How many thunderstorms do we get on average per month?

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Dear Tom,
How many thunderstorms do we get on average per month?
— Jakob Kilmer
Dear Jakob,
Chicago’s annual thunderstorm day tally has been remarkably stable over the years, consistently averaging about 38 a year when thunder is recorded at the city’s official observation site. Not surprisingly, the bulk of the city’s thunderstorm activity occurs during the April-October warm season. June leads all months with seven thunderstorm days, closely followed by July and August, each with six and May with five. April and September typically log four, March and October two, November one and the winter months of December, January and February average less than one.
The most thunderstorm days in any month was 15 logged in May 1944 and the most in any year was 66 in 1975. In contrast, the year with the fewest thunderstorms days was 1875, with just five

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