Mountain Waves / Standing Waves

The Ridge along the north shore of Lake Superior, including Duluth, sometimes induces waves in the atmosphere similar to those produced by mountain ranges. Satellite and personal observations have identified a variety of wavelike features. These features include lenticular clouds sitting stationary above the ridgeline and wavelike patterns in stratocumulus clouds extending well downstream of the ridge. Sometimes higher altostratus or cirrostratus cloud form, similar to mountain generated cirrus associated with the Rocky Mountains. On satellite they appear as smooth or fibrous clouds extending well downstream from the north shore of Lake Superior but with a sharp back edge that aligns itself over the ridgeline. These middle and high clouds form when a wave, which propagates vertically into the middle or upper troposphere, encounters a moist layer. The rising motion in the ridge part of the wave lifts the moist air to form clouds that are then blow downstream by the wind.

A type of wave that meteorologists in Duluth are keen on identifying and forecasting is called a standing wave. A standing wave is a form of mountain wave that remains stationary relative to the mountain or hill that triggers the formation of the wave. Air flows through the wave, sinking in the trough of the wave and rising in the ridge. In Duluth and along the north shore, the chain of hills provides the necessary obstruction. Conditions favorable for a standing wave to form include an elongated hill with a rounded top rather than a sharp narrow peak, winds perpendicular to the hill, winds 25 knots or greater in the lower to middle troposphere, and a generally stable atmosphere. A stable atmosphere is important because buoyancy forces cause the air that was displaced upward by the hill to be restored back toward its previous level after passing over the hill. The series of vertical displacements sets off a wavelike oscillation of the air.

Northwest wind cold air outbreaks often provide favorable conditions for the formation of a standing wave. When enough moisture is in the air, the wave shows up on radar as a band of snow above the ridge crest on the higher elevation scans. The snow lowers downstream until it reaches the ground over the south shore of Lake Superior from the Bayfield Peninsula through Douglas County, WI and into northern Pine County, MN. The snow generated by the wave may be mixed in with lake effect snow. Snow from the wave can be identified from lake effect in circumstances where the snow extends southwest from Superior, WI into northern Pine County, MN. Those locations, when winds are from the northwest, are downwind of land, not the lake. Also note that the Lake Superior north shore ridge extends west-southwest beyond the tip of the lake and thus so does the wave.

References and More Information

Dan Miller, Science and Operations Officer, National Weather Service, Duluth, MN

Forecasting Staff, National Weather Service, Duluth, MN

AMS Glossary definitions
mountain wave
standing wave

“Mountain Waves and Downslope Winds” training module from the MetEd website

Additional Reading

Brady, R. H. and J. S. Waldstreicher, 2001: Observations of Mountain Wave-Induced Precipitation Shadows over Northeast Pennsylvania, Wea. Forecasting, 16, 281-300.

Smith, R. B., 1979: The influence of mountains on the atmosphere. B. Saltzman, Ed., Adv. Geophys., 21, 87-230.
[ Specifically note pages 88 and 89. ]

Kirkwood, P. D., D. M. Gaffin, and S. S. Parker, 2002: An Unexpectedly Heavy and Complex Snowfall Event across the Southern Appalachian Region. Wea. Forecasting, 18, 224-235.