Ihave received a fair number of emails over the last while asking if we are seeing more melting during the winter than we used to see. These types of questions interest me as they will often have two possible answers. The first answer could be that yes, we are seeing more melting, but more often than not it has to do with people’s short-term memory. That is, people tend to remember details about the recent past but forget these details when dealing with longer periods.
For example, after a couple of warm winters, our minds all of a sudden remember that the last few winters have been warm and you have a hard time remembering when we last had a cold winter. On the other side of the coin are the “old-timers,” for lack of a better term, who seem to remember every warm or cold day we have ever had — at least until you call them on it! Either way, it is interesting how our weather memory works.
To try and figure out whether we have seen an increase in the amount of melting occurring during the Prairie winter, I figured the easiest way would be to count the number of days that go above 0 C each winter. I defined winter as the period from Dec. 1 through to the end of February. For this study I chose three different stations from across the Prairies that all had reliable climate records going back until at least 1939. For Manitoba I chose Winnipeg; in Saskatchewan, Regina; and in Alberta, Edmonton. For Alberta I figured Calgary’s data would be too muddled up with chinook winds to give a good representation of any possible increase or decrease in the number of winter melt days.
To calculate the number of winter melt days I simply counted the number of days that went above 0 C for each month of the winter and added them up. The easiest way to look at this data is to graph it out. The first things that jump out are how variable the data is and how many more melting days western regions see compared to eastern regions. Even though Winnipeg is a fair bit farther south than Edmonton and even Regina, Winnipeg sees notably fewer melting days during the winter.
Overall, it is not surprising that all three stations follow a very similar pattern, since the overall weather patterns that cover this area are fairly large in nature. Looking at the graphs it is difficult to say there’s any overall upward trend, although if you look at the area under the line, you could say there does appear to be an increase in the number of melt days beginning in the 1970s. I decided to look at some summary statistics of the data to see if that might tell us a little more. Statistically speaking, 30 years of data is considered a reasonable number, so I compared the first 30 years of data (1939-68) with the final 30 years of data (1982-2011) for each of our stations, as seen in the table here.
Based on this comparison it does look as though there’s been an increase in the number of melt days in recent years compared to the historical data, with the greatest increase occurring in the Winnipeg region. Overall, between 1982 and 2011, Winnipeg experienced about 55 per cent more melt days when compared to data collected between 1939 and 1968! Regina saw about a 30 per cent increase and Edmonton a 24 per cent increase. For you math geeks out there, these differences were statistically significant for Winnipeg and Edmonton, but were not significant for Regina. Maybe those “old-timers” are on to something!