Warm air delivers snow dump
It came, it sat, it took ages to go.
When the snowstorm finally started to relinquish its grip on Christchurch and much of Canterbury last evening, many places were left with at least 10 centimetres of snow and some had close to a metre of the white stuff.
What made this storm different was that warm air, not cold, was largely responsible for the snow.
And, unusually for a Canterbury snowstorm, there was little wind accompanying the bad weather.
Astute snow observers may have noticed that flakes were, by and large, falling straight down. Usually, they are borne on a fresh to strong south to southwesterly wind, and drift and collect in corners.
That lack of wind was a signal the storm was pretty much right overhead and not going anywhere in a hurry.
Weather forecasters call yesterday's event a warm advection snowfall, because the movement of relatively warm air, which can contain more moisture than the same amount of cold air, drives the system and produces the snow.
As the denser cold air, in the form of a southerly, undercuts the warmer air, the snow falling from thick clouds associated with the system cannot melt as it would usually. Instead, the snow is able to make it all the way down to the ground through the frigid layer.
Canterbury, particularly north of Christchurch, became the battleground for this great clash of air masses yesterday and, because we were in the centre of it, there was little wind to hurry it along.
It was a different kind of snowstorm to those in late July and mid-August last year, which were largely the product of bitterly cold Antarctic air.
Yesterday's snow was certainly early, beating by one day a significant fall in June 2008. But it was by no means the earliest – snow fell at Christchurch Airport on April 23, 1987, on April 18, 1990, on May 23, 1988, on May 29, 1989 and on May 20, 1992. Whether it is the only snowfall in the city this winter remains to be seen.