Northwest Winds (North Georgia)

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Mountain/Lee Waves

The wind flows towards a mountain and produces a first oscillation (A) followed by more waves. The following waves will have lower amplitude because of the natural damping. Lenticular clouds stuck on top of the flow (A) and (B) will appear immobile despite the strong wind.
Features of mountain waves
Mountain waves from NW wind blowing over the Appalachian Mountains in satellite image
Mountain waves from NW wind blowing over the Appalachian Mountains in water vapor image
Downwind turbulence and lee waves caused by peaks of the South Sandwich Islands

Winds on inland waters in Georgia have a reputation for being more gusty than the steady breezes at the coast. As inland windsurfers, kiters, sailors, etc. we quickly learn to watch the water upwind for signs of increasing (gust) or decreasing (lull) winds.

Over time, we've noticed that winds from the northwest seem especially gusty. An old joke is that a forecast of "NW 15 to 25" sometimes feels like "NW 15, 2 and 25". Is it just a sailors' complaint or is there something scientific behind this? The answer is in lee waves, also known as mountain or gravity waves. Essentially, as the winds literally roll over the Appalachian Mountains to our northwest in TN and GA, like a water in a brook rolling over rocks, vertical waves form in the atmosphere. We feel those waves as gusts and lulls on the surface.

From Mountain waves are typically observed near large mountain ranges around the world when the large-scale winds are perpendicular to the mountain ranges. These mountain waves can produce very strong wind gusts in a narrow area along the foothills, and can also create strong turbulence which adversely affects aviation. The formation of mountain waves is similar to when fast-moving water flows over a large boulder in a river. As the river current flows over the stationary boulder, waves are formed downstream of the boulder. The atmosphere behaves in a similar fashion when the wind flow encounters a large mountain range (a stationary object) with a stable air mass in place. In a stable air mass, air wants to either remain at its same altitude or descend (much like water always wanting to flow downhill). So, when a strong southeast wind flow is perpendicular to the southwest-to-northeast oriented southern Appalachians, it will be forced to rise over the mountains. With a stable air mass also in place, the wind will behave like water and immediately descend on the other side of the mountains in the form of wave. [1]

Lee waves can travel hundreds of miles from the terrain that generated them (they've been seen to travel up to 700 miles). They can have wave lengths from 5-35 km which explains the period of gusts and lulls that are often 10 to 20 minutes.

Clouds formed by gravity waves

Local Effects

Topo map of eastern TN and NW GA suggesting mountain/gravity waves from various topographical features. Yellow line is simple NW wind direction. Next red line above is suggestion of effects of mountains to NW of Lake Lanier. Top red line includes features from Cumberland Plateau.

On a clear day at Lake Lanier, you can see some of the closer mountains of the Appalachians to the north and northwest. And, beyond them, are ridges of mountains well into TN on the Cumberland Plateau. The strong northwest winds that we often get after a cold front passes have to blow over these and, on the stronger days, it's easy to feel the effect of lee/mountain waves: a very gusty day on the water. Sometimes, these turn into quite frustrating days as it feels like you are never quite dialed into the changing conditions.

It makes gear selection difficult because in one moment, the wind can be gusting well into the upper 20s or more and then the next, a lull almost without whitecaps. Do you rig for the lulls or the gusts? Part of that, of course, depends on your experience. The better windsurfer/kiter will rig for the lulls because he/she has the skills (and can trim their equipment) to deal with the gusts. Those less experienced may opt to rig for the gusts but anticipate waiting out the lulls. For example, a windsurfer might opt for a slightly bigger, more buoyant board in case they can't easily plane in the lulls.

More reading

More viewing

Series of videos on mountain waves from NWS