Hot summertime temperatures in Chicago are often accompanied by high humidity, making it feel much warmer than it actually is. The saying – “it’s not the heat, it’s the humidity” – is often true during Chicago’s summer. The high humidity often results in oppressive heat index values and dangerous heat waves during the summer months. The heat index is an index that combines air temperature and relative humidity to measure the human-perceived equivalent temperature (i.e. how hot it actually feels).

Studies have projected that the region will experience higher dew points in the future, leading to hot days feeling even hotter due to this increased humidity. As a result of higher temperatures and humidity in Chicago, the frequency, duration, and intensity of heat waves are likely to increase substantially in the future. One study projects an increase of between 166 to 2,217 excess deaths per year from heat wave-related mortality in the City of Chicago by 2081- 2100, depending on the climate model.

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