Turbulence and Icing 17&18

Obstructions to Visibility

Weather Related Accidents

  • Causal factors in order:
    Adverse winds
    Reduced visibilities
    Low ceilings
    High density altitude

Categorical Outlooks
AIM 7-1-7 defines 4 categories:
LIFR Ceilings less than 500 vis less than 1 mile
IFR Ceiling 500 to less than1000 vis 1 to less than 3 miles
MVFR Ceiling 1000 to less than 3000 vis 3 to 5 miles
VFR Ceiling greater than 3000 vis greater than 5 miles

There are 3 types of visibility:
In flight
Slant range
Surface vis in Metars is given in SM
Flight vis is reported in SM as well
VFR pilots on average lose control after about 3 minutes

Restrictions to Visibility
Haze and smoke
Dust and blowing dust
Volcanic ash
Blowing sand
Blowing snow

Defined as surfaced based clouds of water or ice crystals
Fog is the most common cause of vis less than 3 miles
Fog can form rapidly dropping vis to less than a mile in a few minutes
Fog is one of the most hardest weather phenomenon to predict

Small temp/dew point is needed
This usually occurs just after sunrise due to small temp/dew point spread
3º and closing usually get fog
condensation nuclei must be present
5/8 to 6 miles is mist BR
Less than 5/8 of a mile is classified as fog

1. Cool air to the dew point
2. Add moisture near the ground

Warm moist air in contact with cooler ground or water
Upslope flow causing adiabatic cooling
Evaporational cooling releasing latent heat
Surface dew point increasing to temp due to evaporation
Air flowing over a moist surface (swamp, lake, ocean, rain soaked ground ect.)
Warm rain falling through colder air
Combination of any of the above

1. Radiation fog
2. Advection fog
3. Upslope fog
4. Precipitation-induced fog/Frontal fog
5. Ice fog
6. Steam fog
Note: Mountain and Valley fog is a variation of one or more of the above

Usually relatively shallow
Also known as Ground Fog
Clear sky, little or no wind, small temp dew point spread
Warm moist air over low flat areas
However it may form in valleys as well
Areas of cold air drainage lend to formation
This type of fog forms almost exclusively at night or daybreak
Look for this fog in the am after an evening/night rain and clearing sky
Or in winter when it gets warm enough to melt snow cover

Terrestrial radiation cools the air close the ground
Wind of 5 kts or less mix the layers of cool air and deepen the fog
Calm wind results in transfer through conduction alone resulting in thinner fog
A wind of +5 kts mixes the moist layers close to the ground with dryer air above preventing formation
Usually only forms over land because of the terrestrial radiation
Usually dissipates quickly if the sun comes out or a wind +5 kts picks up
If high overcast persists it will take longer for the temp/dew point spread to increase

At night a more complicated process is at work
The top of the fog more readily emits IR radiation skyward
Fog droplets absorb some of this IR radiation and reradiate back to the surface
The ground cools more slowly as a result
This shifts the max cooling rate to the top of the fog bank
This results in the fog becoming thicker and lowering vis further

On approach this is what you see
In the flare this is what you see

Moist air moves over colder ground or water
This fog is associated with horizontal air mass movement or advection
The surface over which it moves causes the change in temp
This is referred to as advective cooling
Examples of this occurrence is mild humid air moving over snow covered ground

It also commonly occurs along the coastal areas where warm moist air moves off the ocean over colder land
Deepens with winds up to 15 kts
Depends on the wind to exist (5-15 kts)
More than 15 kts tends to lift it into low stratus
Moves in rapidly with the wind day or night and more persistent than radiation fog

A characteristic of the coastal areas of southern California in summertime is advection fog
It forms as warm, dry air from over the land is moved over the much cooler ocean.
The cold water chills the air to dew point, resulting in low clouds and fog.

Moist stable air cooled adiabatically as it moves up sloping terrain
Depends on wind to exist (5-15 kts)
Mixing of the layers as air is forced up can deepen the fog
Very dense and can exist at high altitudes along the upsloping terrain
Unlike radiation fog it may form under overcast skies
Once the winds stops the fog dissipates

Terrain temp will modify the cooling rate
Very persistent
Covers a wide area
Obscures hills and mountains
If a downslope wind develops fog will dissipate
This fog is found along the west slope of the Cascades, east slope of the Rockies and east slope of the Appalachian mountains

Warm rain or drizzle falling through cool air
Evaporation from the precip saturates the cool air and “poof” fog
Associated with warm fronts mostly
May also form along cold fronts and stationary fronts
Little or no wind
Extra hazards to look for are icing, turbulence and thunderstorms

Very persistent
Covers a wide area
Mostly occurs with warm or stationary fronts
Fog forms on the cool air side of the front
Advective winds may spread the fog into other areas
Formation ends when the precip stops
The fog may persist for several hours after precip stops however

This is an example of frontal fog, forming in southern Indiana as a warm front was encroaching.
Warm and humid air was moving north, and running into cooler air.
Frontal fog is a result of contact cooling between two dissimilar air masses
While referred to as Frontal Fog it is really a form of Advection Fog

Occurs when temp is below freezing and the vapor sublimates directly into ice crystals
Conditions are similar as they are for formation of radiation fog
-25º F or colder so usually found in arctic region or colder winter spots

Also known as Arctic Sea Smoke
Extremely cold and dry air flows over warmer water
Evaporation and heating from the water causes a rise in humidity
Heating from below creates instability
Additionally vapor pressure is high next to the water surface but low in the dry air above
This sets up a vapor pressure gradient
This causes streamers to rise up giving the appearance of steam
Also occurs after a rain on a wet highway
Low stratus clouds
hard to predict bases
scud running not advisable

Most types of fog form in stable atmospheric conditions.
The exception is steam fog, shown in this picture of Maligne Lake, Alberta, Canada, just after sunrise in late summer.
The land cools off overnight while the water retains heat from the summer day.
As the cooled air slips over the lake, heat and moisture are added from below, resulting in a fog that twists and writhes– hence the term “steam fog”.

This type of fog may be persistent
The larger the lake the more persistent and the more widespread
Low level convection must be present for formation
Advective winds may spread the fog downwind

Visibility from 5/8 to less than 7 statute miles
Lower Relative Humidity than fog at about 95-99%
Generally not as restrictive to visibility

Haze – dry particles not classified as dust or something else
Occurs in stable air
Usually only a few 1000 feet thick but may extend as high as 15,000 feet
Often associated with an inversion which gives it a definite top
Haze may lead to the illusion that something is farther away than it really is
Smoke forest fires, industrial areas
Both can be bad under a temp inversion
Can cause visual illusions as well

Precip- drizzle, rain, snow
Snow however may present additional problems
May cause white out conditions or
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 good 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 of ascending or descending when actually flying level.
However, with good judgment and proper training and planning, it is possible to safely operate an aircraft in flat light conditions.

As defined in meteorological terms, white out is 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 aren’t any visual references.
Flying is not recommended in any white out situation.
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 has been the cause of several aviation accidents in snow-covered areas

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 system.
The effect can vary in intensity depending upon the amount of light on the surface.
This phenomenon 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.

The Haboob was covered in the wind chapter
Blowing dust, sand
Winds need to be at least 15kts to pick up your common variety dust
Settles out at the rate of 1,000ft/hour
Usually 3,000 to 6,000 AGL but can extend upwards to 15,000

Surface based phenomena
Classified in 8ths and vertical visibility
Restricted slant range vis
Ceiling may be noted but once below it horizontal vis may be severely restricted

Chapter 19



On 12/30/1997 a United 747 with 374 passengers was enroute to Hawaii from Japan. Dinner had just been served at 33,000 feet when without warning the 747 nosed up then plunged 1,000 feet. Passengers not wearing their seatbelts were flung against the ceiling and walls then dropped. Bags, serving trays and people were all flying about the cabin.  Within seconds it was over, but not after 110 people were injured, 12 seriously.  A 32 year old woman died of serious head injury.


  • 1. Convective currents
  • 2. Mechanical turbulence (Obstructions to wind flow)
  • 3. Wind shear

Convective Turbulence

  • Convective turbulence is most active on warm summer afternoons with light winds resulting from surface heating
  • The hotter the ground the stronger convective turbulence gets
  • Cumulus Humilis and Cumulus Mediocris are good signs of convective turbulence
  • Turbulence will exist up to the base of the clouds and in the clouds
  • Thermals may produce dry convection, watch for the dust devils


  • Descending convective currents occur over larger areas, so lighter turbulence
  • Cumulonimbus clouds = greatest turbulence
  • Gust fronts may push air out as far as 15 miles


  • By definition they involve heat exchange
  • Usually vertical movement of air
  • An example is a Thermal
  • Cold air moving over a warmer surface
  • Different variations in terrain give off different amounts of radiation

Mechanical Turbulence

  • Any obstruction to wind flow may produce mechanical turbulence
  • How long downstream it lasts is a function of the stability of the air
  • Unstable air allows for more intense turbulence but it doesn’t last as long
  • Stable air resists intense turbulence but will allow the turbulence to last downwind a lot longer


  • Usually the faster the wind the more turbulence can be expected
  • Much like water flowing over and around a rock in a stream
  • Watch for buildings along a runway
  • Especially buildings upwind in the final approach path
  • Trees are another good thing to watch

Mountain Waves

  • Mountain waves are a form of mechanical turbulence
  • When stable air passes over a mountain or ridge line waves develop over and downwind
  • The wave may extend as much as 600 miles downwind
  • Turbulence created by the Rockies has been observed in the Midwest
  • May reach as high as 200,000 feet high
  • Standing lenticular clouds may form at the top of each wave form and seem to stand still
  • Rotor clouds may form along the leeward side of the mountain
  • Down drafts may exceed climb rate

Mountain Wave

  • Mountain wave
  • Forms on the leeward side
  • Like waves on a lake
  • 40 kts or greater expect mountain wave
  • 25 kts or greater expect turbulence
  • Double Cap cloud over Mt. Ranier

Clouds Associated With the Mountain Wave


  • Mountain flying
  • Cross a 3000 to 5000 above crest
  • some say 2000 to 4000
  • Take height of mountain above terrain / 2 = crossing height
  • Cross ridge line at 45° angle
  • Mountain passes and valleys, stay right, be prepared know your elevations prior to takeoff and always have an out.


  • May be at any altitude
  • May be at any direction
  • Measured in feet per second
  • Some aircraft manuals have provisions for figuring max speed based on 30 fps or 40 fps gusts.
  • There are 4 main causes of wind shear
  • 1.Low level temperature inversion
  • 2.Clear air turbulence
  • 3.Frontal zones
  • 4.Low level wind shear


  • 1. Low level Temp inversion
  • warm air over cold mixing in the layer
  • check your winds aloft and area forecasts to get the complete picture
  • If you see 25 kts or higher 2000-4000 agl and calm surface winds it’s a good bet for a shear zone.


  • 2. Clear Air Turbulence
  • This can not be seen so no warning
  • Associated with the jet stream
  • The worst shear is found in an upper level trough on the polar side of the jet
  • This is where the temp differential is the greatest in the shortest amount of distance


  • 3. Frontal Zone
  • Wind changes abruptly in a frontal zone
  • Fast moving cold front is the worst
  • There is almost always some indication of a fast moving cold front
  • A squall line may be associated with this type of front
  • A low dust cloud with a sharp leading edge


  • 4. Low level wind shear
  • This is mainly associated with thunderstorm activity
  • Microburst may have downdrafts up to 6000 fpm
  • Like a garden hose pointed at a sidewalk
  • Calm wind shears to a headwind which then shears to a tailwind


  • Pass under jet at least 1000’
  • Sink rate varies 300-500 fpm
  • Near the ground at 100-200’ they move outward at 2-3 kts
  • X-wind of 5 kts holds upwind vortex on the runway
  • Touch down past nose wheel touchdown
  • Takeoff before nose lifts and climb higher or early turn out of the way
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