Preview Course

Mountain waves are most common in the fall and winter, and they can affect flight operations hundreds of miles downwind of a mountain range. Here's why...

1) The jet stream moves south.

In order for a mountain wave to form, you need strong winds aloft, and wind shear from the mountain peaks to roughly 6,000' above the peaks.

In the summer months, the jet stream is located further north along the Canadian border. But as fall and winter arrive, the jet stream makes its way south, flowing across the major mountain ranges of the US, from the Sierra Nevada to the Appalachians.

2) There's more cross-barrier flow.

The strength of mountain waves depends on a few factors, one being cross-barrier flow. When winds aloft strike a mountain ridge at a 90-degree angle, it generates stronger mountain waves.

3) Increased air stability.

The more stable the air, the more likely it is to produce a mountain wave. And in the fall and winter, the US tends to have a lot of stable air.

As an air parcel is lifted over a mountain ridge, it expands and cools making it denser than the air around it. As this lifted air crosses the ridge of a mountain, it starts to descend along the backside of the range, attempting to reach equilibrium. Due to the momentum, it overshoots the equilibrium point and compresses, resulting in the air parcel heating up. This makes the air parcel more buoyant, causing it to rise back toward the equilibrium point. This oscillation creates waves over and over again for hundreds of miles.

Have you experienced mountain wave? Tell us about your experience in the comments below.