The weather in Vancouver this last April was atypical, wasn’t it? A lot of my people are saying so. My mom says April showers bring May flowers, but that obviously can’t hold true everywhere. I wonder where that saying is from. Wikipedia says April showers are a thing in Ireland and Scotland. Anyway, regardless of origin, when it doesn’t rain much in April, this old proverb adds a little extra weight to the unsettled feeling in the air.
This year I’ve transitioned from managing a web product at Peer Giving Solutions to working with data at Chimp Technology. With that shift in my work life, I’m feeling extra motivated in my personal life to find complimentary outlets for my analytical aspirations. You might not believe this, but it feels refreshing to be analytical in my down time. I guess I just miss the years I studied math and physics, when I was known to lock my bedroom door during house parties to continue working on my homework uninterrupted.
I really am a nerd.
With climate change in the back of my mind too, I’m doubly inclined to sharpen my sense of what’s actually happening. I feel uneasy in conversations about how unusual things seem. I don’t really trust my ability to intuit what’s happening, maybe because I don’t trust my memory. I’m much more comfortable talking about facts, especially when it comes to something important like the state of the planet.
So I decided to take a deep dive into this weather question: Was the weather this April atypical?
Did I start by hammering google with search requests? Not even. It’s a pain in the ass to establish trust there too. I went straight to the government’s weather records, specifically data recorded at YVR. It only took an hour to download and format all the daily weather records between January 1937 and April 2016. From there I spent a few hours asking various questions with my favourite tool of inquiry, the spreadsheet.
Ready for science? Here we go:
Figure 1 shows the number of days in April where the daily high broke thirteen degrees Celsius, going back eighty years. Go ahead and take a look. High points indicate warm months.
Here’s the story with numbers:
- 27 days in April 2016 broke the thirteen degree mark. That ties the eighty year record set in 1940.
- On average (last eighty years), April has 14.2 days that see thirteen degrees or higher.
- The standard deviation is 5.7
Since 27 days is more than two times the standard deviation from the average, this reads as quite unusual, statistically speaking. That’s enough for me to be comfortable with the statement that this past April was super warm.
For everyone who just choked on their salad at the term standard deviation, think of it as a kind of ruler stick, a tool which tells you how far away from the rest of the data – and therefore how unusual – a particular data point is.
If you had just choked and coughed up all the half-chewed lettuce in your mouth, most of the chunks would have landed in a general area, and just a few pieces would be farther away. Likewise, in most data sets, most of the data is less than one standard deviation away from the average, and only a tiny portion is more than two standard deviations away.
Let me show you the data from the previous chart without the time scale – its distribution. What we’re looking at in Figure 2 is the number of years (the height of the bars) that had Aprils with a certain number of days that broke thirteen degrees (different bars for different numbers of days). Notice that the distribution kind of looks like a bell curve (a normal distribution).
Near the middle you see a peak around 17 days, the bar there showing that 10 years in the past eighty had Aprils with exactly 17 days breaking thirteen degrees. Two Aprils, this recent one and one from 1940, saw 27 days breaking thirteen degrees, so there’s a bar at 27 (which I’ve circled) showing those 2 years.
I’ve also drawn three blue lines to show you (from left to right) the average, one standard deviation above average, and two standard deviations above average. The crux of the story here is that the circled bar is more than two standard deviations from the average, which indicates statistically that it is very unusual to see 27 days all above thirteen degrees in April.
Let’s talk about rain next. Take a look at Figure 3, which shows the number of days in April that saw rain, for all the years since 1937. Low points show dry months.
The story again with numbers:
- 9 days in April 2016 saw rain
- On average (last eighty years), April has 13.9 days with rain
- The standard deviation is 3.9
What this says is that the rainfall this past April was below average, but not unusually so, since it’s only one standard deviation from the average (see circled bar in Figure 4).
So basically I’ve now convinced myself (and hopefully you too) that this last April was abnormal. Mostly it was unusually warm, and to boot it rained less than average. I ran the same analysis for May, and it’s the same. Not great news for the water supply this year, but it does feel good to have confirmed reports about the weather so far this Spring. Now you can tell people that not only was April unusual, it was two standard deviations unusual, with respect to the number of days above thirteen degrees.
Stay tuned for my proposal that we start dividing the year into eight seasons instead of four. And thanks for reading! Feel free to drop any questions you might have in the comments.