Is Late Summer Heat Becoming More Common?

Many people have said that it seems late summer heat is becoming more common in northern OH. After all, kids are heading back to school weeks earlier nowadays. Current temperatures this year are running warmer than normal with high humidity. So what does temperature history tell us?  

The graph below shows average high temperatures compared to the 20 year average for each period from August 15 to September 15 since 1980 for Cleveland, Ohio.

While we've had some warmer than normal late summers in the early 1990s the overall average high temperature is almost 2 degrees WARMER over the last 2 decades when comparing 2000-2020 to 1980-2000.  So yes, the temperatures over the past 5 years have been well above normal in late August/early September.

The warmest day late summer day in 2019 was 94 degrees on September 11th. How widespread was the heat that day?   Much of the eastern 2/3 of the US was also WELL above normal--lots of 90s.

How about the hottest late summer day(s) in 2018?

How about 1978's late summer heat?

...and 1959?

What about extreme heat...90 degree days in Cleveland?

We've had our fair share of 90s in late summer over the past decade. 2020 had only one, 5 in 2019 and 7 in 2018.  I labeled each year with more than 4 between mid August and mid September. The 1940s had more late summer 90s compared to any decade.

Many people will draw climate change conclusions based on this data. I would refrain on doing so.  It's not that climate change's influence is not present. You shouldn't draw conclusions based on one location's data--in this case Cleveland (with a few years' temperature contour maps for spatial perspective).  You need many other locations' data across a larger area over a longer time frame along with analyses of other variables to derive any climate conclusions. That's way above my paygrade.