Chicago’s summer one of the hottest and driest – a scorching combo

Weather Blog

What a summer we’ve got going in the Chicago area weather-wise!

It’s running hotter than average and is tied with 2012 as Chicago warmest since 1973.

But at the same time it’s also among the drier summer seasons on the books to date. In fact, August 2020’s paltry 0.77″ to date at O’Hare (3.14″ is “normal” to date) is the running 5th driest of the past 50 years.

Some of you have written to say–“How can that be? How is it possible this summer ranks among the top tier of warmest summers to date here? What about the deadly heat in summer 1995. Or how about the scorcher of a summer in 1988 with its 47 days with 90s and seven days of 100-degrees?”

Here’s why this is so.

1995’s July heat wave was historic and responsible for the greatest loss of life on the books produced by a natural disaster in the Chicago area–but it occupied a week’s time. It was a single hot period and not reflective of the average temp over the full season through August 19 which came in under this years 76.3-deg average to date.

It’s true summer 1988 produced the greatest number of 90s and 100s of any warm season since official records began in 1871. But summer 1988 produced also produced historic drought. Moisture was so limited in the Summer of ’88 that the drier than normal atmosphere allowed nights which cooled more than usual from the broiling daytime highs. So when the comparatively “cool” nights were averaged with the hot daytime highs, the average summer temps through August 19 came in well below this year’s 76.3 deg to date.

On the rainfall front, Chicago rainfall in August comes in fifth driest open of any August in the past 50 years (since 1971). The 0.77″ on the books is only a quarter of the normal tally (24.5%). And our city rain tally since July 1 of 3.32″ is a fraction of normal and makes the July 1 to August 19th period the 6th driest in the 100 years since 1921.

Not every corner of our broader Chicago area is as dry. Note the month to date rainfall map and the big totals on the books north toward the Wisconsin border and north.

That area was drenched with big rains with last week’s derecho and mesoscale system, in some places 3 to 4″ and more! Evaporation is pretty extreme this time of year because of the warm summer sun so even big rains and the moist soils they generate produce can be reversed quite quickly. It’s also true that rains that intense fall so furiously, they run off before they can percolate into the soil.

Many thanks to my WGN colleague meteorologist Mark Carroll who crunched the numbers cited in this post.

For the latest weather updates, go to wgnv.com/weather.

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