Hot Weather Information

Heat Safety
Excessive heat is not often recognized as a “natural disaster” or a “severe weather phenomenon.” Floods are considered to be the deadliest natural disaster in the US, with a 30-year average of 120 deaths per year. However, the 20-year average for excessive heat is 190 deaths per year, and people aged 65 and older account for 45% of these deaths. In the past five summers in the Chicago area, 78 deaths were attributed to excessive heat, while only 5 deaths were the result of all other weather-related events. The 1990s turned up two major heat events: July 13-15, 1995, and July 29-31, 1999. Combining these two events, the death toll exceeded 600 in the immediate Chicago area.
It is imperative to know some basic terms related to heat waves, how to stay notified about potential events, and steps to take when bracing for the heat. Being prepared could help save your life or the life of a loved one.
Know the Difference
Watch: Conditions are favorable, good chance that event will happen. Prepare yourself for the event, and keep yourself updated in case a warning is issued
Warning: Weather event that is a threat to life or property is occurring, imminent, or highly likely. Take the necessary precautions immediately.
Advisory*: Conditions are expected to remain below the warning criteria, but are still significant enough to cause inconvenience.
*heat advisories are issued for rural areas, but not Chicago/Cook County
Heat Wave: >3 consecutive days of air temperatures >90º F [>32.2º C]
Heat Index (HI): A measure of how hot it feels when the relative humidity is factored into the actual air temperature. Exposure to full sunshine can increase the heat index by 15º F.
Calculate the heat index using the Meteorological calculator at
Excessive Heat Warning:
Chicago/Cook County: 3 consecutive days with peak HI of 100-105 with a minimum temperature >80, 2 consecutive days with peak HI 105-110, or 1 day with peak HI of >110.
Suburban and rural counties: >3 hours with maximum heat index > 115 with minimum 80
Heat Cramps*: Muscular pains and spasms due to heavy exertion. The loss of water and salt from heavy sweating causes the cramps.
Heat Exhaustion*: Body fluids are lost through heavy sweating due to strenuous exercise in a warm, humid place. Fluid loss causes blood flow to decrease in the vital organs, resulting in a form of shock. Sweat does not evaporate as it should, and as a result, the body is not cooled properly.
Heat Stroke*: Life-threatening. The victim’s temperature control system, which produces sweating to cool the body, stops working. The body temperature can rise so high, up to 105oF, that brain damage and death may result if the body is not cooled quickly.
*visit for symptoms and treatments for these conditions
NOAA’s National Weather Service Heat Index
Preparing for a Heat Wave
-Stay tuned to NOAA Weather Radio or local radio/TV stations when there is the threat of a heat event.
-Be sure to check the forecasts before planning outdoor activities.
-Be sure your air conditioning system is properly maintained and in good working condition.
-If you don’t have air conditioning, check for public cooling shelters or plan to spend the day at a public indoor place such as a shopping mall or a library.
-Check on the elderly and the young, who are especially susceptible to heat.
During a Heat Wave
-Drink plenty of water, even when you’re not thirsty.
-Avoid alcohol and caffeinated beverages.
-Avoid salty foods that will dehydrate you.
-Dress appropriately: wear loose, light colored clothing. Wearing sunglasses protects your eyes from harmful UV rays, and a hat or umbrella will protect your face and head.
-Avoid strenuous activity. Take frequent breaks and try to plan indoor activities as much as possible.
-Always use sunscreen. Sunburns reduce the body’s ability to provide cooling.
-Keep the lights off and blinds closed when possible. Avoid using appliances such as the oven during peak times of the day.
-Do not leave children or pets in a closed vehicle.
Heat Statistics for Chicago and Rockford
Hottest Three Summers
1955, 1995, 1921
1921, 1934, 1983
Max Temperature
105* (7/24/34)
112 (7/14/36)
Average/Max June Temp
79.2/104 (1988)
79.9/106 (1934)
Average/Max July Temp
83.5/105* (1934)
83.1/112 (1936)
Average/Max Aug Temp
81.2/102 (1918)
80.9/104 (1988)
90+ days in 2006
90+ days in 2005
*The highest temperature ever recorded in Chicago was 109 at Midway Airport in 1934, but Midway was not the official observatory at the time.
**Includes a 102 degree reading on 7/24/05
What is the Heat Index Forecast for my location?
From our web page,, go down the main menu on the left to "Forecasts", and click on "Interactive". 
From the "Interactive" menu select  "Hourly Weather Graph". The hourly weather graph may be the most useful way to track the heat index hour by hour. Then point and click on your location on the map.
Then from the "Hourly Weather Graph" menu, choose "Heat Index" and any other parameters you want to view from the list and click on "Submit".

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