Temperature Observations: It's Complicated

Yesterday's maximum at the Salt Lake City International Airport was 106°F, a record for the day and just a degree short of the all-time high (107°F).  The airport measurement site has been much maligned by some as being "jacked" (i.e., too high) or unrepresentative by some.  This is a subject that we've discussed previously in this blog (see What's Up @ KSLC and Records Falling).  What I have noticed in social media is that there is a tendency to assume that the airport site is wrong and the other sites are right.  There is also a tendency to assume that the differences in maximum temperature at the various sites are due to either calibration issues or local siting characteristics.  In reality, there are many many factors that affect maximum (and minimum) temperature readings.  

There are tens of thousands of weather sensors in the United States providing data to the internet.  The MesoWest project at the University of Utah (https://mesowest.utah.edu/), which began as a collaborative endeavor with the National Weather Service in the 1990s, provides access to these observations (there are other sites that do this as well).  On the MesoWest, you can plot the maximum temperatures over the past 24 hours, which I did this morning for the Salt Lake City area.  As you can see in the numbers below, there is incredible variability.  The official airport observing site (KSLC) has the highest maximum (106°F).  It is located just south of the airport on the left-hand side of the map.  Sites near it, however, range from 103 to 105, and just bit to the northeast you can find a 101 and 102 near or at the Rose Park Golf Course.  At the University of Utah, the range is even larger, with stations reporting maximum temperatures as low as 97 and as high as 105 (the 105 is covered).  

Some of these variations are real.  Urban areas involve rich tapestries of land use that affect local temperatures.  The Rose Park Golf Course, for example, is heavily irrigated.  Other areas are concrete and built up.  During one late afternoon last summer, analyses of late afternoon temperature produced by NOAA over Salt Lake City using field measurements and satellite data show the relative coolness of the Rose Park Golf Course compared to areas near the airport or to the east of I-15.  

Source: NOAA/CAPA

But there are other reasons why maximum temperatures vary from site to site.  The reality is that many different types of instruments and processing algorithms are used to measure temperature.  Instruments have varying response times (and accuracies) and differing averaging and reporting intervals are used. This is especially important for a maximum (or minimum) temperature, which can be the result of a relatively brief spike.  

As an example, below is a time series of yesterday's 1-min temperature observations from the University of Utah observing site at the mouth of Red Butte Canyon.  The maximum temperature at this site based on 1-minute averaged temperature observations provided at 1-minute intervals was 105°F.  There were two spikes between 1400 and 1600 MDT when 105°F was reached.

Let's suppose that this station instead reported data hourly.  There are some stations that report hourly averages.  The average temperature for the 1-hour period ending at 1500 MDT was 103.1°F and for the 1-hour period ending at 1600 MDT was 103.3°F.  In this case, MesoWest would have reported a high of 103°F instead of 105°F.

Alternatively, let's suppose that this station reports a 1-min average temperature every hour.  In this case, the 1-min observation at 1500 MDT was 103.3°F and at 1600 MDT was 102.1°F.  This too would have yielded a high of 103°F instead of 105°F.  

Many stations do not report a maximum (or minimum) temperature.  They simply provide temperature observations in discrete intervals.  In this case, the "maximum" temperature is actually the highest reported temperature.  The National Weather Service observing site at the airport, however, does report a maximum temperature every six hours.  This maximum could occur between observation times.   Older reports describing the characteristics of the measurement system (known as ASOS) suggest that these maxima and minima are based on 5-minute averages updated every minute.  If this is still the case, the maximum temperature reported at the airport is based on 5-minute average temperatures calculated every minute.  I am not sure if this is still the case.  

Ultimately, it is important to recognize when comparing maximum or minimum temperatures at observing sites that there is more happening than just instrument siting and calibration.