Blistering heat and oppressive humidity are expected in the city of Chicago and parts of Illinois this week, with excessive heat warnings and heat advisories going into effect.
Forecasters are warning of heat indices exceeding 110 degrees in some locations, with record temperatures also possible on Monday and Tuesday in the city and suburbs.
But what exactly is the heat index, and how is it calculated? We crunch the numbers here.
So how is heat index calculated?
The mathematical formula was created by Lans P. Rothfusz, and is…..quite complicated:
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
In this equation, T is the temperature in degrees Fahrenheit, and RH is the relative humidity.
Thankfully, the National Weather Service has a handy calculator on their website, which simply asks for the air temperature and either the dew point or the relative humidity in a given location.
What does it tell us?
The heat index is a measure of how hot the air feels by combining the temperature and the relative humidity.
In conditions like what Illinois is seeing this week, the air temperature will be in the mid-to-upper 90s, but it will feel like upwards of 110 degrees because of the humidity.
Are there limitations?
Heat index values were devised for shady conditions with light wind, so direct sunlight and gusty winds can impact heat index values by up to 15 degrees, according to the NWS.
A different calculation known as the “Wet Bulb Globe Temperature” factors in wind, humidity, temperature, and even solar radiation, and is viewed as more effective for determining whether it’s safe for outdoor workers to be active during the day, according to officials.
