Was May Really Below "Normal"?

After posting on May 13th about the warmth over the previous 12 months globally and at the Salt Lake City International Airport (see It's Been Warm), there were a few comments concerning there being some below average days recently and May being colder than normal.

Was it?

Certainly not globally, but I'm going to assume those comments were specific to the Salt Lake City International Airport.  Let's have a look at the numbers.

For May 2024, the average temperature at the airport was 58.8°F.  This makes it the coolest May since 2019 (58.0°F) and the 5th coolest since 2000.  

However, whether or not 58.8°F was below normal depends on what you mean by normal.

The National Weather Service climate normals are based on 30-year averages that are updated every 10 years (and there are other national forecast services that do this).  So, when you see "normal" on the news today, it is based on an average from 1991–2020.  For May, that's 60.5°F, making this May 1.7°F colder than normal.  

But lets look at how the May 30-year climate "normals" have changed over time.  For brevity, I'll start with 1941-1970.  

1941-1970: 58.4°F

1951-1980: 58.6°F

1961–1990: 58.8°F

1971-2000: 59.2°F

1981-2010: 59.9°F

1991-2020: 60.5°F

So, May 2024 was above normal relative to the 1941-1970 and 1951-1980 climate normals, at normal for 1961-1990, and below normal relative to the 1971-2000, 1981-2010, and 1991-2020 climate normals.  It was also above normal relative to the 20th century average temperature (58.6°F).  

I've never been a fan of the word "normal" as it is often used in meteorology.  In a stationary climate, "normal" is not a single number since it is normal for the weather to vary.  In a changing climate, or what climate scientists might call a non-stationary climate, what is normal is difficult to define and very dependent on the period used.  

A better synopsis of this May is that it was near average for temperature relative to the temperatures observed in Salt Lake City during the 20th century.  It was slightly below average based on temperatures observed in recent decades.  

This will become an even bigger issue in the future, especially for phenomena that are less well sampled, like extreme precipitation events.