Chicago's historic heat wave may bring warmest stretch of nights in 91 years — what you need to know

With scorching weather Sunday and Monday, temperatures in Chicago have reached the 90s two days in a row, and the National Weather Service said this heat will stick around for the rest of the week. It’s not unusual to have this type of heat in mid-June, but forecasters say it’s unusual to get so many consecutive very hot days — and nights.“What is truly remarkable about this heat wave is its duration,” says Illinois State Climatologist Trent Ford. “Given forecasts, it is likely Chicago will reach into the very upper 80s and possibly above 90 for seven consecutive days.”The outlook isn’t looking good for nighttime cooling, either, Ford said. Temperatures are expected to remain above 70 degrees at night this week. That’s particularly problematic for people without air conditioning who have no way to escape the evening heat.And the cumulative effect of very hot days and extremely warm nights can compound the threat to vulnerable people, especially older people.“Heat wave duration is a very important aspect of the hazard because we typically see the most health impacts during multiday heat events, where the effects build after multiple hot days and warm nights,” Ford said.The last time Chicago had a consecutive streak of seven days of temperatures in the 90s was in June 2005, Ford said.Even more unusual is the lack of relief from lower temperatures.The last time Chicago saw a seven-day streak of low temperatures above 70 degrees was in June 1933. That was just months after Franklin D. Roosevelt was first sworn in as president, and Chicago was hosting its last world’s fair, the Century of Progress Exposition.The heat index measures the combination of heat and humidity. The National Weather Service is forecasting “major” to “extreme” risk of health impacts from the heat and humidity. The heat index is expected to rise above 100 this week, Ford said.Hot, sunny conditions increase the risk of ozone pollution. If you are older or have respiratory health issues, check the government site for the latest updates. State environmental officials issued an alert Sunday warning of poor air quality this week.When weather conditions trap air in place and prevent cooling rain, this can be described as a heat dome. Ford described the air as actually sinking to the ground. The sinking air, he said, makes it difficult for air to rise and create clouds and rain that will cool things off.“The lack of strong winds in the upper atmosphere and sinking air drives the hot, dry weather in Chicago this week,” he said.