The Geography Blog focusing on all things geography: human, physical, technical, space, news, and geopolitics. Also known as Geographic Travels with Catholicgauze!
Written by a former National Geographic employee who also proudly served in Iraq and Afghanistan.
Heat index is the combination of temperature and humidity to determine how hot it feels to the body. High humidity levels negatively impact the body's means for sweat to evaporate and thus interfere with the body's natural cooling mechanism. That's why 90 degrees with high humidity feels worse than a dry 100 degree day.
USA Today made a chart showing negative impacts of heat index levels (hat tip: Daily Caller).