Monday, July 17, 2017

Why Is It So Warm at Night in Salt Lake City?

Some of you have asked me why the overnight minimum temperatures have been so high in Salt Lake City this summer.  In particular, it seems like the minimum temperatures are more above normal than the maximum temperatures.

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:, Utah State Historical Society
The time series since 1928 is more difficult to explain and really requires more careful investigation.  The period prior to the 1970s saw little change in the DTR, but it has dropped significantly since then.  Potential contributors to these trends include the following.

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.

This has undoubtably had a significant impact on local and regional temperatures, although we lack precise knowledge of this impact.  Typically urban heat islands have a stronger influence on minimum temperatures than maximum temperatures (although both increase), leading to a reduction in DTR.

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.


  1. Great minds think alike!

    So many great minds read this blog. And, of course, the author is a great mind, great skier, all around great guy.

    I was just looking at consecutive days where the overnight low was 70 or above. We just beat the record this year--every day since July 3rd, 14 days so far, the overnight low has been above 70.

    13 days was the previous record, achieved in two years, both later in July, 2007 and 2013.

    The day we hit 107 back on July 13, 2002, the overnight low was 69 (though it was 71, 74 and 73 the following 3 days).

    Hopefully now that I've highlighted this record of consecutive days it will stop.

    Got a question on your presentation of data at KSLC.

    Is there a website that calculates the means for selected time periods? You are using July 1 to July 15 here, but you've picked other periods in other blogs. Or do you do that by hand from the detailed daily data as I did for overnight low above 70. I looked at this site

    Doesn't seem like it does calculations for specific periods.

    Thanks for blogging us through this period of "h" "e" double toothpicks.

    The day time I can sort of deal with, it's the hot nights that are driving me crazy

    1. Go to Try "Single Station -> Seasonal Time Series". Select "other" for period of interest. Input the dates you want. Pick what variable and summary you want (very important to get these right). Select a station.

      I don't think this calculates the long-term mean, but it can be done easily with the data provided.


  2. Perfect, cool, thanks.

    What's the trick to plotting two series at once. I can plot either the max or the min, but can't figure out how to plot both as you have.

    Great data!

  3. Actually, as I look at your charts more closely, looks like you are plotting the data in Excel.

    And you have calculated the max less the min (diurnal difference) and then plotted that. I looked for diurnal difference, but couldn't find it ...

    1. Yes, you have to put it in another software package to do specialty plots.

  4. Another possible cause of warmer nights might be increased humidity as a result of more extensive urban vegetation. Trees and lawns give off more water than the native grasses and shrubs, and water vapor is a significant greenhouse gas. Would that make sense?

    1. The irrigation has multiple effects. That is one. It also changes the thermal characteristics of the soil in a way to reduce the diurnal cycle. It also has a cooling effect through evaporation. Many aspects to explore!

  5. I agree that irrigation is a big factor, particularly any changes near the observation site. Even changes in the timing of the watering schedule near the site are a big deal. In a dry climate like SLC, overnight evaporation in watered areas is a huge cooling factor (in contrast to humid climates where overnight condensation keeps the temperature higher). So I would strongly argue that irrigation is a major cooling factor both day and night, but especially at night when there is little vertical mixing of the air. In recent years, there has been a decrease in watering for many urban landscapes as xeriscaping (including rock landscapes etc) are now common. These likely keep overnight temps much warmer, in comparison to areas of irrigated lawn or vegatation. Also consider the prevailing diurnal wind patterns at SLC, which minimize urban effects during the day, and maximize these effects at night when prevailing winds are from the S-SE.

  6. Yes, it cooled dramatically after last night's showers. Thought for sure we would get momentary relief from purgatory, but, alas, KSLC automated site reports last nights min stayed well above 70. Eyeballing looks like 74 at 5:40 was as low as we got.

    15 days overnight low above 70. Uggghhh PURGATORY

    Another theory you neglected, Jim, which you discussed in one of your recent posts was monsoon driven humidity. Casual observation is when we have these daytime highs near 100, BUT, no monsoon, the dewpoint is in the 20s, sometimes the teens, where the past few weeks it seems the dewpoint has generally been in the 30s and 40s. This is the "contrasting humid climate" David mentions where overnight condensation keeps temps higher, I think.

    It will get cold again, won't it?

  7. It would be interesting to analyze the long term climatology of dew point at SLC, in particular the dew point associated with the daily max and min temperatures. Then you would have a correlation between temperature and moisture. Another point here too... in order to get condensation at night you normally need a dew point depression (e.g. difference between temp and dew point) of less than about 10 F which is uncommon here in the summer. If it is more than this you are still generally in a net evaporation situation which cools the surface wherever there is moisture. (There is also the factor of cloud cover and higher mins during summer monsoon periods which complicates things). As far as land surface characteristics, it would be very interesting to do an experiment with a thermometer sensor on the ground for different types of surfaces in the same environment to compare their effects: irrigated lawn, dry surface with native vegetation, pavement, rocks, etc.

    1. I mean thermometer... sorry, my phrasing there was pretty poor. Actually a comparative measurement of temperature and dew point for various surface types would be interesting.