Dear Tom,
When can I be assured the danger of frost and freezing
temperatures in Chicago is past for good?
Jeffrey Rayner, Chicago
Dear Jeffrey,
Light frost can form on plant leaves on a clear, calm night even when air
temperatures a few feet above the ground are as high as 38 degrees. A heavy, killing
frost usually requires a temperature of 32 or lower.
Because overnight temperatures on any given chilly spring night can, in
extreme cases, vary by 20 degrees across the Chicago area, it is difficult to pin down
an exact date. It really depends on your location.
Here are some guidelines: Near the lake, light frost (38 degrees) has never occurred
beyond May 31; heavy frost (32) never beyond May 14. Within about 2-6 miles inland,
the corresponding dates are June 4 and May 13; 7-15 miles inland, June 22 and May
29; farther inland, add seven days.
