Every so often a set of atmospheric conditions comes together to create some really remarkable weather. This is what happened last week in Ontario when cold northwesterly winds combined with the relatively warm waters of the Great Lakes to produce some remarkable snowfalls.
Whenever there is cold air and warm water, you can get what is known aslake effect snow.Here in Manitoba we occasionally see lake effect snows in late October and early November in the areas around Lake Manitoba and Lake Winnipeg, but due to the nature of our lakes we never see huge snowfalls like they do around the Great Lakes.
So, just what are lake effect snows and how do they form? Lake effect snows are exactly that: snowfall created solely by the moisture and energy contained within a lake. Usually when we get snowfall it comes from storm systems, which are areas of low pressure. These systems bring moist air and force it to rise up, condense and then form snow. With lake effect snow you do not need an area of low pressure; all you need is a large difference in temperature between the water and the air.
As cold air moves over a body of water the heat and moisture from the water are picked up by the air. This warm, moist air will then begin to rise up into the atmosphere. As the air rises it expands and cools, allowing the moisture picked up from the lake to condense into ice crystals and eventually form snow. That snow then falls downwind of the lake, or on what’s known as theleeside of the lake. Unlike snowstorms, which usually last only a day or two, lake effect snows can, on occasion, last for several days, creating some truly remarkable amounts of snow.
ADD WARM WATER
Before we look at just how much snow some areas of southern Ontar io received during this last lake effect snow event, I thought we could go into a little more detail on the different mechanisms that come together to produce these events.
The first and most import component that you need is warm water.Here in Manitoba our large lakes have some heat storage, but since they are so shallow they quickly lose their heat in the fall. This means we only have a short period in the late fall and early winter when open water and large amounts of heat are available. Looking at the Great Lakes, they are much deeper and can therefore store huge amounts of energy. With the exception of Lake Erie, it is rare for any of the Great Lakes to totally freeze over during the winter. This means there is plenty of warm, moist air available, especially in early winter, when the lakes are still fairly warm.
The second key ingredient ismoisture.While the open water of the lakes can supply plenty of moisture, what really comes into play is just how cold the air is over the lake. The colder the air, the lower the air’s ability to “hold” moisture. This means any moisture picked up over the lake will be quickly turned into snow as that warm air rises and cools.
The third piece of the lake effect snow puzzle is known aswind fetch.This is basically the length of time the wind is blowing across a lake. The longer the wind can blow across the water, the more heat and moisture it can pick up. In Manitoba, our major lakes run in a north-south direction, so for us to see any significant lake effect snows we need to have winds blowing from the north.
The last couple of mechanisms arefrictionandupslope lift.As the wind blows from the relatively smooth surface of the lake to rougher land surfaces, the air encounters increased friction. Friction slows the wind down over the land, causing the air coming off the lake to pile up. This then forces the air to rise up – it has nowhere else to go – which helps to cool it and form snow. In some areas, the height difference between the lake and the land also helps to lift the air and cool it, thus enhancing the snowfall.
OK, now back to Ontario and last week’s lake effect snow event. The region had all the mechanisms in place for a major event. It is early in winter, so the lakes still had plenty of heat; the air mass was fairly cold; the wind was blowing from the northwest, which gave it a long fetch across the lakes; the winds were quite strong, which produced plenty of frictional lift; and these conditions remained in place for about four days.
By the end of those four days, there were some remarkable total snowfalls. The city of London, Ont. recorded between 65 and 95 centimetres of snow during this period – close to what we see in a whole winter. A little northwest of London the small town of Lucan was hardest hit. During this event, Lucan received an amazing 177 cm of snow, or nearly six feet!
So the next time we get hit by a big storm and are complaining about digging out from 10, 20 or maybe 30 cm of snow, just think about what it would be like to dig out of that much snow every day for four days straight!