Wednesday, July 11, 2018

Salt Lake's July "Human Misery Index" Is Climbing

Human comfort is strongly influenced by a number of factors including temperature, humidity, cloud cover, and wind.  For the purposes of this post, I am going to focus on one that I consider to be especially relevant to human comfort in the Salt Lake Valley, the minimum temperature.

In particular, I am going to declare 70ºF as a critical threshold in comfort, with minimum temperatures below that value generally yielding reasonable comfort, whereas minimum temperatures above that level generally yielding uncomfortable sleeping conditions.  Going a step further, I'll define the human misery index in Salt Lake City as being equal to the number of days with a minimum temperature at or above 70ºF. 

Looking at historical data for the Salt Lake City Area stretching back to 1874, we see a rapid change in the July human misery index beginning around Y2K.  In particular, around that time a clear upward trend began and we have had five Julys since 2007 with 15 or more days with a minimum at or above 70ºF, something that never happened previously. 

Source: NOAA Regional Climate Centers
So, if you think that nighttime comfort around here ain't what it used to be, you are right.

This trends has a variety of implications for everything from human and environmental health to electricity demand. Cooling degree days in July, for example, show a gradual increase for the periods prior to 2000, but since then, we've been experiencing a new normal, with the average number of days clearly higher and several years with values near or above 600.  

Source: NOAA Regional Climate Centers
Thus, in addition to growth and increased use of air conditioners, a hotter valley climate is also contributing to increased electricity usage.  

Development and associated urban heat island effects, global warming, and recent circulation patterns are possible contributors to these recent trends, although I am unaware of a study that has attempted to quantify the contributions of each of these effects.  I suspect it is likely that that both the urban heat island and global warming are contributing significantly to the upward trends.  It is possible that instrumentation moves and local conditions around the airport might also be contributing.  

Personally, I'm starting to find all of this quite depressing.  Mother Nature might perturb the jet stream some summer to give us an unusually cool July, but it is difficult to imagine the long-term trend not being towards an even hotter future.  By the end of the century, Salt Lake could be the new St. George.  Good luck with that.   

Note on Comments:

I usually try to keep up with comments, but for some reason, blogger is no longer sending me an e-mail when you comment.  As a result, I've been missing your comments.  Please keep commenting.  I'm hoping to get this fixed and to be better about responding.  


  1. By coincidence (?), the Times today ran an equally depressing story on the increase in average low temperatures:

  2. To your list of reasons for the recent increase in number of days with minimum above 70 I would add the drought we have been discussing the past few months. There is speculation climate change is causing or at least intensifying the drought. Recalling the DeRose et al data for the Bear River, we would expect the decline in flow since 2000 to be associated with the increase in overnight lows we have been experiencing in July in Salt Lake. My guess is Julys in Salt Lake when the Anasazi disappeared during the 1200s were similarly miserable to what we are now experiencing. My hope, and we'll see how futile it is as the future plays out, is that we are not in the beginning of a 50 or 100 year drought and that we will cycle into a wetter cooler period in the next few years. We can only pray, eh

    1. There are two primary factors contributing to the ongoing persistent drought over the southwest. The first is a lack of precipitation. The second is the high temperatures, which yields greater evapotranspiration (and also has effects on snowpack and runoff). Most of the evidence that I have seen suggests that the lack of precipitation is probably a reflection of internal variability, or what you call a "cycle". It may have happened with or without climate change. However, the warmth is being amplified by climate change and this is essentially a "drought multiplier" due to more rapid evapotranspiration.

      In the past there is strong evidence of persistent drought over the southwest. However, the warming that has been and will continue to occur means that we need to be cautious using past droughts as inferred from proxy records (e.g., tree rings) as an analog for future drought. In particular, during dry periods, drought will come on faster and be more severe in the future.


  3. It may be useful to compare this trend in minimum temperatures to the corresponding (or perhaps just the daily maximum) dew point values, if a graph of dew point trends can be obtained. A lot of nocturnal cooling may be related to surface evaporation, especially in irrigated areas in a dry climate, and therefore to the amount of watering around the measurement site and in the area as a whole. I would speculate that a large-scale change from irrigated grass to rockscapes, etc is a big deal for urban area minimum temps. SLC is a good place to observe these trends because of its dry climate and the location of KSLC relative to noctural prevailing winds over the urban area.