Illinois State Water Survey - University of Illinois at Urbana-Champaign

State Climatologist Office for Illinois

Summer Heat in Illinois

Dr. Jim Angel, State Climatologist

According to statistics from the Centers for Disease Control and Prevention, an average of 384 people die from heat every year. This is more than the deaths caused by floods (135), lightning (85), tornadoes (73), and hurricanes (25) combined. Heat can be especially deadly in large urban areas that can become heat islands. Brick buildings, asphalt streets, and tar roofs store heat and radiate is like a slow burning furnace. Temperatures can be several degrees hotter in the city than in nearby suburbs and rural areas. Of the over 700 heat-related deaths in Chicago in the July 1995 heat wave, a large majority were elderly. Semenza et al. (1996) identified people at high risk including those with known medical problems and those without access to air conditioning. Also at high risk were those who did not leave home each day; who lived alone; and who lived on the top floor of a building.

More on the July 1995 Chicago heat wave

It's not the heat, it’s the humidity

The body cools itself by sweating because the evaporation of moisture has a cooling effect. High humidity reduces this evaporation and hinders the body's effort to cool itself. The dew point temperature is a much more useful measure of the moisture content of the atmosphere than the commonly used relative humidity. During summer in Illinois, dew point temperatures in the 50s are generally comfortable. Most people begin to feel the humidity when dew point temperatures are in the 60s. Dew point temperatures in the 70s are rare and cause significant discomfort. During the peak of the July 1995 heat wave, dew point temperatures were in the upper 70s and lower 80s, unprecedented for the Chicago area.

The Heat Index is an apparent temperature or a measure of how it feels when temperature and humidity are combined. It is the result of biometeorological studies and takes into account body size, core and body surface temperatures, clothing, the skin's resistance to heat and moisture transfer away from the body. The Heat Index assumes an average-sized adult with clothing in the shade with a 5-mph wind. Being in the full sun, or in an area with little air movement can increase the apparent temperature.

Heat disorders

  • Heat cramps: muscular pains and spasms due to heavy exertion. They usually involve the abdominal muscles or legs. It is generally thought that the loss of water from heavy sweating causes the cramps.
  • Heat exhaustion: occurs when people exercise heavily or work in a warm, humid place where body fluids are lost through heavy sweating. Blood flow to the skin increases, causing blood flow to decrease to vital organs. This results in mild shock.
  • Heatstroke: LIFE THREATENING. The victim's temperature control system stops working as the body quits producing sweat. The body temperature can rise so high that brain damage and death may result if the body is not cooled quickly.
  • Sunstroke: Another term for heatstroke.

General relationship between heat disorders and the Heat Index:

  • Values of 80 to 90: fatigue possible with prolonged exposure and/or physical activity.
  • Values of 90 to 105: sunstroke, heat cramps, and heat exhaustion possible with prolonged exposure and/or physical activity.
  • Values of 105 to 130: sunstroke, heat cramps, and heat exhaustion likely and heatstroke possible with prolonged exposure and/or physical activity.
  • Values of 130 or higher: heatstroke/sunstroke highly likely with continued exposure.

The National Weather Service worked with the City of Chicago, looking at past mortality statistics and weather statistics to determine the meteorological thresholds that result in significant numbers of heat deaths. Prolonged heat waves with warm nighttime temperatures appear to be significant factors. Because of the heat island effect and socio-economic problems, the Chicago NWS Forecast Office has established different guidelines for heat warning for the Chicago Metropolitan Area than for other areas of Illinois. An excessive heat watch/warning is issued for any of the following three conditions:

  • Three consecutive days with maximum heat index of 100 to 105
  • Two consecutive days of maximum heat index of 105 to 110
  • One day of heat index greater than 110

For the rest of Illinois, a excessive heat watch/warning is issued for maximum heat index of 110 or greater and minimum temperatures of 75 degrees for at least 48 hours. A heat advisory is issued for a heat index of 105 or greater. Warnings and advisories are issued when the heat is imminent or likely to occur in the first 12 to 24 hours of the forecast. If the heat warning criteria are expected to be reached in 12 to 48 hours, a heat watch will be issued. A heat outlook may be issued for a heat wave that is several days away.

Heat Safety Rules:

  • Check the latest forecast when planning outdoor activities
  • Have a place to cool off. Find out if your community has cooling shelters
  • Plan to spend some time at the library, mall, or with a friend or family member where air conditioning is available
  • Check on the elderly, infirmed, and those living alone.

During the heat wave:

  • Drink plenty of water and natural juices, even if you don't feel thirsty.
  • Slow down. Avoid strenuous activities.
  • Wear loose-fitting, lightweight, and light-colored clothing.
  • Avoid going out in the blazing sun.
  • If you must go out in the sun, use sunscreen and wear a hat.
  • Keep shades drawn and blinds closed, but windows slightly open.
  • Keep lights turned down or turned off.
  • Take cool baths or showers and use cool, wet towels.
  • Eat small meals more often. Avoid foods that are high in protein because they increase metabolic heat.
  • Avoid using the oven.
  • Avoid alcoholic beverages and beverages with caffeine, such as coffee, tea, and cola.
  • Avoid using salt tablets, unless directed to do so by a physician.
  • Especially protect small children from the sun, their skin is sensitive.
  • Do not leave children or pets in a closed vehicle, even for a few minutes. Temperatures inside a closed vehicle can reach 140-190 degrees within 30 minutes on a hot, sunny day.
  • The best way to beat the heat is to spend time in air conditioning. Only two hours a day in an air-conditioned space can significantly reduce the risk of heat-related illness.

Prolonged exposure to intense heat can lead to heat cramps, heat exhaustion, or heat stroke. Heat cramps and exhaustion can be treated by resting in a cool place and drinking sips of cool water. Heat stroke is a severe medical emergency, which can be fatal if emergency medical treatment is not received in a timely manner. Get the victim to a cool place. Reduce body temperature with a cool bath or sponging. Do not give fluids to a heat-stroke victim.