Friday, August 1, 2014

Was July 2014 Cooler Than July 2013?

There's been some discussion in the media about July 2014 being cooler than July 2013, which as you may recall, was the hottest July on record at the Salt Lake City International Airport (see July's Blistering Numbers from August 1st, 2013).  The National Weather Service also released this graphic yesterday showing a dramatic decline in temperature from last July to this July.

Source: National Weather Service
It turns out, however, that the story is a bit more complicated and I think it is questionable that this July was significantly cooler than last July along the Wasatch Front.  

The Salt Lake City airport is a single observation location and therefore it can be strongly influenced by the local climate at the airport or any problems with the temperature sensor.  If we take the difference in average temperature between July 2014 and July 2013 for all available observing sites along the Wasatch Front, we find that the difference at the airport is very anomalous and, in fact, most stations are within 1ºF of last year and some are warmer.

Now there are some warts in this data.  Nephi, City Creek Canyon, and the Salt Lake City Airport have complete records for July 2014, but the other stations are missing 1–3 days.  For instance, Alta is missing July 30, which was very cold.  If this day was available, the average for July 2014 would be cooler and the difference would be smaller but still positive.  Nevertheless, a quick look suggests if this missing data were properly accounted for, all the non Salt Lake City Airport sites would still be within 1ºF of last year and some would be warmer.   My take is that the two July's, at least in terms of average temperature, are largely indistinguishable for the Wasatch Front as a whole.  

So, what gives at the Salt Lake City Airport?  Well, there are times of year when the airport does have a very unique microclimate, such as during the heart of winter when temperatures during inversions can be much lower than found on the east bench.  During summer, however, that's usually less of a player.  

Instead, I suspect the issue is the anomalously high temperatures reported at the Salt Lake City airport during July 2013.  This is a topic we discussed last summer (see What's Up @ KSLC) when it was clear the temperatures being reported at the airport were unusually high compared to surrounding stations.   The instrument has subsequently been replaced and temperatures at the airport appear more in line with those from the surrounding area.  The bottom line is that temperatures last July were "jacked." 

One of the advantages of having a dense network of observing stations is that you can identify and account for these sorts of sensor problems and biases.  All sensors drift and there are times when adjustments need to be made to the observations when examining trends.  People sometimes ask me why we don't use the "raw" unadjusted temperature observations when constructing long-term climate trends and this is an example of why.  Quality control and adjustments need to be made to account for these sorts of sensor problems (here's a discussion of what the National Climatic Data Center does).  

In summary, the average temperatures of the two July's were pretty similar.  In other words, they were both too hot for my blood!

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