Most of us can hardly wait until the snow melts and warm spring temperatures move in for good. It seems as though a lot of us are just assuming spring should be arriving early each year — after all, isn’t that what global warming is supposed to be about? As we learned last March, if the right conditions come together all at once, we can see some pretty remarkable weather! Last year we saw record March warmth and a record early start to spring. This year… who knows?
Just when should we expect the snow to melt and disappear across southern Manitoba? I’ve looked at this a couple of times over the last nine years, but I figured it wouldn’t hurt to look at it again. Using data going back over the last 30 years to determine when spring starts, I decided that for spring to have officially sprung, the majority of snow needs to have melted and daytime 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 14 in 2012 and as late as April 29 in 1996 and April 26 in 1997. When I tried to determine the average date for spring’s arrival, I found the majority of 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. What always amazes me is how quick the snowpack can disappear. Most years, it seems that when spring 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 be literally wiped out in less than a week. An example of this was in 1955, a record year for snowfall. On March 29 that year there were 51 cm of snow on the ground in Winnipeg; 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 with 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 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 temperature 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 high school science, you’ll remember that to change a solid to a liquid (melting) requires energy, and in this case the energy is 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-0 C 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 warming 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 toward space, whereas black soil will absorb the sun’s energy and in turn, heat the air around it — something we really saw last March!
Now, there can always be exceptions to this. Very warm air can move into our region and even with all the snow around, temperatures can get quite warm, but to see temperatures over 10 C you pretty much need no snow on the ground.
Will we see any really warm air moving into our region any time soon? If you take a look at this week’s forecast, it doesn’t look that promising. All I can say is I really hope this spring isn’t the reversal of last spring, but knowing how our weather works, it wouldn’t surprise me if we go from a record-early spring one year to a record-late spring the next! Please don’t shoot the messenger!