Here’s how the heat index works: Why it feels hotter than the temperature

Heat index (National Weather Service)

For whenever we experience “feels like” temperatures of more than 90 or even 100 degrees in Michigan, here is a quick explanation on what the heat index is and how to read it.

The National Weather Service (NWS) explains the heat we feel outside on these hot summer days is not exactly caused by the temperature alone. You have to take into consideration the "heat index."

"The heat index is a measurement of how hot it really feels when the relative humidity is incorporated with the actual temperature," the NWS explains. "Heat indices were designed for use in the shade with light wind conditions. If in direct sunlight, the heat index can increase as much as 15 degrees. With very hot and dry air, strong winds can also be extremely dangerous."

Here's a closer look at how relative humidity and temperature combine to form the heat index and the danger levels:

Duane Friend, an Environmental and Energy Stewardship Educator with University of Illinois Extension, put together this short video explaining the heat index -- worth a watch:

Here are four types of heat disorders that occur due to a high heat index, including their symptoms:

  • Sunburn: Redness and pain. (also swelling, blisters, fever, and headaches)
  • Heat cramps: Painful spasms usually in the legs and abdomen. (also heavy sweating)
  • Heat Exhaustion: Heavy sweating and weakness, along with cold, pale, and clammy skin.
  • Heat Stroke: A high body temperature, hot and dry skin, a rapid and strong pulse, and possible unconsciousness.

The NWS explains an Excessive Heat Warning is issued if the heat index equals or exceeds 105° for at least three consecutive hours. Heat Advisories are posted when the heat index is expected to exceed 100 degrees for three consecutive hour and can be extended into the night if low temperatures are in the 70s or higher.

The NWS suggests the following tips to stay safe during hot weather:

  • Stay out of the sun. (sunburn makes the job of heat dissipation more difficult)
  • Spend as much time as possible in air conditioning. If you do not have an air conditioner, go to an air-conditioned public building, like a library.
  • Slow down. (reduce, eliminate, or reschedule physical activities for a cooler time of the day)
  • Drink plenty of water, even if you do not feel thirsty. Do not drink any alcohol, including beer.
  • Dress in lightweight and light colored clothes. This will reflect the sunlight and heat.
  • Eat smaller meals and less proteins.
  • Do not take salt tablets unless specified by a physician.

5 heat deaths a year in Michigan

According to the NWS, Michigan averages about 5 heat related deaths each year.

"The number of heat related illnesses are difficult to record, but it is fair to say that each year in Michigan there are hundreds of heat related illnesses some of which require hospitalizations," reads a statement by the NWS. "The national average is 134 heat related deaths making heat the number one weather related killer in the United States."