I lived in Bozeman from late 1979 – late 1984, moved back here again in summer 2020 and visited many times in between. I’ve sensed over time that this place is changing. I’m not referring to the obvious growth of the town or its newfound prosperity, I’m talking about its climate. I remember Bozeman of the early 80s having a cool wet summers. We needed a jacket every summer evening once the sun went down and there was no smoke in town … ever. I remember snow in town in September, snowy Februarys at Bridger Bowl and even snowier Marchs. Bitter cold poured into town more regularly and the cold was deeper.
How many of these remembrances are real and how many have been distorted by the passage of time. I wanted to find out. So I asked the Montana Climate Office for daily weather data from the Montana State University reporting station to see what changes were visible in the data. They graciously provided me with data spanning 1960 - 2020. Here’s what I found.
I capture trends in temperature and precipitation in the four figures below. Figure 1 shows number of days with a high temperature above 90 each year. The raw data shows lots of year-to-year variation, but the trendline picks up an unmistakable trend; the number of days each year with temperatures in the 90s is increasing. The trendline shows that the number of days with temperatures in the 90s has, on average, crept up from less than 5 in 1960 to more than 9 now. The last year Bozeman had a summer with no 90-degree temperatures was 1997. I don’t have data for the summers of 2021 and 2022, but the Weather Channel website for Bozeman shows 31 days with temperatures at or above 90 degrees for 2022 and the summer of 2021 was hotter, so the trend is not flattening. Though it’s not shown on the figure, maximum annual temperature has also been creeping up. In 1960 and 61 the highest temperature recorded in the summers were 98 degrees. The maximum temperature did not breach 98 again until the year 2000, and it has happened 5 times since then with the maximum being above 100 in 3 of those years.
Figure 1. Number of days each year with high temperature ≥ 90 degrees
On the flip side, the trendline on Figure 2 shows the number of days where temperatures dropped below 0 has decreased from 16 to 13 on average. The minimum annual temperature has also been creeping up at the rate of about 1 degree every 14 years, making our current minimum temperature more than 4 degrees warmer than it was in the 1960s. There is a lot of variation in the raw data that can obscure these trends somewhat: 2019, for example, had more cold days than any other year in this record, but 2020 compensated for this by having among the fewest.
Figure 2. Number of days each year with low temperature ≤ 0
So what’s the coldest you remember it being in Bozeman: what day was it and how cold did it get? In my memory, the coldest time I experienced in Bozeman was Christmas week 1983. I remember the temps dipping to -42 Fahrenheit on two consecutive days. Give yourselves a minute to come up with your own answers before I tell you what the reporting station data show. Ready …
I was right that Christmas week 1983 was the coldest period from 1960 – 2020, but I was way off on the temperatures. Figure 3 shows the minimum temperatures each night from December 16 – 30. The coldest day occurred on Christmas Eve, but only reached -32.08F. It was also preceded by three days with minimum temperatures between -29 and -30. It was cold, I remember, living in married student housing, and crawling under our lovingly named “Shitty Shack” in the mornings with a blow dryer to warm the pipes and get water flowing.
Figure 3. The coldest week on record in Bozeman since 1960
The precipitation data in Figure 4 shows that inches of rain and snow have held quite steady in town over the past 60 years. This really surprised me as I remember Bozeman having wetter summers and longer winters than it has now. So, I dug into the data further and found that the distribution of both rain and snow have changed throughout the year. As for rain, the biggest change is in August which is down about a half-inch of rain on average. Through 2000, measurable snow fell in September in about half of all years, since then we’ve only had measurable snow in September twice. We still get snow in town pretty regularly in October, but there are more years with only 1 or 2 days of snow and little accumulation. We get more snow in town in February, which is great as it likely means snowy Februarys up at Bridger most years, but unfortunately March snow is way down, which is consistent with what I’ve observed up at Bridger. Many years, come March 1st, the spigot seems to turn off.
Figure 4. Annual rain and snowfall in town
There you have it, this ole guy’s memory is not that faulty— not yet at least. Summers are hotter, August is drier, both of which feed into the smoke we get, assuming that the changes in the Bozeman data are true for the northwest more generally. Winters are getting warmer and shorter, but snowfall is holding up, just try to get your turns in before the end of February.
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