The extremes of Chicago’s first 70 of the year

Dear Tom,

In your response to a recent question you gave the extremes of Chicago’s first 70 of the year. Comparing the dates of the first 70 since 1871, in the last 30 years and the last five years; does the date seem to be earlier?

Fritz Mills,

Dear Fritz,

We asked Chicago climatologist Frank Wachowski to crunch the numbers for us. Prior to 1942, Chicago’s weather records were taken near the lake, but since then, inland, at Midway and O’Hare airports. Since 1871, the city’s long-term average date for the season’s first 70 is March 30 and it remains March 30 , even when computing the lakefront and inland years separately; since early-season 70s are almost always accompanied by strong south winds that negate lake effect. There has been an earlier March 22 onset of spring 70s in the last 30 years, but when considering just the last five years, the date remains March 30.

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