Learning to recognize areas of potential mountain wave turbulence isn't difficult, but you do need to know where to look.

There are two primary types of mountain waves: trapped lee waves, and vertically propagating waves. In this article, we'll focus on trapped lee waves, and the types of turbulence you can expect flying through them.

Trapped Lee Waves

Trapped lee waves are the type of mountain wave that most people think of. You know you're in one when you're maintaining altitude and your plane slowly starts pitching up and down, while your airspeed increases and decreases. It all happens because you're flying through the crests and troughs of the wave.

You can see it in the video below:

Trapped Lee waves are also well known for their ability to travel long distances downwind: we're talking 500+ nautical miles downwind. So how does this type of wave form?

There are three key aspects that need to happen for a trapped lee wave to form:

• Cross-barrier flow of at least 20 knots
• A moderately stable atmosphere
• Significant wind shear from the tops of the peaks to 6,000' above the peaks (a ratio of more than 1.6).

Let's break these down further.

Cross-Barrier Flow: What Is It?

Cross-barrier flow is the angle that an airstream strikes a mountain barrier.

When winds at peak height are 20 knots or greater, and they strike the ridgeline at a 90 degree angle, a mountain wave is more likely to form.

Moderately Stable Atmosphere

In order for trapped lee waves to form, they need to occur in a relatively stable atmosphere. A stable atmosphere, in basic terms, means a parcel of air lifted into the atmosphere (in this case up the side of a mountain) will want to return to its equilibirum or starting point.

This alone is one of the main reasons why trapped lee waves are able to travel so far downwind of the barriers that created them.

Strong Shear: How Do I Determine This?

Strong shear (a ratio more than 1.6) yields more pronounced trapped lee waves. Determining the shear ratio is easy. All you need is a Winds and Temperatures Aloft forecast.

First, you want to look at the wind speeds at peak height. In the Rockies this is roughly 12,000' (6,000' for the Appalachians). You then want to take a look at the wind speeds 6,000' above the peaks, which is 18,000' in the Rockies, and 12,000' in the Appalachians.

Divide the wind speed 6,000' above peak height by the wind speed at peak height, and you'll get shear a ratio.

Any ratio greater than 1.6 is a good setup for a trapped lee wave.

Two Turbulence Zones

Trapped lee waves create two turbulence zones: the Lower Turbulence Zone, and the Upper Zone.

The Lower Turbulence Zone

This zone ranges from about 1,000' to 2,000' above peak height, all the way down to the surface. This region tends to be a higher threat than the upper zone.

As the waves travel downwind of the peaks, rotors form underneath the crests of the ascending wave. These rotors can create moderate to severe turbulence.

So how can you avoid them? If there's any moisture in the atmosphere, the ascending part of the rotor will form a ragged cloud as it rises and cools.

You'll notice that most visible rotors tend to form in long lines that parallel the mountain barrier that's creating them.

Here's an example:

When you see a rotor like this, treat it like a thunderstorm. Assume that it will have moderate to severe turbulence, and avoid flying through it.

Upper Turbulence Zone

This zone occurs from about 1,000' to 2,000' above peak height, continuing up to 15,000' above the peaks. In this zone, you'll primarily find the updrafts and downdrafts associated with the trapped lee wave. You'll see your airspeed fluctuate up and down as you maintain altitude through the crests and troughs of the wave.

In most cases, there isn't much turbulence here. However, if the wave it strong enough, rotors can form under the crests of the wave.

Turbulence Avoidance

On a clear day, you may not have much luck trying to see mountain wave clouds, or even knowing they are there. You'll have to rely on forecast discussions, the Winds and Temperature Aloft forecasts, and PIREPs.

Fortunately, on days that have significant moisture content, you'll see the waves in action through cloud formations. There are three types of clouds you may see that will give you a good idea of where the waves are at, and which spots to avoid.

Cloud 1: Cap Cloud

A cap cloud is formed when a moisture-rich airmass is forced up and over a ridgeline, forming a cloud that 'caps' the mountain peak.

Cloud 2: Lenticular Cloud

Next, a lenticular is formed in a similar fashion, however, the cloud is formed at higher altitudes where the top of the lee wave reaches its crest. Lenticular clouds can form rotors under the peak of the crest, creating turbulence. They also tend to have significant icing in them.

When you see a lenticular cloud, it's best to treat it like a rotor cloud and fly around it.

Cloud 3: Rotor Cloud

Rotor clouds are located in the lower turbulence zone and pose a significant threat to aircraft. You can see them as an eddie of rotating air. Depending on the moisture in the atmosphere, you may see a full rotor cloud or just part of one.

Again, when you see a rotor cloud, assume it will have moderate to severe turbulence and fly around it.

Read The Forecasts, Then See And Avoid

To avoid mountain wave turbulence, start by reading and interpreting the forecasts. Then visually see and avoid the wave clouds. Doing both gives you a good idea of when mountain wave turbulence is present, and where the turbulence is most like to be.

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