Ice Storms at the Head of Lake Superior


Freezing rain events sometimes occur in the higher elevations of Duluth and the north shore of Lake Superior while rain or much less icing occurs at the bottom of the hill. If the water temperature is 34 degrees for example, the air just above the lake will be close to that temperature even if air over the surrounding land is a few degrees warmer. When air rises from the lake up the hill (orographic lift) it cools several degrees to a little below freezing, resulting in freezing rain along the higher terrain.

Based on the dry adiabatic lapse rate, unsaturated air cools 5.37 degrees Fahrenheit per 1000 feet as it rises. Once saturated, the air cools generally half as much. The cooling rate is lower since the condensation process releases latent heat and balances off some of the cooling.

Sustaining a prolonged freezing rain event in any part of the country is difficult since heat is released when water changes to ice. Eventually the warming causes the temperature to rise above freezing unless additional cold air is advected into the area. Strong, persistent east to east-northeast winds off Lake Superior supply an excellent source of air that, when lifted, cools below freezing.

More on lapse rates from AMS Glossary of Meteorology:
Dry Adiabatic Lapse Rate
Moist Adiabatic Lapse Rate