Dear Tom,

Is there a set, definitive temperature range with which it is too cold to have a heat index and too warm to have a wind chill? Assuming the answer is yes, what is it?
Carl Chouinard, Evanston

Dear Carl,

There certainly is. The National Weather Service disseminates a wind chill index when temperatures are at or below 50 and a heat index when the mercury reaches 80 degrees. The time of the year is not important. If temperatures dropped into the 40s during the summer, a wind chill would be reported. Conversely, winter temperatures of 80 or higher would generate a heat index. Chicago has logged numerous temperatures below 50 degrees during meteorological summer, so the city has reported summer wind chills, but Chicago’s highest reading during meteorological winter is 75 degrees, so the city has never reported a winter heat index.