Is it Just Me, or Are Our Summers Getting Hotter?

I realize that it's risky to get overly excited about something during a streak.  Like calling for the firing of a coach during a losing streak.  But as we are well in the middle of a long stretch of hot and dry weather, it got me to wondering if there is any trend in our summer temperatures in the Inland Northwest.

So the first thing I looked at was the average temperature at Spokane each summer.  I defined summer and June, July, and August, knowing that June is often a springtime month here.  But since we've also had some summer-like June's, I felt it was worth including in the search.   The reason for selecting Spokane is that its data goes all the way back to 1881.  However, there are some issues that I'll talk about in a minute.

Here's the average summer temperature (high and low temperatures averaged together) for Spokane since 1881 (blue line).  The green line represents a smoothed trend of those yearly temperatures.

The first thing to notice is that average summer temperatures were on a gradual warming trend from 1881 until about 1930.  After that, there was a cooling trend until about 1960, with no trend until 1990.  Since then, summer temperatures have been steadily rising at Spokane.

So there's a couple of things going on here that make these trends difficult to interpret.  First is that the station location has moved over the years.  In 1881, the temperatures were taken in downtown Spokane.  In 1931, the observation location moved to Felts Field in northeast Spokane.  Then in 1947, the observation moved to Geiger Field which would eventually be Spokane International Airport, which is where it remains to this day.  The reason this is important is that downtown Spokane is about 400 feet lower than the elevation of the airport.  In the summer, this difference in elevation makes downtown about high temperature about 3 degrees Fahrenheit warmer than the airport (but the low temperatures at Spokane Airport are actually warmer than Felts Field in the summer).  Thus, the "cooling" trend in the graph above could be caused at least in part by this change in elevation of the observation elevation. 

The other factor to note is the current location of the weather observation.  Yes, it's at the Spokane Airport.  But look at where it's at (red marker in the image below).

 If you look closely, the location of the thermometer is surrounded by large areas of concrete.  We all know that concrete (and pavement) heat up more than bare dirt or grass.  As Spokane Airport has grown over the past 50 years, more concrete could mean warmer temperatures.

So while Spokane has a long continuous history of data, it may not be the best location to choose for our study.  Thankfully, we do have several other locations to choose from.  But a lot of the city locations suffer the same problem as Spokane: urbanization.  This doesn't have to just be places like Lewiston or Wenatchee.  Even small towns get larger over time.  More buildings, concrete, asphalt, etc.  So are there any locations with data we can trust?  What we need is a place where any temperature trends observed are due to the climate, and not the urban heating effect.

I'm aware of two such stations in our area.  The first is the Priest River Experimental Forest.  It's located about half way between the town of Priest River and Priest Lake.  As the name would suggest, it's a forested area.  It's run by the US Dept of Agriculture and they study forest ecosystems.  What's nice about it, is that it's similar today to its origin in 1898.  There's no urban heating effect.

So what does the data at Priest River look like?  The data from the early years had some gaps, so I've started the graph from 1912.

The smooth curve is a little different than Spokane, but not much.  There was warming in the summer temperatures until about 1930, then a cooling trend through the 1950s.  Warming started again in the 60s, but overall the trend was fairly flat until the late 1990s.  Since then, summer temperatures have been steadily rising.  The amount of warming at Priest lake since the 1990s is about 3 degrees, not quite as much as the 4.4F of warming at Spokane.  

It's interesting to look at the average summer high and average summer low temperatures separately.  First, the high temperatures.

Daytime high temperatures have been warming at Priest Lake since about 1990.

The graph for the low temperatures is a little different.

After a cooling trend from about 1980 to 2000, nighttime temperatures are trending upward.

Earlier I mentioned that there were two observation locations that shouldn't be affected by urbanization.  Priest River Experimental Forest was the first.  The other is the University of Moscow at Idaho.  For those who have been to Moscow, you're probably thinking that the university is right in the middle of town, which is true.  But U of  I has a Plant Science Forest Nursery to the east of Moscow, and that is where the weather observation is taken (red marker below).  Granted, the city of Moscow is growing eastward, but it hasn't yet reached the Nursery.  So there could be a little urban effect at this location.

So what do the graphs look like for Moscow?  Here's the average summer temperature graph.

Once again, rather similar to Spokane and Priest River.  Summer temperatures trended up from 1894 to about 1930.  So that warming signal is consistent at all three locations.  Then there was a  very gradual cooling trend until about 1990, when a slow warming trend began.  Since 1990, there's only been about 1.3F warming, which is much less than Spokane and even Priest River.  But still, the trend is similar at all three sites.

Here's the summer high temperature graph.

Clearly high temperatures have been getting warmer since about 1990.  The change is 3.3F degrees.

The low temperature graph is rather curious.

It actually shows a slow cooling trend that started in about 1930 and continues to the present day.  That is why the average warming trend is so much less than Spokane.

What about precipitation?  Are our summers also getting drier?  Fortunately, there's really not an urban effect on precipitation.  So we don't need to hunt for the perfect location.  But just for consistency sake, I'll show you the summer precipitation graphs for the same three locations.

All three locations show a drying trend in summer precipitation since the 1980s.  Spokane's average summer rainfall has dropped from 2.69" in the mid-80s, to only 1.43" today.  That's nearly a 50% reduction.

While this is far from a definitive study, the three locations I selected would indicate that in the past 30-40 years, summers have become warmer and drier in the Inland Northwest.  It's worth noting that all three sites also had a rather warm and dry period in the late 1920s and early 1930s, followed by a cooling trend in the 1940s and 50s.