Local Winds Flashcards
Describe the general characteristics of a mountain wave set-up with reference to wave lengths.
5-40km wave lengths
Wind 15kts at sea level accelerating as it climbs 20 at ridge top.
90º wind direction (+/- 30º)
Unstable low air, stable ridge crest air for restoring force
Describe the general characteristics of a mountain wave set-up with reference to position and rotation of any possible rotor zones
Found on the crest of the mountain waves. On the lee side of the mountain
Dont move much/ anchored in position
Side furtherest away rotates down
Rotation is opposite for upper level rotors.
Describe the general characteristics of a mountain wave set-up with reference to position and possible cloud development
Cumilform cloud pushing up mountain on windward side of mountain, Triggered by orographic lifting followed by lower level stratiform cloud. As moisture content of air increases ahead of an approaching front, multi-layered stratiform cloud forms with TCu and Cb.
Lenticular cloud forces on crests of wave downstream. As wind at level increase the wave propagate higher into troposphere.
Rotor cloud
Heights of the friction layer
2-3000ft normally. North west arch friction layer mash be 15-18000ft
Areas of probable severe turbulence
Rotor zones on lee side = severe turbulence
In high amplitude waves due to them breaking apart
Strongest rotor is normal the first wave downwind
Areas of possible severe airframe icing
Cb clouds on windward side
Lenticular cloud
Freezing level in visible moisture (any cloud above freezing level)
Factors affecting the wave amplitude
Shapes of the mountain ranges as the wind glows through
Vertical extent of mountain/how steep
Strong inversion at crest will reduce amplitude
Factors affecting wave-length
Lee shape steeper = longer
Increased wind speed = longer
Describe the flight conditions associated with mountain waves
Updraughts portion of wave smooth as long as not broke
Downdraught portion smooth but hard to maintain height
Rotor zones are below lenticular and either side of them
Will be severe turbulence
Explain the rotor streaming process (3)
Same formation as rotor zones except one thing, wind speed decreases above mountain height instead of increasing
Forms at or below ridge top height and is not anchored so moves downstream
Flight conditions associated with rotor streaming
Severe turbulence
Worst immediately in the lee of the range, reduces in strength the further it gets from the range
State the requirements for the development of Fohn wind
Moist air hits the ranges and is forced to rise orographically up windward side.
Lifted air is subject to decreasing pressure as it rises and cools (DALR 3ºC/1000ft) where cloud forms and latent heat is reduced which reduces cooling effect by half - to the SALR (1.5ºC/1000ft form cloud base to top of ranges.
During remaining ascent at SALR, heavy precipitation forms which removes moisture from air.
Once over ridge line, air descends but because air is dry it warms up at the DALR for the whole descent increasing temperature.
Describe the flight conditions when flying in Föhn conditions in the following positions: windward side of mountain
Thunderstorms
Large amounts of rainfall/poor visibility
Severe icing above freezing level
Updraughts
Describe the flight conditions when flying in Föhn conditions in the following positions: mountain range
Weather transitions form thunderstorms/rain to SKC
Air stops rising, levels out and starts descending
Describe the flight conditions when flying in Föhn conditions in the following positions: on lee side of mountain
Gusty wind, warm temp, dry air
