It's a Milwaukee weather term as familiar as "cooler near the lake." It rolls off the tongue as easily as "fish fry Friday." The phrase is "lake-effect snow," something we hear quite a few times during the winter months.

But of the 50 inches of snow Milwaukee averages from October through April, very little of that is due to lake-effect. It's more common on the other side of Lake Michigan, where Muskegon averages 106 inches of snow and Grand Rapids typically receives 72 inches. When Lake Michigan turns on the snow machine for our side of the lake, it can mean a day with white out conditions in Whitefish Bay and sunshine in Waukesha.

Water has an amazing capacity to hold heat. It gives it up grudgingly, as well. Summer sun and hot weather put a lot of heat in the depths of the lake, which is why it takes until August and September before Lake Michigan has somewhat tolerable water in which to swim. Early summer water temperature struggles in the 50s but late summer temperature often reaches the low 70s.

As summer passes to fall, air temperature cools off but water retains the heat absorbed from summer. November water temperatures at the surface of the lake can still be in the 40s while air temperatures can drop into the 20s. The difference in temperature creates a rapid evaporation of water into the air directly above the lake. That is why there is many an autumn morning when we wake up to clear skies but see a mountain of clouds out over the lake just off shore.

Lake-effect snow is created as an extension of that temperature difference and rapid evaporation. If cold air plunges down the length of the lake on northwest, north, or northeast winds, a long "fetch" is created. The fetch is the path the cold air takes over the water surface. A long fetch means the potential for a lot of moisture evaporated into the air. Because Lake Michigan is a north-south oriented lake, a cold north wind creates the biggest lake-effect snow.

The prevailing wind direction in this part of the world in winter is northwest. A cold northwest wind means the potential for a lot of lake-effect snow on the other side of the lake in Lower Michigan. On more rare occasions a northeast wind will blow down the lake, and that is when it is our turn for lake-effect snow. As you might expect, a cold breeze straight out of the north results in a lot of snow dumped along the south end of the lake in northwest Indiana.

Forecasters look for a strong temperature difference between lake water and air. This time of year the buoys floating in the middle of Lake Michigan are reporting surface water temperatures in the mid 40s. Upper air temperature data as measured by twice-daily weather balloon flights will clue us in on the low-level air temperature.

If we see 20-degree air headed our way at 2,000 feet above the ground, then a 25-degree temperature gap exists between water surface and 2,000 feet above it. That is enough to create instability in the low-level atmosphere. Any warm and moisture-laden air that rises from the lake surface will keep rising until it forms clouds and eventually precipitation. By the way, if the temperatures are a little too warm for snow then lake-effect rainfall is possible with some of the clouds that form.

How far inland does lake-effect snow travel? It depends upon the direction and speed of the wind. A strong northeast wind will carry moisture farther inland than a strong north-northeast wind. And a weaker wind speed (under 15 miles per hour) won't carry the snowflakes any farther west than Highway 45. Keep in mind that wind speeds tend to be faster over the lake than over land.

There is less friction over the lake, i.e. no houses, trees, or people to run into to slow it down. A 30 mile per hour wind roaring from the north-northeast will slam onto the lakeshore and immediately run into friction. That causes the wind to suddenly slow down. Just like cars suddenly slowing down in the fast lane of traffic, the wind tends to "bunch up" at the lakeshore. This bunching up causes the air parcels to rise upwards (since they can't go down into the ground). This rising up helps to enhance cloud and precipitation growth.

Perhaps the granddaddy of all lake-effect cities is Marquette, Michigan. It's location in relation to Lake Superior puts it in a prime spot to receive lake-effect snow from wind directions ranging from west-northwest all the way around to east-northeast. No wonder they average a whopping 172 inches per snow season. Marquette's snowiest season was the winter of 1996-'97 when 271 inches of snow fell, with a large portion of it being due to lake enhancement. That is one place you don't want to be caught without a shovel!