A powerful heat wave impacting the Midwest has begun moving eastward as of Wednesday, bringing the threat of record-high temperatures to millions more individuals. Relief is not expected until the weekend, leaving more than 160 million people under extreme heat warnings or advisories as the Fourth of July weekend approaches.

Emergency officials and meteorologists urge the public to heed these warnings. For many, it marks the second or third day in succession of severe heat alerts, advising against outdoor activities during peak daylight hours.

The combination of triple-digit temperatures and high humidity levels is driving up the heat index, making conditions feel even more oppressive. Experts highlight the heat index as a more reliable measure of perceived heat than temperature readings alone. On Tuesday, heat index values soared, reaching 106 degrees in Chicago, 113 north of Milwaukee, 103 in Cleveland, and 113 in Southern Illinois. Predictions for Wednesday suggest similar index values between 100 and 115 degrees from Kansas City to Boston.

Radley Horton, a climate expert at Columbia University, notes that this heat wave is particularly notable for its high humidity levels. “When moisture levels in the air are high, nights tend to stay warmer,” Dr. Horton explains. “The temperature differential between day and night is less pronounced.”