Chicago does experience lake effect snow (LES), though it is generally less frequent and less severe than in areas further east across Lake Michigan. LES occurs when cold, dry air masses move over the relatively warmer, unfrozen waters of the Great Lakes, picking up moisture and heat. This process enhances precipitation, which falls as snow on the downwind shores. Because of Chicago’s geographic position on the southwestern edge of Lake Michigan, the city is typically spared the most intense and prolonged snowfall events that plague locations in Northwest Indiana and Western Michigan.
The Formation of Lake Effect Snow
Lake effect snow is a localized weather phenomenon created by a significant temperature difference between the air and the water. The formation process begins when a mass of frigid, Arctic air, often originating from Canada, moves across the open surface of the lake. This cold air must be at least 13°C (23°F) colder than the lake’s surface temperature at an altitude of about 1.5 kilometers (5,000 feet) to ensure atmospheric instability.
As the cold air passes over the warmer water, the lake acts like a massive energy and moisture source. Heat and water vapor are rapidly transferred from the water’s surface into the lowest layer of the atmosphere. This warmer, moisture-laden air rises quickly, a process called convection, leading to the formation of clouds.
The rising air cools and the water vapor condenses, eventually freezing into snow crystals. These snow crystals are organized into narrow, intense bands of precipitation that are deposited on the leeward, or downwind, side of the lake. The resulting snowfall can be extremely heavy and localized, sometimes reaching rates exceeding 7.6 centimeters (3 inches) per hour in the most concentrated bands.
Why Prevailing Westerly Winds Protect Chicago
Chicago’s protection from the heaviest lake effect snow is primarily due to the prevailing wind patterns in the Great Lakes region during winter. The dominant air flow across the Midwest is a westerly or northwesterly wind. This flow is a result of the large-scale weather systems that move across the North American continent.
When the wind blows from the west or northwest, the air travels a very short distance, or “fetch,” over Lake Michigan before reaching Chicago. The city sits on the windward side of the lake under these common conditions. The air simply does not have enough time or distance over the water to pick up the substantial amounts of heat and moisture required to develop into a strong, organized snow band.
The prevailing westerly winds instead push the developing snow bands away from the city and further east across the lake. The intense snowfall is deposited on the eastern shorelines, directly impacting areas like Northwest Indiana and Western Michigan. These locations are the true leeward shores for the dominant wind direction, making them a part of the regional “snowbelt.”
This dominant wind flow pattern is the reason Chicago’s average annual snowfall is lower than that of other Great Lakes cities positioned on the eastern and southern shores. While Marquette, Michigan, averages over 366 centimeters (144 inches) of snow, Chicago’s average annual snowfall is only about 94 centimeters (37 inches). The majority of Chicago’s winter precipitation comes from large-scale storm systems, not from the lake effect.
Conditions Required for Snow Bands to Hit the City
For lake effect snow to impact Chicago proper, a shift in the regional wind direction is necessary to provide the required distance over the lake. The most favorable wind directions for the city are from the north or northeast. These directions allow the air to travel the full length of the lake, maximizing the fetch and the amount of moisture and heat transfer.
A sustained flow from the north-northeast, for example, allows the snow bands to develop fully and track directly onshore into the Chicago metropolitan area. When this occurs, the bands can be potent, sometimes producing localized snowfall rates of over 5 centimeters (2 inches) per hour. These events can lead to significant accumulation in the city and surrounding suburbs, though they are much less common than the westerly flow.
The impact is often most concentrated along the immediate shoreline. Friction from the land causes the air to slow and pile up, forcing the moisture-laden air upward and enhancing precipitation. While these events can be severe, they are usually shorter in duration than the relentless snowfall experienced on the opposite side of the lake. The resulting accumulation is often focused on the city’s eastern flank, with totals quickly diminishing just a few miles inland.