There are two types of mountain waves: trapped lee waves, and vertically propagating waves. Trapped lee waves are the type most pilots think of, and they affect piston aircraft the most.

Trapped lee waves propagate out horizontally, and they can extend hundreds of miles downwind of the mountain barrier that creates them. Here's how they form, and how you can avoid them.

When Do Trapped Lee Waves Form?

There are three things needed to create a trapped lee wave:

• Cross-barrier flow of at least 20 knots
• Moderately stable atmosphere
• Significant wind shear (more than 1.6)

These three ingredients are most common in the fall, winter, and early spring, which is when you see mountain wave activity the most.

Vertical Extent And Tilt

Trapped lee waves occur in a limited altitude band, generally extending 10,000 to 15,000 feet above the mountain barrier. However, this band is also where piston aircraft operate, creating a hazard most often experienced by GA aircraft.

Upper And Lower Zones

There are two zones in a trapped lee wave: the upper zone, and the lower turbulence zone. Both zones create their own unique hazards to aircraft. Fortunately, both zones are easy to identify.

Hazards In The Upper Zone

The upper zone exists from roughly 1,000' to 2,000' feet above peak height and higher.

Here you'll find updrafts and downdrafts with the wave. Depending on the intensity of the wave, the updrafts and downdrafts may be light, but they can also be incredibly strong and exceed the climb capability of your aircraft.

While you can't "see" mountain wave, there are visual cues when there's moisture and when the zone is producing up and down drafts.

Visual Cues In The Upper Zone

Rows of lenticular clouds that extend downwind from a mountain barrier are an excellent indication that a trapped lee wave is happening.

In the picture below, a wave cloud formed by the continental divide is extending east of Boulder, Colorado. The smooth appearance of the lenticular clouds is a clear indication of the wave.

So how strong are the up and down drafts? There's no perfect answer, because it depends on the strength of the cross-barrier wind, the wind shear from peak height to 6,000' above the peaks, and the stability of the air.

You will have some warning as you approach a mountain barrier, though. As you approach a mountain barrier from the downwind side, the up and down drafts will get progressively stronger, and they will reach their peak amplitude at or just slightly downwind of the mountain peaks.

As you approach a mountain barrier, if the up and down drafts start becoming stronger than you're comfortable with, or anytime they start exceeding the climb performance of your aircraft, turn away from the mountains and fly back in the direction you came from.

Generally, if you see lenticular clouds as large as the image below, it's not a good day to fly a piston aircraft near the mountains.

Hazards In The Lower Turbulence Zone

In the lower turbulence zone, which exists from roughly 1,000' to 2,000' above peak height and below, rotors can create moderate to severe turbulence.

Fortunately, there are visual cues of the lower turbulence zone when there's moisture.

Visual Cues In The Lower Turbulence Zone

In the video below, rotor clouds are forming below peak height and paralleling the mountain range to the east of Boulder, Colorado. The ragged edges of the clouds and their rotating motion is a clear indication of rotors.

Rotors form under the crests of trapped lee waves, and just like the up and down drafts, their strength decreases the further you get away from the mountain range.

Treat any rotor cloud like a thunderstorm, and don't fly through it. Generally, rotor clouds parallel a mountain range. So if they're visible, increase your distance from the mountain range to stay clear of the rotors.

Avoiding Mountain Wave

To avoid mountain waves, start by reading the forecasts. Remember there are three things needed to create a trapped lee wave:

• Cross-barrier flow of at least 20 knots
• Moderately stable atmosphere
• Significant wind shear (more than 1.6)

Then, when you're in the air, look for the visual cues like rotors down low, and lenticular clouds aloft.

Avoid flying through both types of clouds, and you'll set yourself up for a smooth flight.

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