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Meteo 4 > Mountain Waves > Flashcards

Mountain Waves Flashcards

1
Q

What is the definition of a mountain wave?

A

Oscillations on the lee (downwind) side of a mountain caused by horizontal air flow over rough terrain

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2
Q

How fast can the vertical air currents caused by mountain waves get?

A

Vertical air currents can reach over 5000fpm and produce severe turbulence and loss of aircraft control

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3
Q

Where are the downdrafts in mountain waves most severe?

A

Near the mountain or ridges and at the same height as the summit. Horizontal and vertical shear my exist in mountain waves

These downdrafts can be anywhere from 80fps to 5000fps

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4
Q

Hills as small as ___ feet high have the potential to form mountain waves

A

300ft

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5
Q

Clouds may or may not form in mountain waves. How might this be dangerous?

A

If no clouds form, we wont know there are violent waves there until we are in them

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6
Q

Just because there are mountains doesn’t mean mountain waves will form. What ‘ingredients’ does it take to make a mountain wave?

A
  • Wind speed of 25kts or more perpendicular to the mountains
  • Wind direction within 30° perpendicular to the mountains
  • Winds aloft increasing in height (ascending the mountain side)
  • Stable air mass layer, an isothermal layer, or inversion, aloft near the mountain top
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7
Q

Why is stable air aloft a requirement for the formation of mountain waves?

A

Because unstable air would just continue to rise and rise. The definition of stability is for an object to return to its original state after a displacement. So the air rises up the mountain side, then tries to sink back down, then gets forced up the next mountain or hill, etc etc, forming the waves

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8
Q

What are the two key ‘parts’ of a (mountain) wave?

A
  • Wave length (distance from one crest to the next)
  • Amplitude (distance between a valley and the peak of the wave)
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9
Q

Diagram of a mountain wave and possible cloud formations

A
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10
Q

How far can mountain wave be present downstream of a mountain range?

A

A.I.M. says 150NM, this is the exam answer
Now scientists are saying up to 300NM

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11
Q

What are lenticular clouds and what can they tell us?

A

With sufficient moisture in the air, lenticular clouds will form at and mark the crest of each mountain wave.

This can let us know that there are mountain waves present and that dangerous turbulence or wind shear may be ahead

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12
Q

What is the average length of a mountain wave? (From crest to crest)

A

8 NM

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13
Q

What factors affect how long or short wavelengths are?

A
  • Stability - the more stable the air, the shorter the wavelengths
  • Speed - the faster the wind, the longer the wavelength
  • Ridge Spacing - ridges must be at least 5 miles apart
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14
Q

What factors affect the amplitude ( distance between valley and crest of one wave) of mountain waves?

A
  • Smaller/shorter amplitudes found near the ground
  • Larger/deeper amplitudes occur between 3000 to 6000 ft above the ridge
  • Lower stability = shorter amplitudes
  • Larger mountains = larger amplitudes
  • Sharp lee slope = larger amplitudes
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15
Q

What produces the biggest amplitudes?

A

Drops on the lee side of a mountain of more than 3000ft

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16
Q

What is amplification and dissipation in mountain waves?

A

If the mountains/hills are spaced with, they can basically fling the air up higher than it originally was.

Dissipation is when it hits something that breaks it momentum or formation

17
Q

What four types of clouds can form in mountain wave situations?

A
  • Lenticular
  • Rotor
  • Cap
  • Banner
18
Q

What are lenticular clouds and what can they tell us?

A
  • Most commonly at 20000 - 40000 ft (near the tropopause)
  • Formed by the air cooling as it rises up the side of a mountain then heating down the other side and evaporating the moisture, which gives them that saucer shape
  • Form on the crests of waves and can be hundreds of miles long

Lenticular clouds can tell us:
* If there is turbulence within the wave itself
* That the air speed will be fastest just under where the cloud has formed. The higher the crest, the faster the speed.

19
Q

If you have a nice, smooth, saucer shaped lenticular cloud, what does that tell you about the air flow?

A

That it is smooth and steady with similar speed as you go up in altitude

20
Q

What do torn or broken lenticular clouds tell you?

A

That there is a significant difference in wind speed at various heights, causing wind shear and bad turbulence

21
Q

What two types of lenticular cloud will you see in a PIREP?

A

ACSL - Alto-cumulus Standing Lenticular
CCSL - Cirro-cumulus Standing Lenticular

22
Q

What is a rotor cloud?

A
  • Violent and dangerous
  • Caused by wind shear (faster air above, slower air below)
  • Stay away from them
  • Will form downwind of each wave crest
  • Can be forming and dissipating at the same time due to rotation of the air
23
Q

More pictures of rotor cloud

A
24
Q

More rotor cloud info

A
25
Q

If you must pass through the area of a rotor/roll cloud, what should you do?

A

First Choice: Fly over. Expect strong winds
Second Choice: Fly around. But avoid flying beneath

**Clear air does not mean there isnt horrible turbulence

26
Q

What is a cap cloud?

A

Lie overtop and very near the top of the mountain

27
Q

What indication from within the cockpit do you have to indicate you may be travelling through mountain waves?

A

Airspeed indicator is going up and down.

Altimeter may over read (tell you you are higher than you actually are) by as much as 3000ft.

This is due to the drop in pressure associated with the increased wind speeds in mountain waves. This combo of mountain waves and non-standard temperature can result in dramatic over-reading by the Altimeter

28
Q

When will mountain wave activity be depicted on the GFA?

A

When the turbulence is forecasted to be moderate or greater

29
Q

What are the four types of winds that flow through mountains?

A
  • Anabatic
  • Katabatic
  • Glacier
  • Funnel
30
Q

What are anabatic winds?

A

Formed as the side of a mountain facing the sun warms and causes the air over it to rise upward along the slope

31
Q

What are katabatic winds?

A

In areas that are shaded or covered in snow, or at night, air will cool and flow down the mountain slopes and into the valleys.

These winds can seem to ‘pulsate’. As the cooling air builds and then is suddenly released downstream

32
Q

What is a glacier wind?

A

A type of katabatic wind but more extreme. Caused by glaciers which can produce large areas of extreme cooling and can create shallow winds in excess of 80kt

33
Q

What is funnel wind?

A

Calm winds forced through a narrow valley, speeding up, and exploding out the other side

34
Q

How do mountain winds affect your altimeter?

A

It will tell you you are higher than you are

35
Q

A greater intensity of turbulence is found when flying _________ into a mountain wave

A

Downwind

36
Q

What can be expected regarding the airflow near a cap cloud?

A

Extremely strong downdrafts

37
Q
A

Meteo 4 (5 decks)

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