First -25°C of the Winter

A few days ago we looked at when Edmonton gets it's First -20°C of the Winter.  Today we're doing the sequel, as we look at when -25°C first appears.

Recent Years

This year we hit -20°C for the first time on December 24th, and that was late compared to some recent years, although it's not unusual. Then one day later we hit -25°C, on December 25th.

Of the 23 years shown here, there were 11 where the first -25°C showed up within a few days of first -20°C: 2017, 2011, 2010, 2009, 2008, 2006, 2003, 2000, 1998, 1997 & 1996.

And then there are some other winters where the first -25°C didn't show up until late-January or early-February, like in 2005-2006 and 2007-2008.

History

And here is the entire recorded history.

There was one winter in Edmonton that did not have any temperatures below -25°C, and that 1986-1987. You can't really see it in the chart though, because it's pretty buried.

The largest gap here was 1930-1931, with -20°C showing up early on October 19, and then 144 days later -25°C hitting very late on March 11. 1991-1992 was similar, getting it's first -20°C on November 1, and then -25°C arrived 110 days later on February 20. After that 2005-2006 had a 74-day gap, and then there were a few winters at around 60-days: 1888-1889, 1982-1983, 2007-2008.

Looking at the 10 Year averages in red, we can see that the first -25°C has crept up a little bit over the years, and now sits at mid-to-late December.

This histogram shows how things typically break down. November is most common for -20°C, while December is for -25°C. But the two months of November & December combined are where the vast majority of our cold days first hit: 90% of the time for the first -20°C and 79% for the first -25°C.

10-Year Average

Taking a closer look at the averages, on-average there's about a two week gap between the first -20°C and -25°C. That isn't really representative of what's actually happening, though.

As we saw at the start, almost half of the time -20°C and -25°C show up during the same coldsnap, and there's very little separation between them. But then in other years the gap might be two weeks or even two months. The average of all of that is 13 days, but in reality only about 10-out-of-138 have been close to that average.

Now we've looked at the first -20°C of each winter, and the first -25°C. There won't be a version of this for -30°C, because there have been 20 winters without a -30°C, and this type of chart doesn't handle gaps well. There is the threat of -30°C temperatures this week though, so in a few days we'll try to visualize that a different way.