A Look at Max Temps @KSLC

As of August 23rd, there have been 25 days with a maximum temperature of 100˚F or higher at the Salt Lake City International Airport.  This is a new record for a calendar year, eclipsing the 21 days of 100˚F or higher observed in 1960, 1994, and 2021.  

Over the years, I have had many people contact me concerning the veracity or representativeness of observations collected at the airport.  It is something that I have written about previously, including in 2013 (see What's Up @KSLC).  

The reality is non-climatic factors related to instrumentation changes and biases, observing techniques, land-surface change, urbanization, irrigation, and in some instances station moves affect trends and extremes at many observing sites.  Some of these are documented, some not. A good deal of processing is done to identify and adjust for these effects to identify climate trends, a process known as climate data homogenization.  

Climate trends are typically derived after this homogenization, but applying such corrections to daily temperatures is more difficult.  For example, biases can vary depending on the weather (e.g., whether or not the wind is blowing, whether it has rained recently, etc.).  Doing such corrections systematically for daily temperature records is very challenging.  

Additionally, any station measurement is collected at a point and any point measurement is specific to that location.  In an urban environment like Salt Lake City, there can be a great deal of variability in temperature, so even under the best of circumstances the representativeness of observations from KSLC has limits.  For instance, golf courses may be cooler due to irrigation.  Heavily treed areas may see lower temperature than the airport.

Here we focus on July temperatures for two reasons.  First, it is the hottest month of the year, so trends during this month are especially important for human comfort, water demand, and energy use.  Second, it is the month with the least year-to-year weather variability in the Salt Lake City area, so trends may be more detectable.  

Weather records for Salt Lake City date back to 1874, but this is based on observations collected by the US Weather Bureau/National Weather Service near present day downtown Salt Lake City prior to 1928 and at the Salt Lake City airport (KSLC) thereafter.  This observing-site change is clearly evident if you look at the combined time series from the two sites (I have added a vertical bar in 1928 in the time series below).  There is a shift to higher maximum temperatures and a drop in minimum temperatures around 1928.  

Time series graphic source: http://xmacis.rcc-acis.org/

These changes indicate that in the early 20th century, the airport location observed a larger daily July temperature range than the downtown observing site, with higher maximum temperatures and lower minimum temperatures.  This is consistent with the airport's lower elevation, although by 1920 there was already significant development in the downtown area that may have also contributed to higher minimum temperatures at that site.  Some might hypothesize that airport's proximity to the Great Salt Lake might lead to a smaller daily temperature range, but the time series do not support that hypothesis.  

Another feature evident in the time series above is a trend to higher temperatures later in the record.  Precisely when that trend begins is difficult to determine visually, but for minimum temperature it appears to begin in the 1970s.  For maximum temperature the trend is not as large and may emerge from the background variability a bit later.  

Let's take a closer look at what has been happening since 1975. I pick this date in part for convenience because there is a nearby observing site that has a complete record of July observations since that date, Bountiful Bench (big hat tip to that volunteer observer).  The Bountiful Bench site is about 15 km northeast and 775 feet higher than the airport.  

Below are the July average maximum temperatures and linear trends from 1975 to 2022 at both sites.  Visually, there is remarkable correspondence in the year-to-year variability and long-term trends at the two sites.  KSLC observes higher maximum temperatures due to its lower altitude.  Based on linear trends, maximum temperatures at KSLC have increased almost 1˚F per decade, whereas at Bountiful Bench they have increased about 0.75˚F per decade. 

We can take the difference between these two time series to better identify possible shifts in station characteristics.  There seem to be three periods in this time series.  One prior to about 1996ish when the difference between the two stations fluctuates between 4 and 7˚F, then a period from about 1996ish to 2011ish when the difference is between 3 and 6˚F, and then the period after 2011 when the difference is greater than 5˚F.  

At this point, we don't know why those shifts are occurring.  They could be due to instrumentation changes, shifts in the location of the instruments, or other changes at either observing site.  More sleuthing is needed than I have time for today. 

The change after 2011, however, is quite abrupt and something that we've investigated previously (see The Most Boring Summer Ever? from 2013).  Two things happened around that time.  First, the observing site was moved to its current location.  Below is a Google Earth image of the current observing site location from 2009.  This is pre move.  

By 2010, the observing site (in the center of the triangle) appears to be either installed or in the process of being installed.  I am not sure when it became operational.  

Then, sometime between 2013 and 2015, the area was denuded of vegetation.  The image below is from 2015.

It is possible (maybe likely) that either the instrumentation move or the land-surface changes are contributing to the higher maximum temperatures at KSLC compared to Bountiful Bench. The average increase in this difference after 2011 is about 2.5˚F.  Let's suppose that we were to adjust the daily maximum temperatures by 2 or 3˚F so that 102 or 103˚F was the "new 100˚F".  Based on the counts for this year, that would knock the number of 100˚F days down to something between 3 and 8 based on this year's distribution of observed max temperatures through Aug 23. 

Days ≥ 98˚F: 36
Days ≥ 99˚F: 33
Days ≥ 100˚F: 25
Days ≥ 101˚F: 18
Days ≥ 102˚F: 8
Days ≥ 103˚F: 3

But here's another way to look at it.  First, it is clear from the time series above that July maximum temperatures are increasing at both sites.  It is getting warmer, regardless of any non-climatic effects in the KSLC time series.  

Second, let's assume we "only" reached 98˚F on the days we hit 100 this year and that observations prior to 2011 are "truth" and trustworthy (more on this in a minute).  With such a correction, we would be tied for 6th all time for days ≥ 98˚F through August 24th [years at or above days ≥ 98˚F prior to 2012 are 1940 (25), 1960 (30), 1961 (26), 1994 (32), 2003 (28), and 2007 (30)].  Prior to 1960, 1940 is the only year with more than 19 days ≥ 98˚F.  

Now, let me throw a few caveats into this analysis.  First, we should be cautious viewing Bountiful Bench as "truth."  It too can be affected by site or instrumentation changes.  For example, why does the difference between the stations decline around 1996ish?  Is that due to something at the Bountiful Bench site or KSLC?  Often such changes are not well documented.  Second, we should be cautious about treating past observations as well as "truth" because there are some warts there as well.  Was there anything happening in or around 1960 that might affect the representativeness of those observations, for example.  

At this point, I'm exhausted.  Others will need to do that sleuthing for me.