Orographic lift occurs when air is pushed over a mountain or hill. Given enough moisture, clouds and precipitation form as a result of the lift. Orographic lift in Duluth frequently enhances snowfall, mainly inland from the lake shore. Stronger winds and winds more direct into the hill, such as from the southeast, blow the heavier snow farther inland. Although elevation rises in Duluth are minor compared to mountain ranges, the affects are still quite dramatic. One point to consider is that when air is lifted up the hill, a column of air is rising, not just a thin layer in contact with the ground. Cooling and condensation increases liquid water throughout the column and can result in greatly increased snowfall rates.
A good example of orographic lift occurred during a snowstorm on December 14 to 15, 2005. Southeast prevailed during most of the storm. An initial period of moderate to heavy snow fell during the overnight and early daylight morning hours of the 14th. Afterward, light snow continued to fall through the rest of the day and into the evening. Along and just inland of the ridge crest however, relatively heavier and sometimes moderate snow persisted. That put the greater accumulations inland from the city with the Duluth International Airport on the southeast fringe of the heavier snow. The airport is about seven miles inland. Late in the evening and then overnight into December 15th, winds became more east and the heavier snow shifted closer to the shore and over the city itself. Storm totals ranged from 6 to 8 inches at the bottom of the hill near the lake shore to between 15 and 20 inches further inland. The National Weather Service office near the airport measured 15.8 inches. Some of the depressed accumulation at the shoreline was also due to warmth from the Lake retarding accumulation.
Definition of Orographic Lift:
AMS Glossary of Meteorology