LAKE EFFECT SNOW

You might have to click on the image to get it to loop. This image courtesy of Weather Underground.

When my television is turned on I “check out” the Weather Channel fairly often. For the most part I’ve been very impressed by explanations that are given about weather happenings. I’ve never done media weather reporting so can only imagine how frustrating it must be to provide good presentations when time is at such a premium. In the college classroom even though I had an agenda with objectives to cover I had control over the amount of time I spent on individual topics.

Recently I heard a Weather Channel reporter give a less-than-desirable explanation for the cause of lake effect snow events. I imagine that time constraints kept her from being more thorough. This is close to what she said: “The cold, dry air behind this front is moving over the warmer Great Lakes picking up moisture and then dumping snow on the land at the opposite side.”

From my point of view that was far too brief leaving out way too much. But – everything is relative. I suspect that most people would prefer her explanation to an hour lecture on the subject from me. But there would likely be one or two in a large class of meteorology students who would be dissatisfied because I left something out or left them very confused. My dad didn’t have the opportunity to teach me much but one of the things that he tried hard to get through to me was, “You can’t please ’em all.”

So – I’m not complaining about the Weather Channel presentations. I think they do a very nice job. But if I were working for them, here is the minimum that I would insist upon (which is probably one of the reasons why I’m not a suitable candidate to work for them – the brevity necessitated by time constraints would drive me up the wall and my tendency toward long-winded discussions would drive them up the wall – LOL):

Cold, dry air moving over a large, unfrozen lake surface picks up moisture in the form of water vapor made available by evaporation. More often than not unfrozen lake surface water’s temperature is higher than that of the air behind a cold front.

But in most cases clouds that provide precipitation form as a result of air rising and that is most definitely a factor in bringing about lake effect snow events. So – what makes it rise?

The moisture-laden air can rise because of an increase of elevation on the downwind side of the lake but that alone is usually not enough to create snow-producing clouds unless the increase is due to a plateau or mountains. Also, positive buoyancy can cause the air to rise and that can be created in two ways as the air is traveling over the lake water. Picking up heat energy the air can begin to lift (heated air tends to rise); a fair analogy is a hot air balloon. Also, the addition of water vapor to air can increase its buoyancy (as the specific humidity increases, the density increases so long as the temperature of the air does not drop significantly).

The factor most often omitted in a lake effect discussion of why air rises to form snow clouds is this: When air reaches the land it slows down because of the decided increase in friction. When in heavy traffic a car far ahead of you slows down – you, the cars ahead of you, and the cars behind you tend to squeeze closer together. We say the cars are converging (getting closer together). In the case of the fluid air a vertical component of motion is allowed so some of the air “piles up” and therefore moves up. Have you ever heard of cars piling up?

Therefore, even without an elevation increase of the surface over which the air flows and even without an increase in buoyancy, some air on the downwind side of a Great Lake is likely to rise because of convergence. Whether or not snow clouds form is dependent upon a combination of factors.

You’ll have to admit that my explanation is more thorough than “The cold, dry air behind this front is moving over the warmer Great Lakes picking up moisture and then dumping the snow on the land at the opposite side.” The trouble is, it took time, is very “wordy” and you really have to read carefully to pick it all up. Furthermore, as long as my explanation is, it still does not give all of the reasons behind lake effect snows. Also, it makes a blanket statement (as the specific humidity increases, the density increases so long as the temperature of the air does not drop significantly). But it does not elaborate. For most people that sounds like a real paradox – adding moisture to the air can make it less dense and therefore be a contributing factor to its rising! It’s true – and I intend to discuss that paradox on this web-log in the near future. By the way – a friend of mind from way back in high school defines paradox as “two physicians.”

 

YOU MIGHT HAVE TO CLICK ON THE IMAGE ABOVE TO GET IT TO ENLARGE AND LOOP.

In the radar loop above taken from a small time segment (48 minutes) earlier today (12-6-2010), you see the signature of lake effect snows in Indiana. Lake Michigan’s mean elevation is a bit over 577 ft. above mean sea level. The land between Lake Michigan and Fort Wayne is as much as 250 feet higher but I think it’s unlikely that 250 feet of elevation increase is going to create that much snow. Nor is the land heating the air to make it rise via positive buoyancy. In fact, the land surface temperature is colder than the air flowing over it. I believe that it’s convergence of the type I’ve described in this entry that is responsible for much of the snow.

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