While record-breaking temperatures are possible in the Chicago area Wednesday, they'll also lead to dangerous heat index values in the region.
But what is a heat index and how do you determine it?
Here’s a primer.
What is a heat index?
A heat index is a measure of how hot it actually feels outside at a given time, which can be different from the actual air temperature.
According to the National Weather Service, heat index is measured by inputting the air temperature with the relative humidity at a given time.
That number represents the “feels-like” temperature, and helps to determine whether the National Weather Service will issue weather alerts or advisories.
How is the heat index calculated?
If you want to get out a calculator and figure out the heat index yourself, here is the formula, as published by Lans P. Rothfusz in 1990:
HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T - .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH - .00000199*T*T*RH*RH
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In this formula, HI represents the heat index, while T represents the temperature in degrees Fahrenheit. RH represents the relative humidity.
There are other adjustments that can be made in specific situations, but for the sake of simplicity, NWS also provides this website to calculate the heat index.
Why is the heat index important?
Heat indices can have a major impact on the body's ability to cool itself.
"When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature," the NWS reports. "Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body's temperature. When the atmospheric moisture content (i.e. relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions."
What heat index is Chicago expected to see this week?
Heat indices of up to 115 degrees are expected in most of northern Illinois, with high temperatures rising into the upper-90s across the region.
As of 11:47 a.m. Wednesday, the heat index with temperatures at O'Hare Airport was 114 degrees.
What is the highest heat index Chicago has seen?
The highest heat index since record keeping on the metric began with the National Weather Service was 118 degrees on July 13, 1995.
The 114 degrees the Chicago area saw just before noon Wednesday was the highest since July 30,1999.
How does the heat index change weather alerts?
According to the NWS, each office has different criteria, but there are several types of heat-related alerts that can be issued.
An excessive heat warning is issued within 12 hours of the onset of dangerous heat, and the general rule of thumb is the warning can be issued if the heat index is going to exceed 105 degrees on consecutive days.
Excessive heat watches can be issued ahead of such events, generally between 24 and 72 hours prior to the onset of hot temperatures.
A heat advisory can be issued within 12 hours of heat indices of 100 degrees or more for consecutive days.
