Today will be another marathon, but I don't think that we'll be coming back to this topic again anytime soon, so we might as well get it right the first time.
We're going to start with a quick review:
Previously, in Edmonton weather nerdery...
In Part 1 we saw that Edmonton's frost-free season is currently more than a month longer than it was a century ago. More specifically from about 1940 through 1980 the average date of the last spring frost moved from the end of May to May 7th, and the average first fall frost moved from the start of September to September 24th. Since 1980 though, things have been pretty stable.
The results from Part 1 raise a few questions that we're going to try to dig into today:
- how does Edmonton's frost-free season compare to other areas?
- was the dramatic shift during the 1940s-1980s just because of Edmonton's growth as a city?
- what was going on from 1975-1981? There was a string of 7 straight years with unusually long frost-free seasons.
- why did things level off after 1980?
I've read ahead and I know that (spoilers) we aren't going to find all of the answers today. But these are still useful questions to keep in mind as we work through the comparisons.
One other thing to note in the above chart is the lines labelled "25% Chance" and "10% Chance." Frost-free days are typically tracked using a 30-year average, but that average is just the halfway point - some years will be early and some will be late. In Part 1 we added the "25% Chance" and "10% Chance" lines to indicate the likelihood of late spring frosts or early fall frosts.
For the location comparisons today we won't be worrying about the probabilities, and will just focus on the 30-Year average. That's just to keep things simple, but for all of these charts today it's good to remember that using a 10% probability of frost would shorten the season by about 2 weeks at both the spring and fall ends.
Calmar, Campsie & Sion
One of the earliest posts on this blog was Rural Alberta & the Heat Island Effect where we compared the change in Edmonton's winter temperatures to the changes at some nearby, rural stations: Edmonton is currently warmer than its surroundings; but that was also true a century ago; and over the last century the surrounding stations have warmed up by about as much as Edmonton.
Calmar, Campsie and Sion might seem like random location choices, but they were selected because they each had long, unbroken sets of data. Environment Canada has a lot of Alberta weather stations, but many of them only have 5, 10, or 20 years of data. Calmar, Campsie and Sion have 87 years of overlapping data from 1915 through 2003 which allows us make long-term comparisons.
Here we have the 30-Year average dates for the first and last frosts at the Calmar, Campsie and Sion stations.
For the years which we are looking at Edmonton's frost-free season has always been longer than for the other stations. And while Edmonton's season has gotten longer over the years, so have the other stations'. In Part 1 we saw that on-average Edmonton has gained 20 more frost-free days in the spring and 18 in the fall, and here we see:
- Calmar gained 22 days in the spring and 16 in the fall.
- Campsie gained 9 days in the spring and 18 in the fall.
- Sion gained 23 days in the spring and 18 in the fall.
This is another way of looking at how the total length of the frost-free season from spring to fall has changed for Edmonton, Calmar, Campsie and Sion.
- Over the last century Edmonton's frost-free season increased by 38 days, with almost all of that occurring between 1940-1980.
- Calmar's season also increased by 38 days, and like Edmonton its line starts to flatten out around 1980.
- Campsie only increased by 25 days, but that continued through to the late 1990's.
- Sion increased by a whopping 61 days, and that was also fairly continuous through to the early 2000's. That is skewed a bit by some very short (~20 day) seasons, which drag down the average early on.
Going back to our questions from earlier:
Was the dramatic shift during the 1940s-1980s just because of Edmonton's growth as a city?
- It doesn't look like it, with Calmar experiencing a very similar change, and Sion seeing an even larger increase.
- In these charts the jagged lines show the yearly data for these stations, and both Calmar and Sion also had some long seasons during those years. Whatever was going on, it wasn't just Blatchford.
The International Airport, Fort Saskatchewan & Stony Plain
Calmar, Campsie and Sion are good comparisons because they have almost a century of data, but they're also a little obscure. In this chart we're looking at some better known locations, with the Edmonton International Airport, Fort Saskatchewan, and Stony Plain. For these stations the data starts between 1955-1965 and continues to today.
Looking at the 30-Year averages we see:
- the International Airport's frost-free season is about a month shorter than Blatchford's. Its last spring frost is 16 days after Blatchford's, and its first fall frost is 14 days before. And those average dates haven't changed much since 1990
- Fort Saskatchewan's last spring frost is 14 days after Blatchford's, and its first fall frost is 10 days before. It's spring average hasn't changed much, but its fall frost date has moved from September 8th in 1990 to September 14 today.
- Stony Plain's season is quite close to Blatchford, with its spring frost 5 days after Blatchford's, and its fall frost 4 days before. Those also haven't changed much since the 1990s.
It shouldn't be any surprise that Edmonton has a longer frost-free season than the outlying areas. But as we just saw with Calmar, Campsie and Sion the outlying areas changed by about as much as Edmonton. If more data were available for the International Airport, Fort Saskatchewan, and Stony Plain we might see similar changes there as well.
Environment Canada has a few more nearby weather stations, so lets expand this to include all of them:
Many of these stations only have a few years of data, and so for this chart the timeframe has been reduced to show 1980 to today. The dashed lines indicate stations which don't have enough data to calculate a 30-year average.
For stations in the Edmonton region, the average frost-free season from longest to shortest is:
- Blatchford: 140 days - May 7 to September 24
- Stony Plain: 131 days - May 12 to September 20
- U of A: 126 days - May 15 to September 18
- Namao: 118 days - May 19 to September 14
- Fort Saskatchewan: 116 days - May 21 to September 14
- Edmonton International Airport: 110 days - May 23 to September 10
- Woodbend (near the Devonian Botanical Gardens): 104 days - May 29 to September 10
There is a difference of a month from Blatchford at 140 days and Stony Plain at 131, down to the International Airport at 110 and Woodbend at 104. And just between Blatchford and the station at the Univeristy of Alberta there is a two week difference.
And as mentioned earlier, these are just the average dates. If we apply the safety factor for 10% probability of frost that shortens all of these seasons by about a month.
And now is a good time to warn everyone that these calculations are not a perfect match for the dates published by Alberta Agriculture & Forestry. The dates from the Alberta Government were last revised in 1998, and they are generally close to what we're seeing today, but there have been some small shifts.So now we have a pretty good sense of how things work around Edmonton. But what about our favourite neighbour?
The Calgary Region
This chart shows the frost-free season for three of Calgary's Environment Canada weather stations: the International Airport which is about 9km north of their downtown; Canada Olympic Park 12km northwest of downtown; and Springbank Airport 23km northwest of downtown. The records for their International Airport go back to 1885, while the other two stations start in the 1980s and 1990s.
The main point of interest here is Calgary's International Airport which is in red, so lets start there:
- A century ago Edmonton had a longer frost-free season than Calgary, by about 10 days. The difference now is 17 days.
- Calgary's last spring frost has moved by 13 days compared to Edmonton's 20. And Calgary's first fall frost has moved by 17 days compared to Edmonton's 18.
- With Edmonton we've seen that the change of the frost-free season occurred mostly between 1940-1980, but Calgary's change was much more consistent - starting in 1915 and continuing to today.
- Because of those different patterns, the average frost-free seasons in Edmonton and Calgary briefly lined up in about 1930 and then again in 1945.
Looking at Calgary's outlying stations, the average length of the frost-free season at Canada Olympic Park is the same as at their airport: Canada Olympic Park's final fall frost is 2 days later than the airport, but its first spring frost is also 2 days later.
When you travel 11km further down the highway to Springbank things are very different. Its average season is 39 days shorter than the Calgary International. That is a big change in a relatively small distance, and it is larger than anything that we saw in the Edmonton region.
We've looked at Edmonton, and we've looked at Calgary, and if you are a regular follower of this blog you might be able to guess what comes next:
Edmonton, Calgary, Winnipeg & Montreal
Here we have the frost-free seasons for Edmonton-Blatchford, the Calgary International Airport, the Winnipeg International Airport, and Montreal.
To recap what we've seen so far:
- Edmonton has had a pretty dramatic increase in the length of the frost-free season.
- That increase wasn't just limited to the city of Edmonton, but it affected the outlying areas as well.
- Calgary also saw a large increase, but it wasn't quite as large as Edmonton, and it followed a different pattern.
For Winnipeg, a century ago its frost-free season was longer than Edmonton's or Calgary's. And since then Winnipeg's season has gotten a bit longer - on both the spring and fall ends - but not by nearly as much as the Alberta cities.
A century ago Montreal started with a much longer frost-free season than any of the prairie cities - with the last spring frost in late April and the first fall frost well into October. But then over the last 100 years it didn't change much - it actually shortened a bit in the 1960s and 1970s before increasing again recently. For Montreal we've done several previous comparisons in the past - here, here and here - and I think it's fair to say that the weather in Edmonton and Montreal don't have a lot in common.
This chart shows how the total length of the frost-free season has changed for these cities. There is a lot of information in the animated version of this chart, but the main thing to focus on is the solid lines for the 30-Year averages.
- Edmonton's average frost-free season lengthened by 38 days, but that mostly occurred between 1940 and 1980.
- Calgary gained 32 days, spread fairly evenly across the years.
- Winnipeg gained 17 days, a lot of which actually occurred between 1910 and 1950.
- Montreal is off on its own, losing frost-free days and then regaining them, for an average of 1 additional frost-free day over the last century.
A century ago Winnipeg had a longer average frost-free season than Edmonton or Calgary - 4 days more than Edmonton and 15 more than Calgary. But because it has remained relatively flat compared to the Alberta cities, Calgary is now tied with Winnipeg, and Edmonton's season is 17 days longer.
And Montreal used to have a huge gap over the prairie cities - from 64 more days than Winnipeg to 79 more than Calgary. But now that gap has shrunk to 31 days more than Edmonton, and 48 more than Calgary or Winnipeg.
One thing to keep in mind here is the stations which are being used: Edmonton's is downtown~ish; Calgary's is on the edge of the city; Winnipeg's is on the edge of the city~ish; and Montreal's is the furthest from downtown, but it is surrounded by city, and it is near the St. Lawrence. So the absolute numbers that we're seeing here will vary throughout these regions, but as we saw with Blatchford, Calmar and Sion, theoretically the long-term trends will be similar throughout each region?
It's all a bit weird, though: Winnipeg goes up while Edmonton goes down and Montreal is level; then Montreal goes down while Edmonton goes up and Winnipeg is level; and then Edmonton is level while Montreal and Calgary go up. We can speculate about the cause of these differences - proximity to the ocean, proximity to the mountains, elevation, etc. - but there doesn't seem to be much of a common pattern for these cities.
Summary
We started today with 4 questions, so now lets see what's we've learned:
How does Edmonton's frost-free season compare to other areas?
- Blatchford's frost-free season is longer than any of the outlying stations. A station like Stony Plain is 9 days shorter than Blatchford, while the International Airport is a month shorter.
- Blatchford's frost-free season is 2 weeks longer than the Calgary or Winnipeg airports, and its a month shorter than Montreal. But obviously that will vary depending on where you are in the Edmonton region, or in the other regions.
Was the dramatic shift during the 1940s-1980s just because of Edmonton's growth as a city?
I feel like we can say that the answer to this one is "no."
- We saw that the rural stations in Calmar, Campsie and Sion also had large increases in their frost-free seasons during this time period.
- We saw that other big cities - which also would have seen development during this timeframe - didn't follow Edmonton's pattern.
What was going on from 1975-1981?
From 1975-1981 Blatchford had a string of 7 straight years with unusually long frost-free seasons. Those long seasons - and particularly the late fall frosts - also occurred at Calmar, Sion, the International Airport, and Stony Plain. This is another mystery, but it also seems to have affected both the urban and rural regions around Edmonton.
Why did things level off after 1980?
Part of the reason that the average length of our frost-free season leveled off after 1980 is just math, and it's related to those abnormally long seasons in 1975-1981.
With this chart we are pretending that those 7 years didn't exist, and the green line shows what our 30-year average would have hypothetically looked like without them. We're not saying that 1975-1981 didn't exist, or that they were errors, because several of the neighbouring stations had long seasons too. But these seasons were so unusually long that they boosted Blatchford's average upwards by almost 10 days. And because we're using a 30-year average those seasons hang around in the calculation until 2010.
When we look at the orange line for the 30-Year average it dips during 2005-2010, but that is really just a reflection of when those seasons are no longer included in the calculation. If we pretend that 1975-1981 didn't exist and use the green line instead, then from about 1995 onwards we see a steady increase. In particular, from 1990-2017 the slope of the Hypothetical Edmonton line is a really close match to Calgary's
So this is all just fun-with-numbers, but the point is that 1975-1981 were very unusual - they had never happened before, and they haven't happened since. From the original orange line we might assume that Edmonton's frost-free season has stopped changing over the last 40 years, but 1975-1981 heavily skew the average. Without their influence it looks like we've probably still been seeing a slow and steady increase of about 8 days since 1995.
And that brings us to the end of Part 2. There was a tonne of information today, but I feel like we've now pretty much exhausted the topic of frost-free days. In a few weeks we'll be back with Part 3 of our series on gardening, where we'll look at the confusing world of climate hardiness zones.
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