Aviation Fog


Fog Formation:

  • Fog is often associated with cool (or cooling) temperatures because cool temperatures provide the best conditions to condensate (i.e., temperature drops toward the dew point)
  • For fog to form, three basic conditions must exist:
    1. Condensation nuclei (dust, dirt, pollen, etc.) must be present for moisture to condense on
    2. The air must have a high water content (low temperature/dew point spread)
      • Occurs by cooling air to the dew point or adding moisture to the air
      • Fog will form when temperature and dew point are about 5°F/2°C within one another
    3. Light surface winds must be present
      • Winds cause surface friction to cause an eddy, causing more air to contact the ground
      • High winds will lift the layer away from the ground

Radiation (Ground) Fog:

  • Radiation Fog
    Radiation (Ground Fog)
  • Radiation, also known as ground fog, forms over low-lying flat surfaces on clear, calm, humid nights [Figure 1]
  • Typically extends less than 20' off the ground
  • As the surface cools by radiation, the adjacent air also is cooled to its dew point
  • Cloudy nights reflect terrestrial radiation back to the death reducing cooling through a "blanket" effect would prevent formation
  • Radiation fog occurs when the max temperature is reached
  • Winds 5 to 10 knots result in dense fog, while just 5 knots is shallow fog
  • Fog will dissipate due to winds greater than 10 knots or solar heating
  • Surface temperature must rise for the fog to clear, meaning thick layers will last longer as it takes longer to heat the ground
  • Radiation Fog
    Radiation (Ground Fog)

Advection Fog:

  • Advection fog occurs when a low layer of warm, moist air moves over a cooler surface [Figure 2]
  • Some wind velocity is required for formation, and in fact, the fog becomes thicker and denser as wind speed increases up to about 15 knots
  • Stronger winds lift moisture into low-stratus clouds
  • Fog can stay over water for weeks, moving over land late in the day and back over the water the next morning
  • Generally, there are two scenarios which lend to the formation of advection fog:
    • Cold water rising from the depths to the surface cooling the air above it (i.e., the Pacific)
    • Tropical air moving over cold ground (i.e., the Gulf Coast)
  • Solar radiation generally cannot penetrate the thickness to warm the Earth sufficiently, and so the wind is required to push air from a cold surface to a warm surface, reversing the saturation process
    • Especially due to the high specific heat of water

Upslope Fog:

  • Upslope Fog
    Upslope Fog
  • Forms when moist, stable air is forced up a sloping land mass to a level where the air becomes saturated and condensation occurs [Figure 3]
  • Requires wind for formation
  • Fog usually forms a good distance from the peak of the hill or mountain and covers a large area
  • Upslope fog occurs in all mountain ranges in North America
  • Upslope fog occurs during the winter months, when cold air behind a cold front drifts westward and encounters the eastward-facing slopes of the Rocky Mountains
  • As the cold, moist air rises the slopes of the mountains, condensation occurs, and extensive areas of fog form on the lower slopes of the mountains
  • Upslope Fog
    Upslope Fog

Evaporation Fog:

  • This type of fog forms when sufficient water vapor is added to the air by evaporation, and the moist air mixes with cooler, relatively drier air
  • There are two common types of evaporation fog:
    • Steam Fog:

      • Steam Fog
        Steam Fog
      • Steam fog, also called steaming fog or valley fog, forms when cold air moves over warm water
      • When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100%, and fog forms
      • This type of fog takes on the appearance of wisps of smoke rising off the surface of the water, giving it the nickname "seasmoke" [Figure 4]
      • Steam Fog
        Steam Fog
      • Stream fog formss opposite to upslope fog as cool air sinks into warmer, moist air
    • Frontal Fog:

      • This type of fog forms when warm raindrops evaporate into a cooler, drier layer of air near the ground
      • Once enough rain has evaporated into the layer of the cool surface, the humidity of this air reaches 100%, and fog forms

Ice Fog:

  • Ice Fog
    Ice Fog
  • Occurs in cold weather when the temperature is much below freezing, and water vapor sublimates directly into ice crystals [Figure 5]
  • Conditions for formation are the same as radiation fog except for cold temperatures, 25°F and colder
  • It ccurs mainly in arctic regions
  • It an be blinding to someone flying into the sun
  • Ice Fog
    Ice Fog

Flying in Flat Light, Brown Out Conditions, and White Out Conditions:

  • Flat Light:

    • Flat light is an optical illusion, also known as "sector or partial white out"
    • It is not as severe as "white out" but the condition causes pilots to lose their depth-of-field and contrast in vision
    • Flat light conditions are usually accompanied by overcast skies, inhibiting any visual clues
    • Such conditions can occur anywhere in the world, primarily in snow-covered areas but can occur in dust, sand, mud flats, or on glassy water
    • Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and closure rates
    • As a result of this reflected light, it can give pilots the illusion that they are ascending or descending when they may be flying level
    • However, with good judgment and proper training and planning, it is possible to safely operate an aircraft in flat light conditions
    • Flat light conditions can lead to a white out environment quite rapidly, and both atmospheric conditions are insidious; they sneak up on you as your visual references slowly begin to disappear
  • Brown-Out:

    • A brown-out is an in-flight visibility restriction due to dust or sand in the air. In a brown-out, the pilot cannot see nearby objects which provide the outside visual references necessary to control the aircraft near the ground. This can cause spatial disorientation and loss of situational awareness leading to an accident
    • The following factors will affect the probability and severity of brown-out: rotor disk loading, rotor configuration, soil composition, wind, approach speed, and approach angle
    • The brown-out phenomenon causes accidents during helicopter landing and take-off operations in dust, fine dirt, sand, or arid desert terrain. Intense, blinding dust clouds stirred up by the helicopter rotor downwash during near-ground flight causes significant flight safety risks from aircraft and ground obstacle collisions, and dynamic rollover due to sloped and uneven terrain
    • This is a dangerous phenomenon experienced by many helicopters when making landing approaches in dusty environments, whereby sand or dust particles become swept up in the rotor outwash and obscure the pilot's vision of the terrain. This is particularly dangerous because the pilot needs those visual cues from their surroundings in order to make a safe landing
    • Blowing sand and dust can cause an illusion of a tilted horizon. A pilot not using the flight instruments for reference may instinctively try to level the aircraft with respect to the false horizon, resulting in an accident. Helicopter rotor wash also causes sand to blow around outside the cockpit windows, possibly leading the pilot to experience an illusion where the helicopter appears to be turning when it is actually in a level hover. This can also cause the pilot to make incorrect control inputs which can quickly lead to disaster when hovering near the ground. In night landings, aircraft lighting can enhance visual illusions by illuminating the brown-out cloud
  • White Out:

    • As defined in meteorological terms, white out occurs when a person becomes engulfed in a uniformly white glow
    • The glow is a result of being surrounded by blowing snow, dust, sand, mud or water
    • There are no shadows, no horizon or clouds and all depth-of-field and orientation are lost
    • A white out situation is severe in that there are no visual references
    • Flying is not recommended in any white-out situation
    • White-out has been the cause of several aviation accidents
  • Self-Induced White Out:

    • This effect typically occurs when a helicopter takes off or lands on a snow-covered area
    • The rotor downwash picks up particles and re-circulates them through the rotor downwash
    • The effect can vary in intensity depending upon the amount of light on the surface
    • This can happen on the sunniest, brightest day with good contrast everywhere
    • However, when it happens, there can be a complete loss of visual clues
    • If the pilot has not prepared for this immediate loss of visibility, the results can be disastrous
    • Good planning does not prevent one from encountering flat light or white-out conditions

Loss of Visual References:

  • Never takeoff in a white-out situation
    • Realize that in flat light conditions it may be possible to depart but not to return to that site
      • During takeoff, make sure you have a reference point
      • Do not lose sight of it until you have a departure reference point in view
      • Be prepared to return to the takeoff reference if the departure reference does not come into view
    • Flat light is common to snow skiers:
      • One way to compensate for the lack of visual contrast and depth-of-field loss is by wearing amber tinted lenses (also known as blue blockers)
      • A special note of caution: Eyewear is not ideal for every pilot
      • Take into consideration personal factors - age, light sensitivity, and ambient lighting conditions
  • So, what should a pilot do when all visual references are lost?
    • Trust the cockpit instruments
    • Execute a 180° turnaround and start looking for outside references
    • Above all - fly the aircraft

Landing in Low Light Conditions:

  • When landing in a low light condition - use extreme caution
  • Look for intermediate reference points, in addition to checkpoints along each leg of the route, for course confirmation and timing
  • The lower the ambient light becomes, the more reference points a pilot should use
  • Airport Landings:

    • Look for features around the airport or approach path that can be used in determining depth perception
    • Buildings, towers, vehicles, or other aircraft serve well for this measurement
    • Use something that will provide you with a sense of height above the ground, in addition to orienting you to the runway
    • Be cautious of snowdrifts and snow banks - anything that can distinguish the edge of the runway
    • Look for subtle changes in snow texture or shading to identify ridges or changes in snow depth
  • Off-Airport Landings:

    • In the event of an off-airport landing, pilots have used various different visual cues to gain reference
    • Use whatever you must to create the contrast you need
    • Natural references seem to work best (trees, rocks, snow ribs, etc.)
      • Overflight
      • Use of markers
      • Weighted flags
      • Smoke bombs
      • Any colored rags
      • Dye markers
      • Kool-aid
      • Trees or tree branches
    • It is difficult to determine the depth of snow in level areas. Dropping items from the aircraft as reference points are a visual aid only and not as a primary landing reference. Unless your marker is biodegradable, be sure to retrieve it after landing. Never put yourself in a position where no visual references exist
    • Abort landing if blowing snow obscures your reference. Make your decisions early. Don't assume you can pick up a lost reference point when you get closer
    • Exercise extreme caution when flying from sunlight into shade. Physical awareness may tell you that you are flying straight but you may actually be in a spiral dive with centrifugal force pressing against you. Having no visual references enhances this illusion. Just because you have a good visual reference does not mean it's safe to continue. There may be snow-covered terrain not visible in the direction that you are traveling. Getting caught in a no-visual reference situation can be fatal
  • Flying Around a Lake:

    • When flying along lakeshores, use them as a reference point. Even if you can see the other side, realize that your depth perception may be poor. It is easy to fly into the surface. If you must cross the lake, check the altimeter frequently and maintain a safe altitude while still having a good reference. Don't descend below that altitude
    • The same rules apply to seemingly flat areas of snow. If you don't have good references, avoid going there
  • Other Traffic:

    • Be on the lookout for other traffic in the area. Other aircraft may be using your same reference point. Chances are greater of colliding with someone traveling in the same direction as you than someone flying in the opposite direction
  • Ceilings:

    • Low ceilings have caught many pilots off guard. Clouds do not always form parallel to the surface or at the same altitude. Pilots may try to compensate for this by flying with a slight bank and thus creating a descending turn
  • Glaciers:

    • Be conscious of your altitude when flying over glaciers. The glaciers may be rising faster than you are climbing

Case Studies:


  • Fog can reduce surface visibilities low enough to prevent safe takeoffs and landings
  • Learning of fog or potential fog can be found in forecasts such as the Terminal Aerodrome Forecast (TAF) and Area Forecast (FA) [Figure 6]
  • Remember that if a descent must be made through fog, smoke, or haze in order to land, the horizontal visibility is considerably less when looking through the restriction than it is when looking straight down through it from above
  • To recall the types of fog, consider how one might feel after a day of flying through fog with the mnemonic "Runways Are Usually Endlessly Inviting"
    • Runways: Radiant Fog
    • Are: Advection Fog
    • Usually: Up-Slope Fog
    • Endlessly: Evaporation Fog
    • Inviting: Ice Fog
  • Real-time conditions can be found on the Aviation Routine Weather Report (METAR)
  • Although fog is normally associated with weather systems, it is also a function of temperature, potentially catching pilots off-guard when it appears right before sunrise and/or right after sunset
  • Advisory Circular (00-45) Aviation Weather Services, TAF Fog Terms
    Advisory Circular (00-45) Aviation Weather Services, TAF Fog Terms
  • Mist is not fog; mist is defined as visibility 5/8 to 6 SM
  • Improve your weather skills with FAA provided (and WINGS credited) resources by going to https://www.faasafety.gov/ and type "weather" into the search bar
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