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When Should We Expect The Snow To Melt? – for Mar. 31, 2011

The quest ion of when spring is supposed to start seems to crop up in agricultural Manitoba every time we still have a good covering of snow on the ground and it’s getting late into March. This year is no exception. A few years ago I looked into this question, and with what appeared to be a late start to spring this year, I thought it might be a good time to revisit this topic.

Looking back over the last 30 years of data and trying to determine when spring starts, I decided that for spring to have officially sprung, most of the snow needs to have mel ted and dayt ime high temperatures (for the most part) need to be consistently above +5 C or better. Using Winnipeg’s data as an average for southern Manitoba, I found that spring has arrived as early as March 17 in 1981 and 2000 and as late as April 29 in 1996 and April 26 in 1997. We could argue that last spring was the earliest, as temperatures rose to above the 5 C mark and pretty much stayed there starting around the 14th of the month, but it’s a bit of a tougher call as to when all the snow melted.

Unfor tunately, Winnipeg no longer keeps snow depth readings and if we look at Brandon, it lost all its snow by March 18.

When I tried to determine the average date for spring’s arrival, I found that most of the dates fell into the week of April 5-12. When I did comparisons against Brandon’s records, I found that spring usually arrived around one week earlier in the southwestern region, due mostly to lower snow cover and higher elevation. I also found that nearly every year with a late spring had significant snow cover going into April.

STRONG SPRING SUN

What always amazes me is how quickly the snowpack can disappear. For most years it seems that when spr ing decides it’s time to make an appearance, it only takes a few days for it to take hold. Whether it’s March or April, or whether we have 10 or 40 cm of snow on the ground, when spring arrives it’s usually with a vengeance. Under the strong spring sunshine (mid-April sunshine is equivalent to late August in strength), even the deepest snowpack can literally be wiped out in less than a week. An example of this was in 1955, a record year for snowfall. On March 29 of that year there were 51 cm of snow on the ground in Winnipeg, but by April 4, pretty much all of it had melted.

So, when should we expect to see temperatures warm enough to melt all this snow? Well, the basic answer lies within the snow – literally. How many of us have headed out on a warm spring day and have gone for a visit to the local swimming hole or lake? What did you end up experiencing? I would bet that if there were a wind blowing off of the lake you would have noticed how much colder it felt. This is due to the cold water and ice on the lake cooling down the air around it. The same thing holds true when there is snow on the ground.

Snow acts in a number of ways to keep air temperatures down. First of all, snow, being the frozen state of water, is by its very definition, cold. Secondly, if you remember back to your junior-high or senior-high school science, you’ll hopefully remember that to change a solid to a liquid (melting) requires energy, and in this case the energy is heat.

TAKES A LOT OF HEAT

Thirdly, snow is usually white and white objects do a remarkably good job of reflecting sunshine. So if we have a large area, such as Manitoba, covered in snow, and we have above-zero air moving into the region, the snow itself acts to cool the air as it comes into contact with it. Along with this, as the heat of the air interacts with the snow, that heat is used to melt the snow instead of heating the air, and it takes a lot of heat to melt snow. In fact, it takes almost as much heat to melt snow as it does to bring water to the boiling point.

Finally, if the ground is covered in snow it acts as a very efficient reflector of sunshine, reflecting much of the sun’s energy back towards space, whereas black soil will absorb the sun’s energy and in turn, heat the air around it.

There can always be exceptions. Very warm air can move into our region or local conditions could be just right, and even with all the snow around, temperatures can get quite warm, but to see temperatures over 10 C, you pretty much need no or very little snow on the ground.

Will we see any more really warm air moving into our region any time soon? The long-range models show us flirting with some milder air to start off April, but the real question is whether we’ll see any more snow. If we continue to dodge the snowstorms like we have been and we don’t get any more snow, then I think we’ll see the snow gone pretty much right on average, between April 5 and 12.

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Whetherit’sMarchorApril,orwhetherwe have10or40cmofsnowontheground,when springarrivesit’susuallywithavengeance.

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