Why are there often bands of snow streaming from the warmer waters of the Lake Michigan unto the shore instead of a general storm/waves of snow. Restated the question is: Why streaming bands? I have lived my entire life in the great lakes area and know the basic physics of how the cold air on warmer water etc creates snow, I just do not know why the result is usually snow in bands rather than a generalized snow storm? Maybe the heat / moisture transfer is not constant across the surface of the water?
If the bands you’re referring to are over the land, that’s influenced by local geography: Precipitation tends to fall out of air when the air goes over hills.
Sometimes a snow storm (not just bands) will fall. Buffalo, for example, often get snow storms from the Lake effect. Buffalo lies in a favorable position, where the NWly winds come over the Lake directly into the city. I don’t know which city you refer to, but Chicago is mostly NW of Lake Michigan. The sand dunes in Indiana get the brunt of the NWly winds. (After a cold front comes through, the winds are usually NWly.) Chicago will get some lake effect if the winds are favorable enough. Shifting winds, from a more Northerly direction to a more NWly direction will result in occasional snow squalls, as the Northerly contigent hits the downtown area.
If the winds are gusty, you will also get snow squalls as the winds gust, but they will abate or disappear as the winds calm.
…and Toronto, to the northwest of lake Ontario, rarely gets lake-effect snow. We need cold winds from the east or southeast blowing across the open lake for that–unusual to say the least. It has happened though, at the very west end of Lake Ontario, where the shore runs diagonally northeast. In 1999 or 2000, there was a storm with cold east winds, and Oakville got half a metre of snow. It was gorgeous… until I went to go to work.
My cousins on the Bruce Peninsula regularly get snowed in every winter.
This is part of it. The lake doesn’t have a consistent temperature throughout, and neither does the air. And then, there are irregularities of the shore as well. What this adds up to is that certain areas are more conducive toward lake-effect conditions than other areas, so as the air moves over the land, it produces streaks.
Looking at Lake Superior in Qadgop the Mercotan’s link, the bands begin over the lake. The bands are very persistent over the lake, and less so (or at least less well-defined) when they reach land.
In broad terms when you have vertical circulation (i.e. air rising and descending) and a wind blowing, the lift and sink tends to align parallel to the wind, as this represents a lower energy state than when the circulation is not aligned. Glider pilots look for “streets” of cumulus clouds, which occasionally (e.g. on a good day in Texas) allow flight without circling for distances than can exceed 100 miles.
I’ve seen this demonstrated in a pan of boiling water: as the fine bubbles start to rise stirring the water will tend to make them align along the streamlines.
Here’s a site that explains this as Kelvin-Helmholtz instability (though I’m not sure how generally that should be taken).