For most of us it is hard to believe that climatically speaking, spring has already sprung. For a weather person, the four seasons are broken up into four equal portions: spring is March-May, summer is June-August, fall is September-November and winter is December-February. If we look at it from an astronomical point of view, the spring equinox, or vernal equinox, occurred on the morning of March 20. This is the time when the sun is making its transition across the equator, and we are now receiving more than 12 hours of daylight. This date is most often viewed as being the “official” beginning of spring. Right now, I think a lot of us are probably looking outside and wondering just how long it will take for spring to truly show up this year. Thinking back over the years, it seems spring rarely shows up at either of these two times.
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If the climatic and astronomical definitions of spring don’t really work for us, what definition should be used? I had to ask myself this question as I began looking back over the last 40 years of data trying 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 +5 C or better. Using Winnipeg’s data as an average for southern Manitoba I found that the earliest spring has arrived occurred just last year, when the snow was gone and daytime highs pretty much stayed above +5 C starting on March 4. The next-earliest appearance of spring occurred on March 17 in both 1981 and in 2000. Looking at the other end of the scale, the latest springs on record were in 1996 and 1997, when spring didn’t arrive until April 28 and 26 respectively. For those of you who can remember, 1997 was the year we saw that huge early-April snowstorm that brought upward of 75 mm of precipitation (mostly snow and ice pellets) to a large part of southern Manitoba. When I tried to determine the average date for spring’s arrival, I found that the majority of the dates fell into the week of April 5-12. Doing a quick comparison using Brandon’s records, I found that spring usually arrives around one week earlier in western regions, due mostly to lower snow cover in southwestern Manitoba.
Last spring, I discussed how a lack of snow cover will often allow temperatures to warm up more than expected because the sun’s energy can go into warming the air instead of melting snow. To warm up one gram of water by 1 C, it takes one calorie of energy. To melt the same amount of ice (one gram), it takes 80 times more energy, or 80 calories. This extra energy is stored in the water and is known as latent heat or the latent heat of melting. When water freezes it will release that heat. This is one of the reasons why areas around large, open water bodies stay warmer in cold weather. As long as there is significant snow on the ground a lot of the sun’s energy goes into melting the snow, which usually keeps air temperatures cooler.
With this knowledge of the snow cover’s ability to keep temperatures down, I found that nearly every year with a late spring we had significant snow cover going into April — although 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’s March or April, or whether we have 10 or 40 cm of snow on the ground, when spring arrives it is 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 of that year there was 51 cm of snow on the ground in Winnipeg, and by April 4 pretty much all of it had melted.
Now, depending on where you are in southern or central Manitoba, you may be snow free or are still dealing with large areas of significant snow cover. For those dealing with the melt of a deep snowpack and the worry of flooding, or for those who needed the soil moisture; this slow melt has been a good thing. At some point though, even those individuals just want spring to arrive — I know, I am one of them. At some point you start to worry about the ability to get back on the land. For those of you who are now snow free, chances are you are seeing slightly milder temperatures, especially on sunny days. As you know, snow cover is not the only thing that controls spring temperatures. We will also need to see a bit of a shift in our overall weather pattern and to keep on dodging the full impact of spring storms to see a shift in our temperatures. One thing that is always in our favour at this time of year is the angle of the sun overhead and day length, which will keep on increasing, meaning more energy and eventually — more heat!
Next issue we will re-explore some of the biggest spring storms we have seen over the years. Hopefully we won’t be needing to add to that list this year.
