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I have enjoyed watching Mr. Snowbank (Eubank) for years and have learned a lot about the weather from him. He will be missed.
I am recovering from surgery and am up frequently during the night and as I watched the snow a question occurred to me that I don't think has been discussed before (at least that I have seen).
Mark has spoken in great detail about how the cold air moving across the lake creates the snow bands and how it is like a spout and where the winds are blowing it will snow. I understand that and as I went back to bed I was thinking about what other conditions are necessary in order for snow to fall. Cold air is always moving across the lake during the winter so in theory, it should be snowing all the time as long as the air aloft is colder than the surface temperature of the lake. I would imagine it has to have something to do with the barometer or maybe the jet stream but that doesn't seem to fit because the jet is so far to the south of us right now. Anyway, I think you understand my question. One other thing, I know that the same thing happens with the Great Lakes between Illinois and New York but does it happen to a lesser degree with smaller lakes such as Utah lake or any of the reservoirs throughout the state? I imagine that there must be some effect but it is not spoken of. Just curious.
Thank you for your time,
Michael W.
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A late night pondering how the atmosphere works sounds like a fine use of time to us here! Your question certainly is a good one and a lot of people wonder how lake effect works and if ours is the same as in other places.
You ask a few things so we'll break it down. First, for snow to fall, we need precipitation to be below freezing essentially. Of course, there are cases where this doesn't happen like rain that still falls even though the ground temperature is at freezing but for our answers today, you need below 32 F temps to have snow. Cold air and precipitation make snow if it's cold enough.
Lake effect snow as you are mentioning happens when cold air moves over the lake. You need this air to be a lot colder than the lake, not just a little. The upper air temperature and the lake temperature (of the water) need to be far apart. For example, this morning the air moving in high up had a temperature of -4 F, the lake temperature is around 40 degreees F, that difference is enormous! A 36 degree difference between the air and the water!
When the cold air moves over warm lake, the warm lake water will start to evaporate into this air, this cools and condenses and forms clouds and eventually snow. A link on the right will take you to a diagram of how this works.
Yes, cold air is always around in the winter time but you need an air/lake water temperature difference to be very high. The cold air also has to be moving over the lake in such a direction to get more evaporation into it. This means we need the flow to go over the major axis of the lake to get the best shot at lake effect snow (LES). We call this fetch. We want the fetch to be nice and long so the cold air has lots of time to interact with the warm water.
The fetches that work best for the Great Salt Lake are when the air is flowing out of the Northwest. Since we don't have always have the correct temperature difference and fetch, we don't always have lake effect snow even when it's cold outside.
So the barometer and jet stream have about as much to do with lake effect snow as your winter slippers. NADA. Lake effect snow is on a much smaller scale. The jet streams moves our storm systems and weather on the surface of the earth. Behind those weather systems when we get into the northwest flow is where we get our LES. The jet stream is way higher up in the atmosphere than the flow we're concerned with making for lake effect snow. However, the jet stream can determine our general air flow direction aloft. Meaning, if our air flow is out of the NW at 10,000 feet, we may have NW where the jet hangs out (20,000 feet or higher). BUT, this isn't always true either, the flow may be different at 20,000 than at 10,000 feet, so the jet isn't the main factor for LES at all.
Lake effect snow works the same way over the Great Lakes too. And if you get the right conditions, you can have LES on smaller lakes but it's just not as common. You need that special flow and air/lake temperature difference to be just right.
