Weatherz School: Gravity waves
The term “gravity wave” sounds a lot more complicated than it really is. Let’s forget about “gravity” for a second and focus on the “wave” part. We’re familiar with waves in the water.
When you drop a rock in the water, it causes ripples that spread out from the point of impact. What we see is the water rising and falling in response to the momentum of the splash. This simple process is how gravity waves work.
In the atmosphere, rows of clouds are evidence of gravity waves. Clouds form where the air rises, and clear areas form where the air sinks. In our neck of the woods, gravity waves often form from air flowing over the terrain of the North Shore.
What we need is a steady wind perpendicular to the hill, usually out of the west northwest, and stable air. With unstable conditions, rising air is allowed to continue rising. But when the air above us is warmer, it stops upward motion.
As air is forced upward along the high terrain of the North Shore, it hits the stable layer, then sinks back down until it becomes less dense than the air around it, then it rises again. The forcing over the hilltop becomes our rock dropped in the pond. The wave continues downstream toward the South Shore.
We often talk about the South Shore getting lake effect snow, but we can’t give Lake Superior all the credit. Gravity waves can provide extra lift that enhances snowfall for the South Shore.
This happened on November 17th. A narrow band of snow set up for parts of Douglas and Bayfield County that was parallel to the shoreline, and persisted for hours. As snow reports came in, the town of Maple got 8 inches and a snow day.