Yes, Its Not Cooling Off as Much at Night

A lot of people have commented lately that it hasn't been cooling off at night.  Indeed, minimum temperatures have been rising in Salt Lake City and in many other regions.  

Let's have a look at Salt Lake City.  National Weather Service observations of the average June minimum temperature since 1874 (2021 includes only the first 20 days) shows significant variability from year to year, which is a reflection of weather variability.  

Source: http://xmacis.rcc-acis.org/

There is an abrupt drop that occurs, however, in 1928, after which the average temperature seems to be much lower than it was previously.  This drop is almost certainly due to the moving of the observing site from downtown Salt Lake City to the Salt Lake City Airport, which is not only lower and prone to colder overnight temperatures, but back in 1928 would have been in a highly undeveloped area.  

After 1928, average minimum temperatures seemed to fluctuate around with little long-term trend for a couple of decades before they began to rise over the last 50 or 60 years.  

And minimum temperatures are increasing faster than maximum temperatures.  The average minimum June temperature from 1951-1980 was 53.2˚F.  From 1991-2020 it was 57.4˚F, an increase of 4.4˚F.  For the same periods, the average maximum June temperatures were 82.9˚F and 83.9˚F, an increase of 1˚F.  As a result, the average size of the daily temperature range has decreased from 29.7˚F to 26.5˚F.  This decrease in the daily temperature range has important implications for human health and energy consumption as the lack of cooling at night increases stress on individuals without access to adequate air conditioning during heat waves and energy consumption for the air conditioning of homes, buildings, etc. 

The rise in temperature and the decline in daily temperature range are consistent with expectations from urbanization and global warming.  However, I do not know know the size of the contribution of each of these in Salt Lake City and they may not add up in a way that is linear.  In other words, there could be some feedbacks between the two of them that amplify the warming (or reduce it) compared to what you would expect if you could just add the net warming from each effect together.  

Regardless, these effects are making Salt Lake less comfortable during summer, especially during heat waves.  

Addendum

I should have added that although this June has featured a couple of very warm nights, the average diurnal temperature range is 28.9˚F, which is actually larger than the 1991-2020 average (but a bit smaller than the 1951-1980 good old days).  I suspect this is in part due to the dry conditions (low humidity and low soil moisture) and less cloud cover than we would see in a typical June.