To answer this question, let's take a look at the long-term trends in mean maximum and mean minimum temperature in Salt Lake City during the first half of July since 1874. As can be seen from the chart below, both exhibit a long term warming trend. If you look carefully, however, you can see a surge in minimum temperature during the early 1900s, and abrupt drop in minimum temperature in 1928, and then another rapid increase in minimum temperature, at a rate faster than the increase in maximum temperature, beginning in the 1970s.
These changes are better illustrated if we plot the mean difference between the maximum and minimum temperature, which meteorologists call the diurnal temperature range (DTR). Prior to 1928, you can see a gradual decline in the DTR, followed by an abrupt increase in 1928. Then there is another decline beginning in the 1970s.
How can we explain these DTR characteristics? Let's begin with the abrupt increase in DTR in 1928. It is my understanding that this is when the official Salt Lake City observing site shifted from downtown Salt Lake City (near the present Vivint Smart Home Arena) to the airport. Thus, the rapid increase in DTR reflects this change, with the airport featuring a larger DTR because of its lower elevation (favoring lower minimum and higher maximum temperatures) and rural character.
The decline prior to 1928 is interesting and I suspect is an urban heat island effect related to the development of downtown Salt Lake City, which was quite extensive by 1920.
|Salt Lake City in 1920. Source: https://www.ksl.com/?sid=39827737&nid=148&title=thowback-thursday-how-salt-lake-citys-skyline-has-evolved-over-the-years, Utah State Historical Society|
1. Urban Heat Island. There has been dramatic growth of the population of Salt Lake City, Salt Lake County, and the Wasatch Front in recent decades. For example, the population of Salt Lake County has grown from under 200,000 in 1928 to over 1,000,000 today.
2. Site Characteristics at the Airport. Temperatures are also strongly dependent on the land surface characteristics and the precise location of the instruments at any given observing site. The Salt Lake City International Airport is dramatically different today than it was several decades ago. In addition, the location of the observing site at the airport may have changed. Even small changes in location can make a difference (ask any golfer or hiker who walks around in an open area on a clear morning).
3. Instrumentation Bias. Over the years, the instruments used by the NWS have changed and this too can affect long-term trends. The DTR decrease since the 1970s is, however, more than 5ºF and likely can't be fully explained by bias.
4. Global Warming. A decrease in the DTR and a more rapid increase in minimum temperatures than maximum temperatures are consistent with an enhanced greenhouse effect and expectations of global warming.
5. Regional Climate Change and Variability. Characteristics of the trends since 1928 might also be influenced by regional climate change and variability, such as variations in large-scale circulations, including the North American Monsoon.
All of these factors may play some role, although their relative importance is unclear and to my knowledge unquantified. In addition, explaining shorter-term trends, such as why trends in the DTR shifted so abruptly in 1970 is also difficult. The 1970s did mark a significant shift in trends in global temperatures, and it is also possible that growth along the Wasatch Front and/or near the airport reached a "critical mass" around that time. Careful investigation is needed by people who are smarter than me.