I couldn’t stop thinking about the big snowstorms we discussed in my previous post so I went digging for snowfall data on the National Weather Service’s website, trying to see if you really could “see the climate breathing, inhaling and exhaling, like something alive.” I was speaking metaphorically of course, but to my great delight it turned out that you actually can see the climate breathing if you look at a long enough record. NOAA’s National Centers for Environmental Information keeps an extensive online archive of historical weather data going back over a hundred years for many places and it’s free to access. If you’re interested in what past weather was like in thousands of places across the country then that’s where you want to be. It’s all there, stored behind a strangely organized batch processing system.
It took me some trial and error to figure out how the system worked but soon I had my hands on a monstrous file containing all the weather records for all the weather stations in the Flagstaff area. The longest running weather station in Flagstaff is the Pulliam Airport station (the white box visible in the above photo from December, 1967) with records going back to 1893 so I began with it.
I slurped all the records into an Excel spreadsheet and started filtering and sorting the data, trying to recreate the Arizona Daily Sun’s list of top-ten record snowfall events from my previous post. It turns out that that’s difficult. The Pulliam Airport weather station had some serious problems with absenteeism, unfilled jobs, faulty data entry, and decades-long gaps, presumably because of WWII. Almost all of the 1940s were missing and most everything before 1930 was spotty and incomplete, so I wasn’t able to do it. That’s a bummer.
However, despite the missing data I was able to make a second spreadsheet of all the Flagstaff Pulliam snowfall data stretching back to the 1930s, skipping across the missing decade of the 1940s. So about 80-90 years of historical data. I figured that’d be long enough to get a view of what winters were like in the old days. From that list I was able to learn that the deepest snow measured at Pulliam Airport was an astounding 83 inches on December 20th, 1967, followed by the relatively weenie 44 inches recorded on January 23rd, 2010 and a paltry 40 inches on February 2nd, 1979. The most snow to fall in a single day was 28 inches on January 16th, 1895, 27 inches on December 13th, 1967, and 26 inches on March 1st, 1970.
Bill Cole’s comment on the previous post got me to wondering just what, exactly, makes for a snowy winter. Is it the amount of snow that falls over a short period – the proverbial “snow apocalypse” that the papers sometimes write about? (I didn’t think so as those events are actually pretty rare.) Or, more likely, is it some function of the depth of the snow accumulated on the ground and how long it persists before melting that makes us perceive one winter as “worse” than another?
I didn’t have enough patience to pick through 30,000 individual records so I sorted everything by the accumulated snow depth and by how much snow had fallen over 24 hour periods and then broke it out by decade. I then computed various things like
- How many days per decade measurable snow was on the ground.
- How many days per decade snow was actually falling.
- How deep the accumulated snow was per decade.
- What kinds of snow events occurred.
- What the average high and low temperatures were when there was measurable snow on the ground.
I then made some very enlightening charts. The most interesting and useful is probably this one, a graph of how many days per decade there was measurable snow accumulated on the ground:
This tells us that over decadal scale, the 1950s – 1970s were very snowy compared to the 1930s and post 1980s. Most of the time there was only about 6 inches on the ground. In recent decades we have begun to recover from the depths of the 1980s and 1990s when not much snow was on the ground during the winter.
Another interesting chart is this one, how many days per decade measurable snow was actually falling and what kind of snowfall it was:
Again, this tells us that the 1950s thru the 1970s were very snowy, with far more snowfall days during those decades than in the 1930s and 1990s onward. The 1980s are interesting though as it snowed quite a lot but it didn’t stick on the ground as much (compare with the previous chart of accumulated snow depth.) The 1990s and 2000s saw a crash in 6″ snowfall days, but the “big dump” days of 12″+ stayed about the same. The 2010s have seen a rapid recovery in 6″ snowfall days.
Here’s another interesting graph, this time showing the total snowfall in inches by decade:
And finally, here’s what the average high and low temperatures were on the days when there was snow accumulated on the ground:
This is in some ways the most interesting of all the graphs as it clearly shows that the wintertime highs and lows have been trending warmer over the last 90 years or so, at least on those days when there was snow on the ground. This is kind of misleading though as one of the warmest of the decades, the 1980s, had a ton of snowfall days. When you look at the individual days when snow was falling it was hundreds of of little 1″ and 2″ days. When it’s actively snowing the low temperature for the day tends to be higher, hence the higher lows during that decade. Also, the more days with snowfall during a year the more likely those days will fall into the warmer shoulder months of Spring and Fall.
The main takeaway from all of this is that the old timers aren’t kidding when they say winters used to be a lot snowier here. Over the ~90 years of records the winter climate has generally gotten warmer with the more recent decades yielding less total snowfall and the composition of the winter snows shifting away from the “big dumps” of yesteryear and long cold months of snow on the ground towards shorter duration, smaller total accumulation events. In other words, it’s gotten warmer and drier in terms of snowfall since the 1990s or so, but we’re starting to recover somewhat. Since the depths of the late 1990s we’ve been trending higher on most measures, but it remains to be seen what will come of that.
A second takeaway from this is that the climate is not a static thing, it’s always fluctuating around over time, breathing in and out. In the graphs you can clearly see it inhaling and exhaling as the regional winter snowstorms respond to some larger forcing agent. I strongly suspect that that larger driving force is the Pacific Decadal Oscillation (PDO), a medium-term climate cycle in the waters of the Pacific Ocean, but I’m not a climate guy so I can’t say for sure.
The PDO actually lines up remarkably well with my first chart, the Days per Decade with Measurable Snow on the Ground. Strongly negative PDO readings correlate with big snow decades here in Flagstaff. I’d love to take credit for this fascinating insight but I have to give the hat tip to Bill Baker’s book Fire Ecology in Rocky Mountain Landscapes for first exposing me to the concept. Bill’s got a whole chapter in that book devoted to remote teleconnections between ocean temperatures and wildfires in various places around the West. I wasn’t expecting to see such a strong connection between the PDO and good old Flagstaff’s winter snowfall though.
Here’s a clipped PDO Index chart from Wikipedia and my graph of Flagstaff snow days so you can see the relationship for yourself (be mindful of the horizontal scales as they aren’t quite the same):
A third takeaway from this (and the one most pertinent to this blog’s focus of late) is that perhaps all that snow back in the 1950s – 1980s had something to do with the generation-long lack of large wildfires in this part of the southern Colorado Plateau. During those decades of increased snowpack the spring and fall dry seasons would’ve been compressed, leading to shorter, weaker wildfire seasons with less acreage burned. Trees would’ve grown very quickly, suffered less mortality from fire and insects, and the forests as a whole would’ve become denser as they took advantage of the increased moisture. This wet period was of long enough duration that a whole generation of foresters would’ve come into the field and worked their entire careers never having known anything different but rapidly growing and regenerating forests. My dad was one of them, but barely, as he came to the timber side of the house late in his Forest Service career just as the wet period was ending. The people who trained him were solidly ensconced in the wet period’s version of forestry, though.
When the regional climate coin (PDO) flipped to the positive phase in the 1980s the kind of nasty wildfires that’d been previously rare reemerged and began to eat into the thickly grown forests. Fire suppression policies and attitudes predicated on previous decades’ experience began to break down. Trees burned and men died in huge never before seen conflagrations driven by extreme weather and high fuel loadings. But perhaps what they were really experiencing was simply the flip side of the growth phase that had occurred during the 1950s – 1980s. Maybe the big, hot wildfires were simply Nature’s way of clearing out thirty or forty years of accumulated biomass and it should’ve been expected and planned for, even welcomed.
I actually mentioned this theory to a Forest Service hydrologist and a silvaculturalist one day during a forest thinning walk and they acted dumb, like the idea that trees might naturally tend to grow fast and dense if exposed to decades of enhanced moisture had never occurred to them. One said he’d look into it and get back with me (which he never did, but I wasn’t expecting him to.) Pretending to be ignorant is one of the tactics agency people use to avoid engaging with members of the public who hold informed differing views on these subjects. I’ve had it happen several times in discussions with them.
Okay, so we’ve gotten pretty far from the original topic of winter snowfall so I’ll close this post out. If you enjoyed this one give thanks to Bill Cole as it was his interesting comment on the top-ten list of Flagstaff snow events that sent me down the rabbit hole looking for weather data and eventually to the PDO/snowfall/wildfire relationship.