What You Need to Know: Flying in Fog

Jason Blair, ATP, CFI-I, MEI-I, FAA Designated Pilot Examiner, AGI

February 2024

Modern avionics systems help us with lots of things as pilots. They can help us with traffic separation, more accurately navigate approaches to minimums, better navigate enroute, and give us better situational awareness. But one thing they can’t do (yet, and for most aircraft) is to mitigate the risk associated with flying in fog.


At some point, we need to see enough to land. Fog can be something that leaves us unable to complete that last, potentially most important, task in a flight, landing. At some point, we still need to be able to land with enough visibility to make sure you are aligned with and stay on a runway.


Know When it Occurs


Fog commonly emerges when moist air cools rapidly, leading to condensation and the formation of tiny water droplets suspended in the air. This process occurs when warm, moist air moves over a colder surface or when colder air infiltrates an area, causing the air temperature to drop to its dew point—the temperature at which the air becomes saturated and can no longer hold its moisture, resulting in fog formation. Pilots encounter different types of fog, such as radiation fog, advection fog, and upslope fog, each with distinct formation mechanisms, yet all equally challenging to navigate.


When fog settles around an airport or along a flight route, it significantly reduces visibility, sometimes to mere feet. During takeoff or landing, fog poses immense risks.


The Federal Aviation Administration (FAA) highlights four primary types of fog due to their significant impact on aviation safety in FAA weather publications that include:


Radiation Fog: This type forms during calm, clear nights when the Earth's surface loses heat by radiation, causing the air near the surface to cool rapidly. As the temperature drops to the dew point, fog forms. It often occurs in low-lying areas and valleys and dissipates as the sun rises and heats the ground.


Advection Fog: Advection fog develops when warm, moist air moves horizontally over a colder surface. As the warm air cools to its dew point, fog forms. This type of fog is prevalent in coastal areas where moist air from the ocean moves over cooler landmasses.


Upslope Fog: This type occurs when moist, stable air is forced upward along sloping terrain, such as hills or mountains. As the air rises and cools, fog forms along the slopes. Upslope fog can persist for extended periods, especially in regions with consistent wind patterns.


Steam Fog: Also known as evaporation fog, steam fog develops when cold air moves over warmer water bodies, causing the water to evaporate and create fog above the water's surface. This type of fog often appears as wispy patches or steam-like layers over lakes or rivers in colder weather conditions.


Knowing where and what types of fog are likely to occur can help a pilot evaluate the probability of encountering it in their particular flight operations. With that knowledge, they can better plan and mitigate the risks associated with fog.


Here are a couple of strategies that might be good to keep in mind.


Have an Alternate Option


If you are traveling somewhere where there is a potential for fog, have a plan B.  An example might be a pilot traveling to an airport near a body of water where fog commonly occurs. Having an alternate airport in mind that is 50 miles inland where fog is less likely would be wise. Another example could be a pilot planning to land in the morning when fog may develop or still be hanging on. They might be able to have a little extra fuel in the tanks and wait for clearing. However, they should also have an alternate airport in mind where fog is less likely and be prepared to divert there to wait for the fog to burn off if necessary.


Adjust departure and arrival times


If your departure and arrival times are expected to occur when temperature and dew point spreads are greater, you can reduce the risk that fog will be present during your flight time by changing when you fly. A good review of forecasted weather with a focus on potential fog development can help a pilot better understand if and when fog is likely to occur at their intended destination. Dig in a little deeper in a preflight review of the weather if you think it might be a possible occurrence for your flight.


In a general sense, early evening and morning tend to be times when fog most commonly affects flight operations. You can use information on recent weather trends in the area you are planning to land to determine if you are likely to encounter fog around your expected time of arrival and adjust accordingly.  Just timing your arrival or departure times to mid-day periods where peak temperatures, winds, and sun dissipate fog can be an effective strategy to avoid risks associated with fog.


Don’t be afraid to drive the last leg


A pilot might even choose to use an airport as their destination that is further away from water, at a higher or lower elevation, or in a location where a little wind may keep fog from forming as easily.


Avoiding landing at an airport down in a valley can sometimes avoid needing to deal with fog that might develop. It might be worth landing at an inland airport and renting a car to travel to that coastal destination. The same might hold true if your destination is an island.


Every once in a while, pilots get surprised by unexpected fog formation. I remember personally an occasion where I had taken off a little before sunrise, with all local METARs reporting great visibility and no ceilings. The dew point was close to the temperature, and the air was humid. As the sun came up, fog over the ground quickly formed and left me flying in great VFR a couple of thousand feet above the ground but with little ability to see the actual ground references, and it would certainly have left me unable to make a landing in the nearby area. I was able to fly to my destination about 70 miles away, which was further away from the lakeshore, and make a good safe landing, but I also was able to clearly make that decision by gathering some updated weather information from my destination through ADS-B delivered weather services. This allowed me to continue to monitor if the fog that had developed under me was also doing so at my destination.


Our modern in-flight access to weather information helps us better make decisions and allows us to make them sooner than in most of aviation history. Not waiting until the last couple of miles to update weather information allows us to keep more options available to keep us from getting into trouble instead of just helping us get out of it as pilots. Utilize these resources throughout your flight to be proactive with all your flying decisions, including those that might be needed if fog is encountered or might affect your flight plans.


As the aviation industry evolves, so too will the strategies and technologies employed to combat the challenges posed by fog. But until technology allows general aviation pilots to see through the fog and eliminate it as a risk entirely, knowledge of the risk and developing mitigation strategies is the key to ensuring that pilots and their passengers don’t fall victim to the dangers of fog.


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Jason Blair is an active single- and multi-engine instructor and an FAA Designated Pilot Examiner with over 6,000 hours total time, over 3,000 hours instruction given, and more than 3000 hours in aircraft as a DPE. In his role as Examiner, over 2,000 pilot certificates have been issued. He has worked for and continues to work with multiple aviation associations with a focus on pilot training and testing. His experience as a pilot and instructor spans nearly 20 years and includes over 100 makes and models of aircraft flown. Jason has published works in many aviation publications, a full listing of which can be found at www.jasonblair.net.


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