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Chapter 18
Flight Hazards Over
High Ground
Effect on a Warm Front
• When a warm front passes a mountain range, the air, which is lifted
over the mountain, will strengthen the formation of clouds and the
precipitation on the windward side.
‘Stau’
Effect on a Cold Front
• A cold front lifted over a
mountain is always
strengthened
- Cold front passing a mountain,
coldest air at the leeward
side (top picture) coldest air
at the windward side
(bottom picture).
Coldest
Coldest
Effect on a Cold Front
- If the coldest air is found
behind the cold front, the
extra speed given to the cold
air downhill may strengthen
the cold front some distance
behind the mountain, where
the “warmer” air is pushed
up, forming a Cb cloud.
Lenticular Clouds Generated by Standing
Waves over the Highlands
Mountain Waves Over The Alps
Mountain Waves (MTW)
• Mountain waves are characterized by two parameters:
• wavelength and
• amplitude.
• Wavelength is known to be directly proportional to wind speed and
inversely proportional to stability.
• Typical wavelength values are 2 to 25 NM extending 150 to 300 NM
downwind from the mountain crest.
Standing/Mountain Waves
Conditions Favourable for
Standing Wave Formation
• High ground or mountain range with gentle
rise to windward
• Low level wind at least 15 kt at crest.
• Little change in direction with height
• Wind increasing with height.
• Wind at 90° (30°) to line of terrain.
• A marked stable layer at the level of crest
with less stable layer above.
Additional Facts
• Usually extend 50 to 100 nm downwind.
• Average wavelength is about 5 nm in the
troposphere.
• In the stratosphere wave length may be 15 nm
typically but 60 nm have been measured.
• Double amplitude commonly 1500 feet.
• Vertical velocities of 1000 fpm common.
• 2000 fpm max in UK and 5000 fpm over
Rockies in USA.
Visual Detection
• Characteristic ‘lenticular’ clouds form in crests of
waves downwind of ridge.
• Roll clouds with severe turbulence usually form in
the first wave down wind level with the ridge.
• Standing waves are frequently associated with
Nacreous clouds.
• Cap cloud forms over the ridge.
Roll Cloud over the Scottish Highlands
Turbulence
• Strong wind over rough terrain may produce sev.
turbulence.
• Rotor zone and area below are strongly turbulent
and severe structural damage may occur.
• Reversed flow often observed at the surface.
Turbulence
• Strong wind in lower troposphere with wind in
middle or lower layers suddenly dropping or
reversing produces severe turbulence and ‘rotor
streaming.’
• These rotors may stream down wind some distance
at low level.
• Turbulence in jet streams is increased by high
ground causing flow disturbance and increased wind
shear.
Further Hazards
• Downdraught to leeward of high ground.
• Severe clear icing in lenticular cloud down
to -27° C.
• moderate to severe turbulence may extend
well into the stratosphere.
Flight Information
• Presence forecast on significant weather
charts.
• Do not enter rotor zones.
• Maintain a height at least equal to height of
ridge above the terrain & cross at 90º to line of
high ground
Flight Information, cont’d
• Be prepared for icing.
• Choose flight levels with care to avoid CAT.
• Use turbulent air penetration procedures.
• At levels above FL300 margin between stall
and Mach 1g buffet may be small and jet upset
may occur.
CAT and Wave Induced Turbulence with low level windshear
 If a jet core crosses the mountain just below the tropopause, there is a very
great risk of CAT about 5000 ft above and below the tropopause.
Venturi
Effect
Alteration of
Geostrophic
Wind
Effect on Low Level Flow
Low Level Flow
Disturbance
Summary
– When flying among mountains always be very cautious of the
weather situation and plan carefully. If you are not familiar with the
area, you should avoid flying at low level if the wind speeds at the
level of the peaks exceed 20 KT. You take great risks when flying in
winds exceeding 40 KT.
Summary
– If you are going to cross a mountain range at low level, do so at an
approach angle of 45° to the mountain to be able to rapidly change
your heading, and at least double the height of the mountain above
surrounding terrain, if it is blowing more than 20 KT.
Summary
– As a summary we can state that the frontal activity strengthens
on the windward side of a mountain and weakens or even
disappears on the lee side due to the subsidence.
– Icing conditions intensify on the windward side of all mountains
due to the increased lift and adiabatic cooling.
Summary
– Generally speaking, there is always a risk of abrupt changes in
weather/wind in the mountains, if there is any weather/wind at
all. Weather services unless locally based tend to miss these
local effects.