Introduction:

  • Fog is nothing more than a low cloud, which has its base within 50' of the ground
  • Fog forms and is therefore dependent on the air becoming temporarily supersaturated
    • Air containing more moisture than it can hold for that temperature
    • Excess moisture then condenses resulting in fog
  • Fog can be extremely important to a pilot, especially when flying to an unfamiliar region
  • Types of fog include:

Fog Formation:

  • In order for fog to form, 3 basic conditions are required:
    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 dew point or adding moisture to the air
      • Fog will form when temperature and dew point are about 5°F/3°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, also know 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 actually 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 in order for the fog to clear and so thick layers will last longer as it takes longer to heat the ground

Radiation Fog
Figure 1: 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 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

Figure 2: Advection Fog In San Francisco Bay
Upslope Fog
Figure 3: 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
  • This type of 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
  • This usually 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 up the slopes of the mountains, condensation occurs and extensive areas of fog form on the lower slopes of the mountains

Steam Fog
Figure 4: Steam 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
  • The two common types are:
    • Steam fog, and;
    • Frontal fog
  • Steam Fog:
    • Steam or, steaming 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]
  • 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 cool surface, the humidity of this air reaches 100% and fog forms

Ice Fog
Figure 5: 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 temperature, 25°F and colder
  • Occurs mostly in arctic regions
  • Can be blinding to someone flying into the sun

Flying in Flat Light 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 actually 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
  • 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 down wash picks up particles and re-circulates them through the rotor down wash
    • 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)
      • 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 degree 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 a number of 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.)
      • Over flight
      • 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 areas that are level. Dropping items from the aircraft to use as reference points should be used as 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 that 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 you still have 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 look out 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:

Conclusion:

Advisory Circular (00-45) Aviation Weather Services, TAF Fog Terms
Figure 6: Advisory Circular (00-45) Aviation Weather Services, TAF Fog Terms

References: