Still working on finishing up the look at this past winter and, in particular, whether it was the shortest winter on record. As I worked on defining just what constitutes ‘winter,’ I came across a previous article I wrote about when spring usually arrives. So, I figured I would revive that article and begin our look at this past winter by seeing how this spring’s arrival compares to previous years.
Looking back over the last 30 years of data and trying to determine the start of spring, I decided that for spring to have officially sprung, most of the snow needs to have melted and daytime high temperatures (in general) 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. Using the same logic and looking at this year’s data, spring would have begun on about March 4. From March 4 to April 9, we only had six days that did not see high temperatures warmer than +5 C. So, this year looks to be by far the earliest start to spring on record, or at least in the last 30 or so years.
When I tried to determine the average date for spring’s arrival, I found that the majority of dates fell within 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 in nearly every year with a late spring, there was significant snow cover going into April. What always amazes me is how quickly the snowpack can disappear. For most years it seems that when spring decides it is time to make an appearance, it only takes a few days for it to take hold. Whether it is 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 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 of that year there were 51 cm of snow on the ground in Winnipeg, and by April 4 pretty much all of it had melted.
As most of us saw this year, if you do not have much snow on the ground, the air tends to warm up quickly. 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 the air around it. The same thing holds true when there is snow on the ground.
Refrigerate and reflect
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 high school science, you’ll hopefully recall that to change a solid to a liquid (melting) requires energy, and in this case the energy is heat. This means that when there is snow on the ground, heat energy goes into melting the snow instead of warming the soil and air. 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 air above 0 C 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 heat 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.
Now, there can always be exceptions to this. 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 snow on the ground — something we have definitely experienced this spring.
Since we are now about halfway through April, and the spring sunshine is getting pretty darned strong, chances of seeing any late-season snowfall lasting for any length of time are relatively small. If we continue to dodge the snowstorms like we have, we don’t get any snow and the ground remains dry, I wouldn’t be surprised if we see daytime highs pushing the upper 20s by late in the month.