The Science Behind Summer Snow: What's going on in Alberta this week?
Wednesday, September 17, 2014, 9:27 AM - It was a rough start to the month in Calgary and the rest of southwestern Alberta, with the beautiful summer weather vanishing in a flash of snow, forcing residents to pull the winter clothes out of the closet and dig through the garage for the shovel. Why? Snow ... SNOW ... IN SEPTEMBER ... with 11.8 centimetres of snow falling on Calgary, just 1 millimetre shy of the record set back in 1921.
(EDITOR'S NOTE: This article was originally published after the September 10, 2014, snowfall in Calgary.)
So, what exactly is going on here, that's giving southwestern Alberta an early blast of wintry weather when we're not even into fall yet? It is some kind of freak event? Is it climate change?
Well, it's definitely pretty freaky to see snow in September, but it's not the first time Calgary and the surrounding area has seen this. The record snowfall amount for September 8th in Calgary is 11.9 cm, set back in 1921, and there was another significant snowfall on September 19, 1895, when 19 cm of snow fell in the city. As for climate change, while some recent studies have been showing that the increased amount of open ocean in the Arctic - due to global warming melting more of the ice there - has been causing deeper 'dips' of the jet stream (recall the 'slip' of the polar vortex this past winter and the numerous cold-snaps that sparked from it), it's difficult to point to any single weather event and say it was 'caused' by global warming and the resulting climate change. The most that can be said right now is that it's possible that global warming and climate change are having some influence on the situation, but there's really no way to be sure that this wouldn't have happened anyway.
However, let's look at what's happening here.
The main culprit here is the cold front that swept through the area early in the day on Monday. Unlike the typical scenario for a cold front passing by, where it moves across roughly from west to east, this particular front passed through southern Alberta from north to south. Lined up behind that front is a fairly steady stream of cool, humid air flowing down from the northeast, out of northern Canada.
Now, this flow isn't just affecting southern Alberta. It stretches right across the Prairies at this time. However, the issue here is all about the angles.
Firstly, the winds through Manitoba and most of Saskatchewan are flowing slightly more from the north, and as you get into western Saskatchewan and Alberta, it's flowing more from the northeast (trust me, that's relevant).
Topographic map of Canada. Source: Wikipedia
Secondly, if you travel from east to west across Manitoba and Saskatchewan, you'll find that you're going slightly uphill the whole way, with Saskatoon roughly 250 m higher elevation than Winnipeg (at 230 m AMSL). Travel directly north or south from any point along your route through those provinces and you will see roughly the same gradual slope from north to south (for example, Regina about 100 m higher than Saskatoon). Even going as far west as Medicine Hat (elevation 690 m) and you still have the same gradual rise in elevation, but go further west than that and land slopes upwards much, much more abruptly. Pass Lethbridge and go through Calgary and now you're suddenly up to around 1,000 m above sea level. Keep going to Canmore, on the road up into the mountains, and you're closer to 1,500 m elevation.
So, while the winds are flowing down over Manitoba, Saskatchewan and even southeastern Alberta, the gradual rise in elevation isn't having much of an effect on them. However, when the northeast winds reach that abrupt elevation rise going into southwestern Alberta, that sudden lift has a profound effect. Not only are the winds running up against the physical 'block' of the mountains, causing them to 'pile up' on themselves, which concentrates the water vapour and raises the humidity, but the lift is cooling everything down at the same time.
Called orographic lift, the first thing that happens as the air is forced upwards is that it cools significantly, at a rate of around half a degree Celsius per 100 metres. This cooling causes the water vapour contained in the air to condense out quickly. While this condensation releases latent heat, and while that does put some warming into the system, the overall cooling from the large-scale flow up-slope dominates. Since the temperatures just from the wind flow itself were already down in the single digits as the front passed through, it didn't take much for this lifting to push them down to the freezing level. So, rather than all that water vapour condensing into rain, it fell as snow and made it all the way to the ground.
The warm temperatures over the weekend meant that the ground was still quite warm through most of the day, so the snow didn't stick everywhere (mostly on grass and trees), but as temperature remained at the freezing level throughout the day, the 'advantage' switched over to the snow's side, and it began to accumulate.
The official weather station in Calgary saw close to 12 cm for the day on Monday, but some residents measured up to 15 cm in other locations around the area. That's not all, though, more snowfall warnings are in effect for today, with another 10-15 cm expected in the foothills and around Calgary, and possibly up to 20 cm further up into the mountains. So, it's not over just yet!