Lake effect snow
During the fall and winter, many areas of the eastern United States and Canada experience tremendous amounts of snowfall. This snow, known as "lake-effect snow," is generated from the temperature contrast between the cold arctic air moving over the relatively warm waters of the Great Lakes (or other large body of water). Unlike most winter storms, lake effect snows do not build their foundation upon strong areas of low pressure. Instead, they are fueled by the same dry arctic air that is responsible for clearing skies over land in other parts of the country. Specifically, cold arctic air passing over the Great Lakes picks up moisture and deposits it as snow inland from the downwind shore. So while other parts of the northeastern United States are clearing up after a recent cold frontal passage, communities near the Great Lakes wait for the lake effect snow machine to fire up! Barny Wiggin, former Meteorologist-In-Charge at the NWS Office in Buffalo, said it best when he claimed that the ‘weather often "clears up stormy" to the lee of the Great Lakes during the winter.'
Lake effect snow cloud bands are remarkably persistent and have been known to cause continuous snowfall for as long as 48 hours over a sharply defined region—an amount that often exceeds that of a typical winter storm (i.e., one associated with a low pressure). Lake effect snows yielding as much as 193 centimeters (cm) (76 inches) of light-density snow in 24 hours and fall rates as high as 15 cm (6 inches) per hour have been reported. Furthermore, 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. In general, lake effect snowfall contributes between 30 and 60 percent of the annual winter snowfall on the eastern and southern shores of the Great Lakes.
How does it work?
In the late fall and early winter, the waters of the Great Lakes become increasingly warmer relative to the cold dry arctic air masses that flow down from the north and northwest. When this air traverses the lake, the lower levels of the atmosphere pick up moisture and warmth. This air (along with the moisture it picked up from the lake below) is now lighter than the air above it and starts to rise as it continues its journey across the lake (a condition known as "convective instability"). As the air rises, it cools and the moisture that evaporated into it condenses (into tiny droplets or ice crystals) and forms clouds. Depending on the degree of instability of the air mass (i.e., how much warmer the lake water is than the air), bands of either stratus, stratocumulus, or heavy cumulus clouds form over the water and travel with the wind toward the downwind shore. When enough moisture condenses out of the air, it falls in the form of snow over the water and the lee side (downwind side) of the lake.
Variability of lake effect snow
While the principle is simple, actual lake effect snow is quite variable in terms of time, space (location), and intensity. This variability of lake effect snow is of particular danger to motorists.
- Time: It is not uncommon for sunny skies at a particular location to be quickly replaced by blinding, wind-driven snowfall in a matter of minutes.
- Space: The particular location of the snow depends on the wind direction, with places downstream of the largest expanse of water subject to the most snow. Therefore, lake effect snow is often very local with some places receiving a couple inches and others getting more than a foot.
- Intensity: Lake effect snow may take the form of a single strong snow band or several weaker snow bands. Likewise, individual squall of heavy snow may remain over one small area for several hours and then, with a shift in the wind direction, move to drop its snow on another area.
Variables contributing to lake effect snow
The distribution of lake effect snow is dependent upon several factors—many are associated with the prevailing meteorological situation, while others are a function of the physical geography of the land and water in the immediate area.
- Temperature Difference Between Lake Surface and Overlaying Air: The temperature difference between the lake surface and overlying air promotes "convective instability" that provides the basic energy source for lake effect snow. Ideally, the ambient air temperature should be 15 °C to 25 °C cooler than the surface of the lake, and the dew point differential between the 850-millibar (mb) level and the surface must be at least 13 °C. In general, the greater the temperature difference between the cold air and the warm water, the heavier the snow showers will be. If the temperature contrast is great enough, the rising air will have enough buoyancy to form thundersnow (i.e., thunderstorms characterized by snow as opposed to rainfall).
- Wind Speed: Sufficient wind speed is necessary to advect arctic air over the lake, and transport sufficient amounts of warm, moist air to the shore. Increased wind speed also increases turbulent fluxes, which enhances the vertical mixing required for lake effect snows. A minimum wind speed of 5 meters/second is generally required for significant lake effect snow formation over land (otherwise, the snow that forms will fall over the lake or along the immediate shoreline). On the other hand, if the wind speed is too strong, residence time over the lake is reduced and the air passes over the lake too quickly to pick up sufficient heat and moisture needed for lake effect snow.
- Wind Direction (and Duration): Local surface wind direction determines where lake effect snow will fall, with the leeward or downwind portion of the lakeshore receiving the most lake effect snow. Wind direction also helps determine the fetch (see definition below).
- Stability: Stability affects the depth through which mixing and convection will occur. Deeper mixing allows deeper, more intense convection that intensifies lake effect snow. Stability also plays a role in determining cloud band structure.
- Latent Heating: Latent heat release from condensation (cloud formation) provides an important additional energy source for convective instability and subsequent lake effect snows (as it is a significant heat input to the mixed layer).
- Relative Humidity: The relative humidity of the upstream air determines the amount of moisture required to saturate it. In general, the dry arctic air masses that generate lake effect snow requires longer residence times over water to saturate.
Physical geography variables
- Extent of Ice Coverage: The surface of the lake must be unfrozen and its temperature must be at least 0 °C for lake effect snow to form. When the lakes become covered with ice, it significantly decreases the process that allows heat and moisture to evaporate into the air. Since the Great Lakes are some of the largest bodies of non-ocean water in the world, their surface temperature remains above freezing well into the fall and even the early winter seasons.
- Fetch: The wind must pass across at least 80 kilometers (km) of lake surface before lake effect snow will form. This distance is called the "fetch" and (in most cases) the larger the fetch, the greater the probability of a lake effect snow event.
- Shoreline Characteristics:
- Surface Roughness (friction): Increased surface roughness over land decelerates air that flows off the lake, which induces convergence and enhanced vertical lift that contributes to lake effect snow.
- Mechanical Lift: If there are hills or mountains downstream of the lake, the upslope flow enhances the lake effect due to the mechanical lifting that forces the warm moist air to rise, condense, and form snow at an accelerated rate.
Other areas affected by lake effect snow
Lake-effect snows are common over the Great Lakes region because these large bodies of water can hold their summer heat well into the winter, rarely freeze over, and provide the long fetch which allows the air to gain the heat and moisture required to fuel the snow squalls. However, lake-effect snows are not restricted to the Great Lakes shorelines (i.e., Buffalo, Syracuse, Rochester, Erie, Muskegon, and South Bend), but are most common and heaviest there. In fact, any large lake may produce lake-effect snow downwind if it remains essentially ice-free and there is cold air channeling across its relatively warm waters. Other locations on Earth where these snows occur, include such diverse places as the Great Salt Lake, Eastern shores of Hudson Bay, Gulf of St. Lawrence in the U.S., as well as parts of Japan, Korea and Scandinavia, to name a few.