What are mountain waves?
by David Gaffin
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. Mountain waves generally occur during the cooler months of the year from late fall through early spring (mid October to mid April), when large low pressure systems and stable air masses are more common. The peak month for mountain wave activity in the western foothills of the southern Appalachians is December. These events generally do not occur during the summer months.
Strong wind gusts from mountain waves generally only affect a narrow zone in the foothills where the bottom of the wave intersects the ground. During a mountain wave event, many people outside of the foothills will not experience much wind and may wonder "what is all the fuss about?". However, those people who live in the narrow corridor along the foothills where these waves intersect the ground can sometimes experience hurricane-force wind gusts. Wind gusts around 100 mph have been routinely measured at Cove Mountain in the Great Smoky Mountains National Park during strong mountain wave events when strong southeast winds occurred within a stable air mass. Wind gusts in excess of 80 mph have also been measured at Camp Creek in southeast Greene County during mountain wave events. While high winds due to mountain waves occur all along the foothills, the Cove Mountain and Camp Creek observation sites are places that routinely experience strong winds due to their favorable terrain location. A favorable terrain profile for mountain waves is one where steep slopes are found on the leeward side (side facing opposite the direction the wind comes from) with more gently-rising slopes on the windward side (side facing the direction the wind comes from). Mountain waves also can occur along the eastern foothills of the southern Appalachian Mountains when the wind flow is from the northwest. While mountain wave events occur more frequently on the eastern side of the mountains (due to the more frequent occurrence of northwest winds versus southeast winds), it is likely that high winds due to mountain waves are stronger on the western side because of the steeper slopes that quickly descend into the Great Tennessee Valley. However, due to the lack of observations near the mountains, this theoretical claim can not be fully verified.