. Ground observations and METARs are reliable methods of determining whether fog exists at a given location. Depending on the type of fog, you may be able to anticipate its presence lingering in the area hours later, or know it will reliably dissipate in time for your arrival.
Even if fog may not be present at your destination when you take off, know the predicted conditions before you take off to determine whether or not fog may challenge your flight plans when you arrive. As with all other weather conditions, careful planning and review of all available resources is key before beginning any flight.
Related CTS training:
- Aviation Weather Theory
- Aeronautical Information Manual
- Brownout, Whiteout, and Flat Light Conditions