Editing Lake-effect snow (section)

== Formation ==
[[File:Lake-derived-snow.svg|thumb|right|Lake-effect snow is produced as cold winds blow clouds over warm waters.]]

Some key elements are required to form lake-effect precipitation and which determine its characteristics: instability, fetch, wind shear, upstream moisture, upwind lakes, synoptic (large)-scale forcing, orography/topography, and snow or ice cover.

=== Instability ===
A temperature difference of approximately {{convert|13|°C-change}} between the lake temperature and the height in the atmosphere (about {{convert|1,500|m|ft|abbr=on|disp=or|sigfig=1}} at which barometric pressure measures {{convert|850|mbar|kPa|disp=or|abbr=on}}) provides for absolute instability and allows vigorous heat and moisture transportation vertically. Atmospheric [[lapse rate]] and convective depth are directly affected by both the mesoscale lake environment and the synoptic environment; a deeper convective depth with increasingly steep lapse rates and a suitable moisture level allow for thicker, taller lake-effect precipitation clouds and naturally a much greater precipitation rate.<ref>{{cite web |url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld010.htm |title=Lake Effect Snow: Instability |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20090617013142/http://www.comet.ucar.edu/class/smfaculty/byrd/sld010.htm |archive-date=2009-06-17}}</ref>

=== Fetch ===
The distance that an air mass travels over a body of water is called fetch. Because most lakes are irregular in shape, different angular degrees of travel yield different distances; typically, a fetch of at least {{convert|100|km|mi|-1|abbr=on}} is required to produce lake-effect precipitation. Generally, the larger the fetch, the more precipitation produced. Larger fetches provide the boundary layer with more time to become saturated with water vapor and for heat energy to move from the water to the air. As the air mass reaches the other side of the lake, the engine of rising and cooling water vapor pans itself out in the form of condensation and falls as snow, usually within {{convert|40|km|mi|abbr=on}} of the lake, but sometimes up to about {{cvt|100|mi|km|order=flip|round=50}}.<ref>{{cite web |url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld012.htm |title=Lake Effect Snow: Fetch |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead|archive-url=https://web.archive.org/web/20080515101954/http://www.comet.ucar.edu/class/smfaculty/byrd/sld012.htm|archive-date=2008-05-15}}</ref>

=== Wind shear ===
Directional [[Wind shear|shear]] is one of the most important factors governing the development of squalls; environments with weak directional shear typically produce more intense squalls than those with higher shear levels. If directional shear between the surface and the height in the atmosphere at which the barometric pressure measures {{convert|700|mb|kPa|abbr=on}} is greater than 60°, nothing more than flurries can be expected. If the directional shear between the body of water and the vertical height at which the pressure measures {{convert|700|mb|kPa|abbr=on}} is between 30° and 60°, weak lake-effect bands are possible. In environments where the shear is less than 30°, strong, well organized bands can be expected.<ref name="Wind Shear">{{cite web |url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld014.htm |title=Lake Effect Snow: Wind Shear |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead|archive-url=https://web.archive.org/web/20080511162642/http://www.comet.ucar.edu/class/smfaculty/byrd/sld014.htm |archive-date=2008-05-11}}</ref>

Speed shear is less critical but should be relatively uniform. The wind-speed difference between the surface and vertical height at which the pressure reads {{convert|700|mb|kPa|abbr=on}} should be no greater than {{convert|40|kn|km/h}} so as to prevent the upper portions of the band from shearing off. However, assuming the surface to {{convert|700|mb|kPa|abbr=on}} winds are uniform, a faster overall velocity works to transport moisture more quickly from the water, and the band then travels much farther inland.<ref name="Wind Shear" />

[[File:Lake effect deltat chart.svg|thumb|right|Temperature difference and instability are directly related, the greater the difference, the more unstable and convective the lake-effect precipitation will be.]]

=== Upstream moisture ===
A lower upstream relative humidity lake effect makes condensation, clouds, and precipitation more difficult to form. The opposite is true if the upstream moisture has a high relative humidity, allowing lake-effect condensation, cloud, and precipitation to form more readily and in a greater quantity.<ref>{{cite web |url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld015.htm |title=Lake Effect Snow: Upstream Moisture |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20080509133403/http://www.comet.ucar.edu/class/smfaculty/byrd/sld015.htm |archive-date=2008-05-09}}</ref>

=== Upwind lakes ===
Any large body of water upwind impacts lake-effect precipitation to the lee of a downwind lake by adding moisture or pre-existing lake-effect bands, which can reintensify over the downwind lake. Upwind lakes do not always lead to an increase of precipitation downwind.<ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld016.htm |title=Lake Effect Snow: Upstream Lakes |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20080509135856/http://www.comet.ucar.edu/class/smfaculty/byrd/sld016.htm |archive-date=2008-05-09}}</ref>

=== Synoptic forcing ===
Vorticity advection aloft and large upscale ascent help increase mixing and the convective depth, while cold air advection lowers the temperature and increases instability.<ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld019.htm |title=Lake Effect Snow: Synoptic-Scale Forcing |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20080516000533/http://www.comet.ucar.edu/class/smfaculty/byrd/sld019.htm |archive-date=2008-05-16}}</ref>

=== Orography and topography ===
Typically, lake-effect precipitation increases with elevation to the lee of the lake as topographic forcing squeezes out precipitation and dries out the squall much faster.<ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld021.htm |title=Lake Effect Snow: Orography/Topography |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20080509071700/http://www.comet.ucar.edu/class/smfaculty/byrd/sld021.htm |archive-date=2008-05-09}}</ref>

=== Snow and ice cover ===
As a lake gradually freezes over, its ability to produce lake-effect precipitation decreases for two reasons. Firstly, the open ice-free liquid surface area of the lake shrinks. This reduces fetch distances. Secondly, the water temperature nears freezing, reducing overall latent heat energy available to produce squalls. To end the production of lake-effect precipitation, a complete freeze is often not necessary.<ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld022.htm |title=Lake Effect Snow: Snow/Ice Cover on the Great Lakes |first=Greg |last=Byrd |date=June 3, 1998 |website=University Corporation for Atmospheric Research |url-status=dead |archive-url=https://web.archive.org/web/20080515221206/http://www.comet.ucar.edu/class/smfaculty/byrd/sld022.htm |archive-date=2008-05-15}}</ref>

Even when precipitation is not produced, cold air passing over warmer water may produce cloud cover. Fast-moving mid-latitude cyclones, known as [[Alberta clipper]]s, often cross the Great Lakes. After the passage of a cold front, winds tend to switch to the northwest, and a frequent pattern is for a long-lasting [[low-pressure area]] to form over the [[Canadian Maritimes]], which may pull cold northwestern air across the Great Lakes for a week or more, commonly identified with the negative phase of the North Atlantic Oscillation (NAO). Since the prevailing winter winds tend to be colder than the water for much of the winter, the southeastern shores of the lakes are almost constantly overcast, leading to the use of the term "the Great Gray Funk" as a synonym for winter.{{Citation needed|date=March 2010}} These areas allegedly contain populations that suffer from high rates of [[seasonal affective disorder]], a type of psychological depression thought to be caused by lack of light.<ref>{{cite web |url=http://www.wunderground.com/health/mood.asp |title=Health Advisories: Weather and Mood |website=The Weather Underground |access-date=2007-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20070221231441/http://www.wunderground.com/health/mood.asp |archive-date=2007-02-21}}</ref>{{citation needed|date=November 2017}}