What is the average number of days in August at or above 90 degrees?

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Dear Tom,
What is the average number of days in August at or above 90 degrees?
D. Corrigallas,

Berwyn
Dear D.,
In Chicago, in the full period of record from 1871 through 2019, the average number of 90+ days in August is 3.9 days. At Midway Airport, from 1929 through 2019, it’s 5.8 days. At O’Hare International Airport, from 1959 through 2019, it’s 4 days. The official number of August days at or above 90 degrees in the period 1981-2010 is also 4 days.
The official number is calculated every ten years to include the most recent 30-year period; it will be recalculated again in 2021 to include the years 1991-2020. The number of hot days has gradually been increasing due to the growth of the “heat island” effect of metropolitan Chicago and the influence of global warming.

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