Adverse Wind & Fog 15 & 16

Chapter 16
Mountain Weather

“Aviation in itself is not inherently dangerous, But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.”
If you want to be the best pilot in the world, kill yourself in a plane crash
They will all be saying so at your funeral
“What a great pilot”
“He died doing what he loved”

Mountain Waves and Adverse Winds

  • Things get really interesting when the wind picks up in mountainous terrain
  • Unstable air will usually lead to cumulus development and possible thunderstorms
  • Stable air will usually lead to mountain wave formation, gravity waves, roll and rotor clouds
  • If little or no moisture is present these wind currents will not be easy to identify
  • The more wind, the higher the likelihood of dangerous flying conditions

Gravity Waves

  • As covered in an earlier chapter, gravity waves need stable air to develop
  • As wind blows across a mountain range or single peak, air is displaced upward due to orographic lifting
  • As the wind descends, it gains inertia, momentum increases and it overshoots its original level of buoyancy
  • It rebounds upward and the cycle repeats until energy is returned to a state of equilibrium
  • This can produce turbulence on the downward side of the mountain structure
  • The severity of the turbulence is governed by many factors, some of which are:
  • The speed of the wind
  • The height of the mountain
  • The degree of stability of the air

Kelvin-Helmholtz Waves

  • Sometimes referred to as gravity-shear waves these are visible when moisture is present, but may go unseen if no moisture is present
  • These occur when a layer of slower winds shears to an area of stronger winds
  • Essentially what is happening is the tops are getting blown downwind by a stronger wind than found at the cloud base
  • Since the turbulence found here is based on shear, changing altitude is your best bet

Vertically Propagating Waves

  • The leeward side is where the action gets dangerous
  • The book mentions a hydraulic jump formation that can cause severe to extreme turbulence
    Winds of 100kts may accompany this jump and may be present 10,000 feet or more above the ground

Trapped Lee Waves

  • When wave energy is confined below a certain altitude they are referred to as Trapped Lee Waves
  • A stronger faster wind at altitude serves as the mechanism that does the trapping, confining the energy of the wave into a narrow vertical space
  • These may extend downwind from the ridge line or mountain range many miles causing low level turbulence and a rough climb to altitude
  • Clouds are typically 1,000 to several thousand above the ridgeline
  • Look for strong shear and turbulence below the trapped lee wave
  • If moisture is present roll and rotor clouds may exist to tip you off

Roll, Rotor and Jump

  • When approaching a ridge or mountain range with a strong headwind, your whole day could be ruined by a downdraft stronger than your rate of climb
  • Approach the ridge at no lower than Vy + 5kts
  • Approach a ridgeline at a 45 degree angle, allowing for a shorter turn away from the higher terrain
  • Once cresting the ridge, turn 90 degrees to the ridge line to minimize exposure to the highest terrain
    If you cannot outclimb the terrain make the shortest turn and execute a climb away from the higher terrain.
  • Watch for terrain growing
  • If you can see more and more terrain on the other side of the ridge, you’re higher than the ridge
  • If the ridge is growing in your windscreen and you see less and less terrain on the opposite side of the ridge, you are lower than the ridge

Vertical Axis Vortices

  • Flow over vs Flow around
  • Reaching 150 kts or more these are no joke
  • Same conditions as the horizontal flow we have been talking about but in the vertical

Boras

  • Cold air flowing downslope with very little compressional heating
  • This is unlike the Chinook wind where compressional heating is a bigger factor
  • 2 primary causes of the Bora:
  • Cold front moving over a mountain range
  • Cold outflow from T-storms that spills over the opposite slope

žCHAPTER 17

Tropical Weather

LOCATION

  • Latitude of 23.5º N to 23.5º S
  • Intertropical convergence zone – where the northeast and southeast trades converge (near the equator)
  • Lots of moisture to great heights
  • As a result severe thunderstorms are possible if air is unstable

SUBTROPICAL HIGH PRESSURE BELTS

  • Semi-permanent highs occur over the water
    generally good weather
  • Lows form over the land because of temp. differential
    land is generally warmer
  • Commonly an inversion is set up under the subtropical high
  • The inversion is strongest when the east side of the high is over the west edge of a continent
  • Dry weather – California
  • The inversion is weakest and highest when the west side of the high is over the east edge of the continent
  • Wet and thunderstormy in Florida

TRADE WIND BELTS

  • Northeasterly in the Northern hemisphere
  • Southeasterly in the Southern hemisphere
  • Flying wx is generally good over the ocean (uniform temp)
  • Where they blow from land to sea generally dry terrain
  • Where they blow from sea to land generally wet terrain

MONSOON, THE MOTHER OF ALL SEA BREEZES

MONSOON

  • India gets average of 400 inches of rain a year from monsoons
  • June to October is the monsoon season
  • In summer, wind blows inland (wet air)
  • In winter, wind blows seaward (dry air)

SHEAR LINE

  • This happens when the air mass has been modified to conditions similar to the present mass it replaces
  • All that is left is a front line or difference in wind direction
  • This may happen to polar front in the U.S.
  • May influence storms in the Atlantic or Gulf of Mexico

TROUGH ALOFT

  • AKA Tropical Upper Tropospheric Trough (TUTT)
  • Spreads extensive clouds to the east of the trough line
  • Lots of rain – 460” of rain on Mt Waialeale Hawaii

TROPICAL WAVE

  • Forms on the southeast side of the subtropical high
  • Travel from east to west
  • Preceded by good wx
  • Followed by cloudiness in a North South line
  • Provide the building blocks for hurricane formation

TROPICAL CYCLONE

  • Any low that is born in the tropics
    1. Tropical depression up to 34 kts
    2. Tropical storm 35 – 64 kts
    3. Hurricane or Typhoon above 65 kts
    Super typhoon winds 130 kts or greater

HURRICANE

  • Low level convergence, high level divergence
  • Sets up a chimney effect
  • Large quantities of water release lots of latent hear furthering the updrafts
  • Rise in temp. lowers surface pressure which increases the low level convergence
  • Generally move in a west/northwest path

Saffir-Simpson Hurricane Scale

Strongest Hurricane

  • 10/23/2015
  • In addition to its unprecedented 200-mph (320-kph) sustained winds, Hurricane Patricia now holds the record for lowest pressure in any hurricane on record.
  • With a minimum central pressure of 880 millibars (25.99 inches of mercury) at the 4 a.m. CDT advisory,
  • Patricia broke the record of 882 millibars set by Wilma almost exactly 10 years ago.