Mountain Waves

Question 1

What is the definition of a mountain wave?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

Where are the downdrafts in mountain waves most severe?

Answer 3

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 80fpm to 5000fpm

Question 4

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

Answer 4

300ft

Question 5

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

Answer 5

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

Question 6

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

Answer 6

• 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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

Diagram of a mountain wave and possible cloud formations

Answer 9

Question 10

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

Answer 10

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

Question 11

What are lenticular clouds and what can they tell us?

Answer 11

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

Question 12

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

Answer 12

8 NM

Question 13

What factors affect how long or short wavelengths are?

Answer 13

• 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

Question 14

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

Answer 14

• 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

Question 15

What produces the biggest amplitudes?

Answer 15

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

Question 16

What is amplification and dissipation in mountain waves?

Answer 16

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

Question 17

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

Answer 17

• Lenticular
• Rotor
• Cap
• Banner

Question 18

What are lenticular clouds and what can they tell us?

Answer 18

• 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.

Question 19

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

Answer 19

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

Question 20

What do torn or broken lenticular clouds tell you?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

What is a rotor cloud?

Answer 22

• 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

Question 23

More pictures of rotor cloud

Answer 23

Question 24

More rotor cloud info

Answer 24

Question 25

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

Answer 25

First Choice: Fly over. Expect strong winds Second Choice: Fly around. But avoid flying beneath **Clear air does not mean there isnt horrible turbulence

Question 26

What is a cap cloud?

Answer 26

Lie overtop and very near the top of the mountain

Question 27

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

Answer 27

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

Question 28

When will mountain wave activity be depicted on the GFA?

Answer 28

When the turbulence is forecasted to be moderate or greater

Question 29

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

Answer 29

* Anabatic * Katabatic * Glacier * Funnel

Question 30

What are anabatic winds?

Answer 30

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

Question 31

What are katabatic winds?

Answer 31

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

Question 32

What is a glacier wind?

Answer 32

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

Question 33

What is funnel wind?

Answer 33

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

Question 34

How do mountain winds affect your altimeter?

Answer 34

It will tell you you are higher than you are

Question 35

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

Answer 35

Downwind

Question 36

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

Answer 36

Extremely strong downdrafts