From shortly after noon until early evening Sunday, temperatures hovered at or over 100° across the Chicago metro area. Strong westerly winds drove a warm tongue of low-level heat from the mid-Mississippi Valley across northern Illinois, but these strong winds did alleviate the potentially deadly effects of a higher heat index by mixing drier air aloft to the surface.
As a result, dew points dropped from the mid 70s into the lower 60s—in effect cutting the potential 115°-120° heat index values back into the 103°-107° degree range. For example, veteran weather observer Frank Wachowski reported at Midway Airport that the dew point at 8 a.m. was 75°—the highest of the summer—but as the strong winds mixed in drier air from aloft, the dew point slowly but steadily fell, reaching 63° by mid-afternoon. This diluting of moisture near the ground significantly reduced reported heat index values by approximately 15 degrees.
WGN-TV Chief Meteorologist Tom Skilling and the WGN Weather Center staff provide daily coverage of weather in the Chicago area.
