What exactly are 'normal' weather conditions? Find out, here

Sunday, January 1, 2017, 4:50 PM - For a province that has a reputation for its enviously mild winters, residents of coastal British Columbia – particularly those in the Lower Mainland – may be feeling like the past month has been anything but mild. After all, it snowed no less than five times in downtown Vancouver, and on more than one occasion the temperature didn’t even get above freezing during the day.

So much for its mild reputation!

What gives? Are these conditions normal? What do we even mean by "normal?" This article aims to provide context for the frequently used (and oft-abused) term of "normal," and endeavours to highlight how your own memory can alter your perception of the weather conditions around you by providing statistical comparisons between December 2016 and December 2014/2015 in Vancouver.

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"Normal" in context

Let’s be honest here: it’s simply not that uncommon to record temperatures at or below 0 C during the winter months along the B.C. coast, despite its mild reputation.

Nor is it that unusual for cities like Vancouver to receive accumulating snow in December and January, despite all the jokes about the city grinding to a halt after a few centimeters of the white stuff has fallen.

In fact, on average December is the coldest month of the year for Vancouver, with average daily max/min temperatures of 6.3/0.8 C and a total of twelve days recording temperatures of 0 C or below. Perhaps more impressively, roughly 15 cm of snow accumulates on average during the month – the snowiest month of the year.

Surprised? I’d be willing to bet you’re not alone.

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The problem arises from the fact that most people have a sense of what weather is “relatively normal” for a given time of year by comparing the current month or season to what it was like at that time last year, or two years ago.

Beyond that, it becomes very difficult for even the most savvy of weather enthusiasts to keep an unbiased mental log of how previous years compare to the present. This is where meteorologists begin to rely on the “climatological norma:l” the computed average of all the daily recorded temperatures and snowfall / rainfall accumulations over the course of thirty years for a given location. These are the “average” values presented to you above, which for the sake of completeness correspond to Vancouver International airport (in Richmond) and not to downtown Vancouver itself.

When you begin to average daily weather conditions on a decadal scale, extremely mild months and extremely cold months can end up cancelling each other out over time, leading to, well, average values – which are not necessarily representative of actual conditions experienced by residents of that city as they were happening. However, any proper statistical analysis requires a baseline value for comparative purposes – ideally a value obtained from a large collection of data over a long period of time – and the climatological “normal” is just that. Nothing more.

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With this context now provided, let’s compare the temperatures and snowfall accumulations of December 2016 in Vancouver to both the thirty-year climatological normal and then to the “relative normal” of the most recent Decembers of 2014 and 2015. All values presented here are from observations at YVR, and come from Environment Canada’s online weather database.

December 2016 in context

This December, twenty-two days had recorded temperatures at or below 0 C in Vancouver. Three of those days had sub-zero daytime highs (the coldest being -2.0 C), which led to an average daily max temperature of only 3.8 C for the whole month. The extreme daily max – the warmest temperature recorded in the whole month – was 8.8 C, and a total of 23 cm of snow fell over the course of five separate events.

Climatologically speaking, this was by definition both colder and snowier than average: compared to the thirty year average between 1981 and 2010, December 2016 had daily highs that were on average 2.5 C colder than the normal of 6.3 C, and received 8 cm more snow than the normal of 15 cm.

While this may not seem overly extreme (naturally some years will be colder and snowier than others), it gets significantly more interesting when you compare December 2016 to the anomalously mild years of 2014 and 2015 just passed.

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In 2015, for example, there were only seven days in December that recorded temperatures at or below zero (and just barely, at that). Not a single centimeter of snow accumulated in the whole month, and the coldest daytime high temperature was 2.6 C, leading to an average daytime high of 7.6 C. As an embellishment, the extreme daily maximum temperature was 13.9 C – a full 5 C above December 2016.

So in just one year, residents of Vancouver went from a relatively balmy, snow-free December to one with over 20 cm of snow and temperatures well below normal. No wonder this December seems so much colder and snowier!

The situation is the same when you look at the values for December 2014: that year there were only ten days that went below zero, only a trace of snow was recorded. The coldest daytime high temperature was 1.6˚C, although the average daytime high was 7.9 C. Perhaps most impressively, however, was the extreme maximum temperature of 15.1 C.

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Evidently, having two consecutive Decembers almost entirely snow-free with warmer-than-average temperatures would only serve to enhance the perception of December 2016 being unusually cold and snowy – which I doubt many residents would dispute! However, it’s always nice when climatological statistics can support your perception.

Although there are multiple contributing factors as to why December 2016 was significantly colder than that of 2014 / 2015, they will not be elaborated upon here as they go beyond the scope of this article.

Perspective is everything

With 2017 mere hours away, inevitably a brand new set of statistics will be used or thrown at you to compare certain weather conditions to “normal”. It is my sincere hope that a bit of context has been provided for you to better understand how a “climatological normal” can differ drastically from your own perception of “relative normal” – especially when a month or year goes from anomalously warm to anomalously cold.

Stay warm, my friends, and all the best to you in 2017!

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