Lake Effect Snow
Lake effect snows occur when a mass of sufficiently cold air moves over a body of warmer water, creating an unstable temperature profile in the atmosphere.
As a result, clouds build over the lake and eventually develop into snow showers and squalls as they move downwind. The intensity of lake effect snow is increased when higher elevations downwind of the lake force the cold, snow-producing air to rise even further.
The most likely setting for this localized type of snowfall is when very cold Arctic air rushes over warmer water on the heels of a passing cold front, as often happens in the Great Lakes region during winter.
Winds accompanying Arctic air masses generally blow from a west or northwest direction, causing lake effect snow to fall on the east or southeast sides of the lakes.
Whether an area gets a large amount of snow from lake effect is dependent on the direction of the winds, the duration they blow from a particular direction, and the magnitude of the temperature difference between the water and air.
Since cold air can hold very little moisture and the low level of the atmosphere is quite unstable, clouds form very rapidly, condensation occurs and snow begins to fall. Lake effect snow is lighter than snow that forms from frontal stratus or nimbostratus.
Areas of relatively high elevation downwind of the Great Lakes generally receive heavier amounts of lake effect snow than do other locations in this region.
For example, residents of the Tug Hill Plateau in New York State east of Lake Ontario can spend the winter months digging out of anywhere from 200 to 300 inches of snow. Likewise, the mountains of West Virginia can receive over 200 inches of snow in a winter, helped by the lake effect.
The only other lake that produces significant lake effect snow in the United States is the Great Salt Lake in Utah.
Cape Cod Bay in Massachusetts and Chesapeake Bay in Maryland and Virginia, on occasion, produce what is called bay effect snow. Bay effect snow forms in the same manner as lake effect snow, only over the ocean.
Steps to Lake Effect Snow
- Cold air streams across the warm lakes. Air warms and becomes more humid.
- As the air warms, it becomes less dense and rises.
- As air rises, it cools.
- Cooler, moist air may form clouds and cause precipitation.
- After the air has moved some distance over the lake, convection and turbulent exchange have transported the moisture aloft to form clouds. Snow may fall.
- Once over land, moisture in the air condenses into snow. Snow created in this way is called lake effect snow.
- As the warmed air reaches the shoreline, additional lifting may occur as the air begins to “pile up.” Air moves more slowly over land than over water, due to increased friction.
- Hills and high lands on down-wind lake shores force air upward. Air cools further, encouraging cloud formation and greater snowfall.
As the cold air streams across the warm lakes, it is warmed and becomes more humid. As the air warms, it becomes less dense and tends to rise cooling (as it rises). Whenever moist air rises, as previously noted, clouds may form and precipitation may result. Fog results from the intense evaporation or transfer of moisture from the warm water to much colder air when the cold air initially makes contact with warm water. After the air passes from some distance over the lake, convection and turbulent exchange have transported the acquired moisture aloft to form clouds and snowfall may occur.