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All About Weather

What are feels-like temperatures?

I have some pet peeves about weather casts. Temperature is one of them. During the summer when it gets hot and muggy, we start seeing weathercasters talk about “feels-like” temperatures. Media folks assume we know how they got those temperatures, which are always higher in summer than what the temperature actually is. It’s confusing to a lot of people. 

What is a “feels like” temperature?

Feels-like temperatures are calculated through something called a Heat Index. With high temperatures and humidity, it becomes potentially dangerous to do a lot of physical activity outdoors. Over exertion in these conditions can lead to muscle cramps, heat exhaustion, or heat stroke.

Things that go into the calculation include the pressure exerted by water vapor, human body volume, skin surface area, clothing, body temperature, temperatures of the skin and clothing, activity, wind speed (which is much more important in the winter and the wind chill index), and rate of sweating. You’re already glazing over so I’ll stop there. This is one reason the “feels like” temperature is used, because it’s information people can relate to.

How does humidity play into this?

What do we do in the summer to cool off besides have a cool beverage? We sweat. Moisture on the skin surface will evaporate, removing heat from the skin surface, which then lowers our overall body temperature. If there is already a lot of moisture in the air, that will slow the rate of evaporation, and will slow the cooling, or even stop it. If you continue with physical activity under these conditions, your body temperature will increase. We’ve all heard of football players in August that have gotten sick, or in rare instances died, from over exertion in hot muggy conditions. 

When does it become dangerous?

The heat index is divided into four categories: caution, extreme caution, danger, and extreme danger. The extreme danger category is so high we really shouldn’t have to deal with that one, but we can easily get into the extreme caution range of 91 to 103 degrees. Heat cramps, heat exhaustion and sunstroke are possible with prolonged activity. 

The danger category is a feels-like temperature of 103 to 124 degrees.  As might be expected, heat exhaustion and sunstroke are likely with physical activity for most people under those conditions.

Keep your cool

Here’s where common sense comes in. If you’re getting hot, then stop what you’re doing and get cooled off. If you start having muscle cramps, get to a cool place and drink some cool water or sports drink that has some electrolytes. If you are experiencing nausea, light headedness, or other symptoms, it may be advisable to seek medical attention. Don’t keep pushing yourself.  

In six months I’ll discuss the wind chill index. Something to look forward to. Not.

The following references include additional information on the topic: 

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Duane Friend is an energy and environmental stewardship educator with University of Illinois Extension, serving the organization in many roles since 1993. Duane provides information and educational programs to adult and youth audiences in the areas of soil quality, weather and climate, energy conservation, and disaster preparedness. These programs provide practical solutions for families, farms, and communities.  He assists families in creating a household emergency plan, farmers with the implementation of soil management and conservation practices, and local government officials and business owners with energy conservation techniques.

All About Weather is a blog that explores the environment, climate, and weather topics for Illinois. Get in-depth information about things your weather app doesn't cover from summer droughts to shifting weather patterns.