Fog can be challenging to deal with, even for experienced IFR pilots. This is because fog is often unpredictable, sometimes dangerous, and always requiring you to stay on top of your situational awareness.
While fog may seem harmless on the ground, its various forms can impact visibility in distinct ways. That can present unique challenges for you as pilot. This is why it's important to learn about the different types of fog, and recognize when they could complicate your next flight.
Let's learn about them now!
SUMMARY
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Radiation fog forms when warm, moist air cools rapidly at night near the ground.
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Advection fog develops when warm air moves over a colder surface.
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Freezing fog and ice fog occur in freezing temperatures, creating ice hazards.
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Upslope fog, frontal fog, and evaporation fog form due to rising air, weather fronts, or evaporating water.
1. Radiation Fog
The first type of fog we'll cover is radiation fog. Radiation fog forms when warm, moist air cools rapidly at night as the ground loses heat. The temperature drops to the dew point, forming a dense, ground-level fog.
To give you more context, heat radiates from the ground surface at night. Then it cools the bottom air until it saturates.
Radiational cooling at the top of the fog layer, deepening it. The radiation fog can form at the surface, and gradually get thicker as it continues to cool.
Key Points:
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Where it forms: Common in low-lying areas, valleys, and regions with minimal wind.
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When it forms: Typically on calm, clear nights, especially during fall and winter.
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Impact on aviation: Unlike advection fog, radiation fog is usually shallow, but when thick, it can reduce visibility to near zero, complicating taxi, takeoff, and landing operations.
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How it clears: It often dissipates with the morning sun as the air warms and relative humidity decreases.
2. Advection Fog
The next fog you'll need to know about is advection fog. Advection fog occurs when warm, moist air moves over colder ground surface.
Advection fog can occur day or night and often spans large areas, making it both persistent and at times dangerous for pilots. This type of fog is common near coastal regions where warm air masses encounter colder water surfaces.
As a pilot, you can monitor temperature differences between the surface and incoming air to anticipate the formation of advection fog.
Key Points:
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Where it forms: Common along coastlines where warm air masses pass over cooler waters.
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When it forms: Often during transitions between seasons or with onshore breezes.
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Impact on aviation: Persistent and widespread, making it a visibility hazard, especially near coastal airports.
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How to prepare: Monitor surface temperature differences and coastal wind patterns to anticipate its development.
3. Steam Fog (Evaporation Fog)
The third type of fog we'll dive into is steam fog. Steam fog forms when cold air moves over a warm water body. It then causes rapid evaporation and condensation. Steam fog also called "sea smoke" for it's appearance when it occurs over large water bodies.
This type of fog is usually more common in colder months when the air temperature is lower than the water temperature. Steam fog is known by its dramatic and almost wispy appearance as it rises from the surface.
Key Points:
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Where it forms: Over lakes, rivers, or oceans, especially in colder months.
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When it forms: Typically in winter when there’s a stark temperature difference between water and air.
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Impact on aviation: Can form quickly and unexpectedly, reducing visibility near water crossings or low-altitude approaches.
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Unique feature: Often visible as wispy, rising columns of fog, which can look dramatic but pose real hazards.
4. Upslope Fog
Our #4 slot goes to upslope fog. Upslope fog forms when moist air is forced to rise up a slope, cooling as it ascends. This type of fog requires consistent wind to drive the air upward.
Key Points:
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Where it forms: Common along mountain ranges or sloped terrains.
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When it forms: Can occur at any time, depending on terrain and wind conditions.
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Impact on aviation: Persistent fog on mountainous approaches increases the risk of controlled flight into terrain (CFIT).
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Preparation tips: Monitor winds and terrain conditions for early signs of upslope fog formation.
5. Ice Fog & Freezing Fog
Coming up at #5 is ice fog & freezing fog. We lumped these two together because of their similarities, but have individual explanations for them because of key differences you'll read about further.
Ice fog is composed of tiny ice crystals, forming in extremely cold conditions. It requires temperatures below 14 F (-10°C) and typically occurs in arctic or high-altitude environments.
Another important thing to keep in mind is that ice fog can develop when hot, moist exhaust gases from vehicles or aircraft mix with cold air. On clear, cold days in the Arctic, an aircraft taking off can produce enough ice fog to greatly reduce visibility, and that can prevent other aircraft from being able to take off.
Key Points:
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Where it forms: Ice fog is common in polar regions, but can also occur at high altitudes in temperate zones.
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When it forms: During extremely cold, clear conditions with high humidity.
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Impact on aviation: Reduces visibility and poses a hazard for engine intakes and airframe icing.
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How to mitigate: Use proper cold-weather procedures and monitor conditions in polar and subpolar regions.
This might seem similar to Freezing fog, but there are some differences:
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Freezing fog involves supercooled liquid droplets that freeze on contact.
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Ice fog consists entirely of ice crystals formed in extremely cold environments.
Freezing Fog
Freezing fog occurs when the temperature reaches 32°F (0°C). Water droplets in the fog are supercooled and freeze upon contact with surfaces. This creates a thin layer of ice, known as rime ice, which can be hazardous to aircraft and airport operations.
Key Points:
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Where it forms: Often in arctic or subarctic regions but can occur in temperate climates during winter.
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When it forms: When surface temperatures drop below freezing, with sufficient moisture in the air.
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Impact on aviation: Poses a severe risk of airframe icing and icy runways, especially during takeoff and landing.
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Preparation tips: Always check weather reports for icing conditions and ensure proper de-icing procedures.
6. Frontal Fog
The next type of fog is frontal fog. Frontal fog develops when warm rain falls through cooler air, evaporating and saturating the air with moisture. This type of fog is often associated with warm fronts and steady precipitation.
Key Points:
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Where it forms: Along weather fronts, especially warm fronts.
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When it forms: Most common during rainy weather when warm air overrides cooler air.
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Impact on aviation: Can extend across vast regions, reducing visibility for en-route flights and approaches.
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How to anticipate: Pay attention to METARs and TAFs for frontal activity and precipitation.
7. Precipitation Fog
The final type of fog on our list is precipitation fog. Precipitation fog forms when rain falls through cooler air and evaporates, increasing the air's humidity until it reaches saturation. This type of fog commonly occurs in conjunction with frontal activity, particularly warm fronts.
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Typically forms during steady or light rain.
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Reduces visibility significantly, often covering large areas.
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Usually combined with other weather hazards, such as low clouds and turbulence.
Frequently Asked Questions
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What is the rarest type of fog?
The rarest type of fog is ice fog, as it requires extremely cold conditions below -20°F where water vapor freezes directly into ice crystals. -
What are the 4 types of aviation fog?
The four primary types of fog affecting aviation are radiation fog, advection fog, upslope fog, and steam fog. -
What is a very thick fog called?
A very thick fog is often referred to as dense fog, where visibility drops to less than a quarter mile. -
What is frozen fog called?
Frozen fog is known as freezing fog when supercooled droplets freeze on surfaces, and ice fog when vapor freezes into ice crystals in the air.
Takeaway
Fog can sneak up on you fast, and each type forms under different conditions. Pay attention to changes in temperature, rising air, or moisture near water and terrain. Knowing how fog develops helps you stay prepared, spot hazards early, and keep all of your future flights safe.
Stay sharp, stay informed, and always plan ahead for low-visibility challenges.
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