ASK TOM WHY: When will our first 70-degree and 80-degree day occur?

Dear Tom,

I’m always ready for the return of spring’s mild weather. About how long should I expect to wait for our first 70-degree day, and 80 degrees?

— Jerome Kennedy, Chicago


Dear Jerome,

Proximity to Lake Michigan delays the arrival of 70-degree and 80-degree days by at least two weeks relative to their arrival at inland locations. The lake retains the chill of winter well into the spring, and lake-chilled air often spills westward across the city. The lake’s cooling effect is felt most strongly at the shore, progressively less so farther inland. On average, the first spring occurrence of 70 degrees is March 26 beyond five inland from the lake, or 42 days from today, but it is delayed two weeks, to April 9, near the shore, or 56 days from now. Inland, 80 degrees arrives about April 21, in 68 days, but not until May 7 near the shore, or 84 days ahead.

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