Mountain Wave Dangers And How To Avoid It?

Mountain waves come in two varieties: trapped lee waves and vertically propagating waves. Most pilots consider trapped lee waves more severe as they usually affect piston aircraft more directly.

Trapped lee waves travel horizontally and can travel hundreds of miles away from their source, such as an abrupt mountain barrier. Here’s how they form and how you can avoid them:

Three elements are necessary to generate 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, when you see mountain wave activity the most.

Trapped lee waves usually form within a narrow altitude band extending 10,000-15,000 feet above mountain barriers; this area also coincides with piston aircraft operating altitude, making this threat particularly common among GA aircraft.

A trapped lee wave contains two distinct zones of turbulence – upper and lower turbulence zones – both presenting unique threats to aircraft. Thankfully, both zones can easily be identified.

Risks in the Upper Zone | High Point University The upper zone extends approximately 1,000 to 2,000 feet above peak height and higher.

Your aircraft could become trapped by updrafts and downdrafts within the wave, depending on its intensity, which can range from light to extremely strong, potentially exceeding its climb capability.

Though mountain waves cannot be directly seen, visual indicators exist when moisture exists, and the zone creates up-and-down drafts.

Rows of lenticular clouds moving downwind from a mountain barrier indicate an impending lee wave.

As seen below, a wave cloud formed by the continental divide extends east of Boulder, Colorado. It can be identified by the smooth appearance of lenticular clouds, which is evidence of its existence.

What exactly are the strengths and weaknesses of up-and-down drafts? There’s no definitive answer, as it depends on factors like cross-barrier winds, wind shear from peak height to 6,000′ above the peaks, air stability, etc.

As you approach a mountain barrier, the drafts gradually intensify until reaching their maximum amplitude at or slightly behind their summits.

If the up- and down-drafts become uncomfortable or exceed your aircraft’s climb performance as you approach mountains, turn away from them and return in the same direction you came.

However, flying a piston aircraft near mountains on days with large, lenticular clouds is only sometimes considered ideal.

Rotors can produce moderate to severe turbulence in the lower turbulence zone, generally between 1,000-2,000 feet below peak height and below.

Visual cues indicate when there’s moisture present, and thus, an upper turbulence zone exists.

Below is a video depicting rotor clouds forming below peak height, paralleling a mountain range east of Boulder, Colorado. Rotor-shaped clouds with jagged edges and rotating motion can be found below this point.

Rotors form beneath the crests of trapped lee waves, and, similar to up-and-down drafts, their strength decreases with distance from the mountain range.

Anytime rotor clouds appear, treat them like thunderstorms and steer clear. Rotor clouds often follow mountain ranges, so if any are visible, increase your distance away from them to stay out of harm’s way.

To successfully avoid mountain waves, first start by reviewing weather forecasts. Remember three factors are required to create trapped Lee waves:

Cross-barrier flows of 20 knots or greater generally indicate moderately stable environments with significant wind shear (more than 1.6). When in flight, look for visual cues such as low-down rotors and lenticular clouds overhead to help identify trouble spots.

Avoid flying through both types of clouds for an uninterrupted flight experience.

Mountain weather may conjure images of places like the Rockies. But Eastern Ohio hills also produce mountain weather all year round; just fly near the Appalachians, and you may have witnessed its effects without realizing it!