A heat wave is a period of abnormally and uncomfortably hot and usually humid weather. In 1900, climatologist A. T. Burrows defined it as a spell of three or more days on each of which the high temperature reaches or exceeds 90 degrees. By that commonly accepted standard, Chicago experiences an average of three heat waves per year, and this year's first heat wave is now in sight.
But first, Chicago's temperatures will have to recover from the sharp, 17-degree cool-down that took Monday's high of 88 degrees down to 71 on Tuesday.
A major weather pattern change is now under way because the primary west-to-east jet stream is shifting into Canada. A jet stream typically forms between hot and cold air masses and its northward shift signals an expansion of heat across the United States. For Chicago, 90s arrive Saturday and persist into next week.