Types Of Adverse Winds
- Variable wind
- Sudden wind shift
- The book says you’re most at risk during takeoff and landing
- It also says little planes are affected most
- This wind blows from one side or the other
- Breaks off the landing gear
- This is where the wind speeds up suddenly (10kts or more)
- Breaks or bends the spar
- This wind blows up your tail
- Runs you off the end of the runway or into the trees on takeoff
- A 10% increase in landing speed will result in at least a 21% greater landing distance
- Variable wind/Wind shift
- This is when the wind is variable and is shifty
- Wind shear
- This one is trickier than the rest, it pretends it’s your friend then whammy
Computing X-wind & Headwind
- For x-wind use the sin of the angle between the wind and the runway
- Wind 280@20 runway 36
- Sin(80)=.984807753 x 20=19.69kts
- For the headwind use the cos of the angle between the wind and the runway
- Wind 280@20 runway 36
- Cos(80)=.1736481777 x 20=3.47kts
Obstructions to Visibility
Weather Related Accidents
- Causal factors in order:
- Adverse winds
- Reduced visibilities
- Low ceilings
- High density altitude
- 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 loose 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
Fog Formation Mechanisms
- 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
Types of Fog
- 1. Radiation fog
- 2. Advection fog
- 3. Upslope fog
- 4. Precipitation-induced fog/Frontal fog
- 5. Ice fog
- 6. Steam fog
- 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
PRECIPITATION INDUCED FOG FRONTAL FOG
- 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
PRECIPITATION INDUCED FOG FRONTAL FOG
- Very persistant
- 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 AND SMOKE
- 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
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 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