Wednesday, August 28, 2013

The Most Boring Summer Ever?

Yesterday evening's rain notwithstanding, this has been a boring summer of weather in Salt Lake City and, at least for minimum temperature, possibly the most boring summer in at least the past 50 years.

We already know that this summer has been hot and that we have had very few days with a below average minimum temperature (only one day in July and August has had a below average minimum temperature, see previous post).  But did you also know that the day-to-day fluctuations in minimum temperature have also been much smaller than usual?

To illustrate this, Trevor Alcott sent me the analysis below showing the average (mean) and standard deviation of minimum temperature at the Salt Lake City International Airport during the heart of summer (1 July to 15 August) since 1958.  This year not only has the highest average temperature, but by far the lowest standard deviation.

Source: NCDC/Trevor Alcott
Standard deviation is a statistical measure of variability.  In a year with a high minimum temperature standard deviation, there are larger day-to-day fluctuations.  In a year with a small minimum temperature standard deviation, there are smaller day-to-day fluctuations.  This year's standard deviation of just over 2ºF is incredibly small compared to anything else over the past 50 years or so.  

However, if you look at the graph below, you will notice that before 2011, the lowest standard deviation in any year was about 4ºF.  Since 2011, however, we've seen standard deviations at or well below that level.  

It is my understanding that the observing system at the airport (known as ASOS) was moved about three years ago and I wonder if this could be playing a role in the decline.  This year has been boring, with limited weather variability, but the remarkably low variability in minimum temperature at the Salt Lake Airport might partly reflect characteristics at the new instrument location.  More digging is needed to test this hypothesis.  


  1. Another possibility is changes in watering near the station, if there is any of this. If a station is near a grassy area that is being watered at night, that can cause large local temperature variations (sometimes 10 or more degrees F) when the air is dry and winds are light. I remember at least one airport measurement site in Idaho where the temperature would tend to dip precipitously at about the same time every evening along with a spike in dew point (probably when sprinklers came on nearby) then gradually return to "normal" later. In a dry climate it is probably ideal to have a large area that is xeriscaped around an official measurement site instead of grass, since watering patterns vary so much and can produce very artificial temperature readings.

  2. I have noticed on my home weather station that this summer had warmer nighttime temperatures than is usual for my location. I am in Holladay and temperatures are usually cooler than the airport both day and night during the summer and this year that has continued to be the case. I have not run any statistics on this, but morning lows were noticeably warmer than past summers. I would agree that if the measurement site was moved at the airport it might certainly account for a change in its measurement values. However, it is my observation that a similar change seems to have occurred at my location in the valley as well.