Friday, July 31, 2020

July Enters Cold, Leaves Hot

On July 1st, the low and high temperatures at the Salt Lake City International Airport were 53˚F and 88˚F, quite comfortable by northern Utah standards.

In contrast, overnight minimum on this last day of July was only 73-75˚F (the official minimum won't be reported until noon) and the forecast high for this afternoon is 100˚F.  


I've mentioned in several previous posts that this July hasn't seemed too bad (i.e., hot) compared to recent standards, but that it has been hot by 20th century standards, and that appears where it will end up at the close of business today.

With one day left in the month, the average temperature has been 80.8˚F, which ranks as the 16th warmest all time. 

Source: NOAA Regional Climate Centers

There are only three July's ahead of it that occurred during the 20th century, 1988 (80.9˚F), 1989 (81.1˚F), and 1960 (81.2˚F).  There's a chance we'll move up ahead of 1988 today.  I'm not sure we can catch 1989 or 1960.  In any event, this July would have ranked at least in the top 4 during the 20th century.

However, many of the 21st century July's were hotter including (in order of increasing average temperature): 2014, 2008, 2002, 2019, 2012, 2006, 2016, 2018, 2003, 2007, 2013, and 2017. 

So basically, the "baseline" for July in Salt Lake City has shifted.  This July didn't seem too bad by 21st century standards, but it was quite hot by 20th century standards.

Scientists sometimes refer to this as climate nonstationarity.  This is a phrase you will hear more of in the coming years as past climate normals become irrelevant for future climate normals.

The National Weather Service uses 30-year averages for climate normals.  These are used, for example, for the "normal" high and low temperatures reported on the news.  They are updated every 10 years, so right now, the climate normals reported on the news are based on the 1981-2010 average, but as can be seen in the chart above, that was a period of increasing temperatures during July with the decade of that period relatively cold and the last decade relatively hot.  

Those averages are no longer relevant normals for July in 2020.  Year to year variations are not bouncing around that average, but a higher one.  

The National Weather Service will issue new climate normals after 2020, based on the 1991-2020 averages.  These will also become obsolete, probably even quicker than the 1981-2010 normals.  


Keep cool and carry on.  

1 comment:

  1. The 700 mb temperature measured by this morning's weather balloon was 18.6C, which appears to be a record for the 8/1 12z sounding (previous record was 18.4).