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.