What's Up @ KSLC?

Through yesterday, the average temperature for June, July, and August at the Salt Lake City Airport (KSLC) was 81.2ºF, nearly 2ºF higher than the hottest summer on record, 2007, when the June–August average temperature was 79.3ºF.

Source: http://xmacis.rcc-acis.org

While there is little doubt that this has been a hot summer, many of my meteorological friends have wondered if there is something odd about the temperatures at the Salt Lake City airport, which seem unusually high.  For example, at Tooele, which also has a very long temperature record, 2013 is hot, but not as anomalous as it is at the airport.

Source: http://xmacis.rcc-acis.org

In addition, a comparison of the temperatures at KSLC (top graph below) with those measured by a weather station less than a mile away (bottom graph below) shows that KSLC is consistently about 2ºF higher.

Source: MesoWest

What gives?  I personally find it difficult to draw conclusions from comparisons between surface stations in areas of complex topographic and land-surface contrasts.  For example, perhaps the extreme nature of the temperatures at KSLC reflects the lack of water in Farmington Bay (the southeast "corner" of the Great Salt Lake), whereas Tooele is still getting a lake influence from the main Gunnison Bay.  Perhaps the station less than a mile from KSLC is biased or the difference in temperature simply reflects a contrast in site characteristics.

So here's a different way to address the problem (special thanks to Trevor Alcott for providing these figures).  Compare the temperatures measured at KSLC with those measured by weather balloons at 700 mb (approximately 10,000 feet above sea level).  During the summer, there's usually a strong correspondence between these measurements, so what we are looking for is a shift in that correspondence.

Observations for the past two summers (2012 and 2013, red stars and lines below) show that maximum temperatures for a given 700-mb temperature were slightly warmer than during the 1960–2011 period.  The difference, however, is less than 1ºF, and might not survive the usual significance tests, which I haven't bothered conducting.  Let's just say the change is small.

Comparison of daily maximum temperature and 0000 UTC 700-mb temperatures at the
Salt Lake City airport (KSLC) during 1960–2011 (blue) and 2012–2013 (red).  Courtesy
Trevor Alcott.

When we look at minimum temperature, however, we see a much larger difference, with minimum temperatures for a given 700-mb temperature clearly warmer in the past two years.

Comparison of daily minimum temperature and 1200 UTC 700-mb temperatures at the
Salt Lake City airport (KSLC) during 1960–2011 (blue) and 2012–2013 (red).  Courtesy
Trevor Alcott.

Now, we should be cautious about assuming this phenomenon occurred in the past two years.  Our choice of 2012–2013 is purely arbitrary, and perhaps with a bit more analysis we would find that this is a trend that has emerged over many years.  I leave that work for others.

Why this change has occurred?  Potential contributors have been discussed in prior posts (see Intricacies of Local Temparture Trends Part I and Part II) and include Global Warming, Urbanization, changes in the land surface at the airport and the adjoining area, and instrumentation changes or biases. In addition, the meteorology in the past two years may have simply favored higher minimum temperatures (e.g., more wind, high clouds, etc.). Finally, we haven't adjusted at all for instrument bias, and that could play a role.  For example, the NWS has changed instruments for measuring upper-air temperatures many times over the years.

It should also be noted that KSLC is typically one of the coldest places in the Salt Lake Valley in the morning.  Thus, there is more potential for large trends and variations in minimum temperature at KSLC than on the benches.  For example, the difference in minimum temperature between nights with clear and cloudy skies is larger at KSLC than at bench locations.  This is an important consideration when doing comparisons with other climate stations.

In conclusion, this has been a hot summer.  The analysis above supports this fact, but also illustrates the unique nature of observations at KSLC where it appears at least minimum and possibly maximum temperature have been somewhat anomalous for at least the past two summers (possibly longer) relative to long-term relationships with 700-mb temperature.  Further work is needed, however, to ascertain if this is a recent shift or a long term trend and to explain its causes.  The bait has been dangled.  Get to work!