What is Lake Effect Snow?

winter-Utica New York State
Winter in the Adirondack Mountains, New York. Chris Murray/Aurora/Getty Images

Lake effect snow (LES) is a local weather event that occurs when a cold air mass passes across an expanse of warm water creating convective snow bands. The phrase "lake effect" refers to a body of water's role in providing moisture to air that would otherwise be too dry to support snowfall.

Lake Effect Snow Ingredients

To grow a snowstorm, you need moisture, lift, and below-freezing temperatures. But for lake effect snow to occur, these special conditions are also required:

  • A lake or bay of 100 km wide, or larger. (The longer the lake, the greater the distance the air must travel over it, and the greater the convection.)
  • An unfrozen water surface. (If the water surface is frozen, the passing air is unable to pick up little moisture from it.)
  • A lake/land temperature difference of at least 23 °F (13 °C). (The greater this difference, the more moisture the air will take in and the heavier the LES.)
  • Light winds. (If winds are too strong, say over 30 mph, it limits the amount of moisture that can evaporate from the water's surface into the air above.)  

Lake Effect Snow Setup

Lake effect snow is most common over the Great Lakes region during November to February. It often forms when low pressure centers pass near the Great Lakes regions, opening the way for cold, arctic air to rush southward into the U.S. out of Canada.

Steps to Lake Effect Snow Formation

Here's a step-by-step explanation of how cold, Arctic air interacts with warm bodies of water to create lake effect snow.

As you read through each, look at this LES diagram from NASA to help visualize the process.

  1. Below-freezing air moves across the warm lake (or body of water). Some of the lake water evaporates into the cold air. The cold air warms and picks up moisture, becoming more humid.
  2. As the cold air warms, it becomes less dense and rises.
  1. As air rises, it cools. (Cooler, moist air has the ability to form clouds and precipitation.)
  2. As the air moves some distance over the lake, moisture inside of the cooler air condenses and forms clouds. Snow may fall -- lake effect snow!
  3. As the air reaches the shoreline, it "piles up" (this happens because air moves more slowly over land than over water due to increased friction). This, in turn, causes additional lifting.
  4. Hills on the lee side (downwind side) of the lake shore force air upward. Air cools further, encouraging cloud formation and greater snowfall.
  5. Moisture, in the form of heavy snow, is dumped on the south and east shores.

Multi-Band vs. Single-Band

Two types of lake effect snow events exist: single-band and multi-band.

Multi-band LES events occur when the clouds line up lengthwise, or in rolls, with the prevailing wind. This tends to happen when the "fetch" (the distance air must travel from the upwind side of the lake to the downwind side) is shorter. Mulit-band events are common to Lakes Michigan, Superior, and Huron. 

Single-band events are the more severe of the two, and occur when winds blow cold air along the entire length of the lake. This longer fetch allows more warmth and moisture to be added to the air as it crosses the lake, resulting in stronger lake effect snow bands.

Their bands can be so intense, they can even support thundersnow. Single-band events are common to Lakes Erie and Ontario.

Lake Effect vs. "Ordinary" Snow Storms

There are two main differences between lake effect snow storms and winter (low pressure) snow storms: (1) LES are not caused by low pressure systems, and (2) they're localized snow events.

As a cold, dry air mass moves over the Great Lakes regions, the air picks up lots of moisture from the Great Lakes. This saturated air later dumps its water content (in the form of snow, of course!) over areas surrounding the lakes.

While a winter storm may last a few hours to a few days on and off and impact several states and regions, lake effect snow will often produce snow continuously for up to 48 hours over a particular area. Lake effect snows can precipitate as much as 76 inches (193 cm) of light-density snow in 24 hours with fall rates as high as 6 inches (15 cm) per hour!

Because winds accompanying arctic air masses generally originate from a southwest to northwest direction, lake effect snow typically falls on the east or southeast sides of the lakes.

Only A Great Lakes Event?

Lake effect snow can happen wherever the conditions are right, it just so happens that there are few locations that experience all the needed ingredients. In fact, lake effect snow only occurs in three places worldwide: the Great Lakes region of North America, the east shore of Hudson Bay, and along the west coast of the Japanese islands of Honshu and Hokkaido.

Edited by Tiffany Means

 

Resources & Links:

Lake Effect Snow: Teaching Great Lakes Science. NOAA Michigan Sea Grant.

http://www.miseagrant.umich.edu/lessons/lessons/by-broad-concept/earth-science/lake-effect-snow/