How rare and extreme are 80 degree dewpoints in Chicago?

As hot as it was yesterday and today, the really crazy part of this two-day heat wave has been the humidity associated with it. Chicago’s first 100 degree temperature since 2012 occurred today, but an even more rare temperature occurred yesterday, and that was the 80 degree dew point.

View of the Kennedy Expressway and Skyline of Chicago, Illinois, United States. Image: Shane West

As a climate historian, I am fascinated by the weather events that shape our history and culture. One of the most remarkable phenomena that I have studied is the occurrence of 80 degree dewpoints in Chicago, which is a measure of how much moisture is in the air. Dewpoint is the temperature at which the air becomes saturated with water vapor and condensation forms. The higher the dewpoint, the more humid and uncomfortable it feels.

A fair warning, I am based out of the Chicago area, and thus take a particular interest in this climate, so I do apologize for those of you outside the area, as this is not the first, nor will it be the last, Chicago centric blog.

According to the data from the National Weather Service, Chicago has experienced 80 degree dewpoints only 8 times since 1945. That means it is a very rare and extreme event, especially for a city that is located far from the tropics and near a large body of water. The highest dewpoint ever recorded in Chicago was 83 degrees on July 30, 1999, which was also the day of the highest heat index (a combination of temperature and humidity) of 125 degrees. The most recent 80 degree dewpoint occurred yesterday, August 24, 2023, which was the first time since 2016 that Chicago had such a high level of humidity.

As usual, WGN and Tom Skilling had the best information on this piece of Chicago weather history!

It should be noted that this is based on the data from the O’Hare Airport weather station, which is the official station of Chicago; warmer dewpoints have undoubtedly occurred, especially in agricultural areas where irrigation of crops can create a microclimate of humidity greater than more developed areas of the region. By the official weather station reading, it appears Chicago narrowly missed a second straight day of 80 degree dew points, as today’s peaked at 79 degree around 10am.

So what causes such high dewpoints in Chicago? The main factor is the transport of warm and moist air from the Gulf of Mexico by southerly winds. This air mass can reach Chicago when there is a high pressure system over the eastern U.S. and a low pressure system over the western U.S., creating a clockwise flow of air around them. This pattern can also produce thunderstorms that can enhance the moisture content of the air.

Another factor that can contribute to high dewpoints is the urban heat island effect, which is the phenomenon of higher temperatures in urban areas compared to rural areas due to human activities and structures. The urban heat island effect can increase the evaporation of water from lakes, rivers, and vegetation, as well as from human sources such as irrigation, fountains, and cooling towers. This can add more water vapor to the air and raise the dewpoint.

The urban heat island effect also affects the daily temperature range in Chicago, which is the difference between the maximum and minimum temperatures. According to a study by NOAA, Chicago has one of the largest urban heat island effects in the U.S., with an average difference of 5.9 degrees Fahrenheit between urban and rural areas. This means that urban areas tend to have warmer nights and cooler days than rural areas. As a result, Chicago’s temperatures during the summer are becoming more humid, especially low temperatures, partly as a result of the urban heat island effect.

It definitely felt like this today despite there being no rain! Image: Arina Krasnikova

The impacts of 80 degree dewpoints on human health and well-being are significant. High dewpoints can reduce the ability of the body to cool itself by sweating, leading to heat stress, heat exhaustion, and heat stroke. High dewpoints can also worsen respiratory problems, such as asthma and allergies, by increasing the concentration of pollutants and allergens in the air. High dewpoints can also affect mental health, by causing irritability, fatigue, and depression. The combination of heat and humidity is what lead to the major heat wave deaths of 1995 in the city. For an excellent synopsis of the failures of the city to find individuals involved in that event, I highly recommend the book Heat Wave by Eric Klinenberg.

As climate change continues to warm the planet, it is possible that 80 degree dewpoints will become more frequent and intense in Chicago and other parts of the world. This will pose serious challenges for public health, infrastructure, and energy systems. Therefore, it is important to monitor and understand the causes and consequences of high dewpoints, and to take adaptive and mitigative measures to cope with them.

You can also check out one of our more recent blogs: How Climate Change Is Making Mosquitos More Annoying And Dangerous. Thanks as always for reading!