MOUNTAIN WAVE

The Aeronautical Information Manual, paragraph 574 states, “Your first experience of flying over mountainous terrain, particularly if most of your flight time has been over the flatlands of the Midwest, could be a never-to-be-forgotten nightmare if you are not aware of the potential hazards awaiting … Many pilots go all their lives without understanding what a mountain wave is. Quite a few have lost their lives because of this lack of understanding. One need not be a licensed meteorologist to understand the mountain wave phenomenon.” 

Perhaps other than IFR weather, nothing affects the pilot flying in the mountains more than the mountain wave.

To develop an understanding of the mountain wave, we need to ask and answer some questions:

  • What is a mountain wave?

  • What forms it?

  • Why is it of concern to pilots?

  • What are its distinguishing characteristics?

  • How do we deal with it?

 

The most distinctive characteristic of the mountain wave is the lenticular cloud. This is a "signpost of the sky" indicating that mountain wave activity is present.

Someone has come up with all kinds of names for the mountain wave. There is the:

  • Mountain wave

  • Standing wave

  • Lee wave

  • Gravity wave

  • Standing lenticular

  • ACSL (altocumulus standing lenticularis)

  • Or just plane "wave"

  • Pilots have developed a few names of their own, but we can't mention them here.

The wave that forms over the mountain is more properly called the "mountain wave." The waves downwind from the mountain are the "standing wave" or "lee wave." Pilots have come to accept all of these names for wave activity, regardless of position of the lenticular clouds.

How does the atmosphere go about setting up a mountain wave condition? It needs three elements:

  • Wind flow perpendicular to the mountain range, or nearly so, being within about 30 degrees of perpendicular.

  • An increasing wind velocity with altitude with the wind velocity 20 knots or more near mountaintop level.

  • Either a stable air mass layer aloft or an inversion below about 15,000 feet.

Because of these elements, the weather service is able to predict the mountain wave condition with over 90-percent accuracy.


Figure 1

Figure 2


Figure 3

In figure 1, we have likened an atmosphere with low stability to a flimsy spring that offers little resistance to vertical motion. So while the lower coils move easily up and over the mountain, the jolt received at ground level is not transmitted very far upward.

Figure 2 represents a stable atmosphere that is similar to a tough, heavy spring. This air, when it strikes the mountains, tends to suppress internal vertical motion. It is essentially too tough for oscillations to be set up.

In figure 3 we have an arrangement of a strong coil sandwiched between two weaker springs to simulate an atmosphere with a stable layer sandwiched between areas of lesser stability. With this arrangement it is conceivable that the strong spring will continue to bounce up and down for some time after the parcel of air has crossed the mountain ridge. With a stable layer (or inversion aloft) the air stream is both flexible enough to be set in vertical motion and elastic enough to maintain that motion as a series of vertical oscillations.

As the air ascends, it cools and condenses out moisture, forming the distinctive lenticular clouds. As it descends, it compresses and the heat of compression reabsorbs the moisture. It goes through this up and down action many times forming a distinctive lenticular cloud at the apex of each crest, providing there is sufficient moisture present for the cloud formation.

The up-and-down action forms a trough at the bottom of its flow and a crest at the top of the flow. The distance from trough to trough (or crest to crest) is called the wave length. The wave length is directly proportional to wind velocity and inversely proportional to stability.

The wave length is used for visualization. In the area from the trough to the crest is an area of updrafts. The area from the crest to the trough is predominately downdrafts.

In the intermountain west the wave length can vary from about 2 nautical miles to over 25 nautical miles. It averages 8 miles and extends downrange about 150-300 nautical miles. Satellite photos have shown the wave capable of extending over 700-nautical miles downwind from the mountain range.