How a Heat Index Works

Here we are in the dead of summer. The dog days, if you will. Why do we cal them the dog days? Is it because it's so hot, it feels like a dog is breathing on you when the wind blows? Actually, that's not it. Its also not because we are all lying around like lazy dogs in the oppressive heat. Instead it has to do with the positioning of the Dog Star, Sirius, (like Harry Potter) in the morning sky. In late July the star rises just before the morning sun. This lead to all sorts of legends in ancient Greece– so you can go ahead and thank those robed ruminators for that saying. Heat In the summer, oppressive heat typically builds under a ridge of high pressure. Or that big "H" we meteorologists put on the map. Under a ridge, air sinks and compresses. Some of you already know that by compressing a fluid, or in this case air, it will heat up. If you didn't know, now you do. The reason this happens is because the sinking compresses all of the little molecules together that make up our air. So they are much closer together. Since they are so close together they generate heat from friction as they move about. Think of it like rubbing your hands together to warm them up. This is just like that, but on a much small scale. Since the air all around is doing this, it heats up even more than it typically would just from the heat of the sun! Humidity It's not the heat, it's the humidity. Humidity is caused by the moisture in the air. This moisture can come from many different places, so to save us from yet another boring explanation I'll just skip that part and we can assume there is water vapor in the air. Hot air holds more moisture. So on those hot days the dewpoints can climb to levels that they otherwise would not be able to hit on cooler days. Humidity is dangerous because it makes all of that sweating we do on a hot day less efficient. The moisture in the air keeps your sweat from evaporating. Evaporation is what helps your sweat cool you. Because of this, it actually feels hotter outside than it actually is! Photo: Kullez/Flickr Heat Index The "feels like" temperature on those hot summer days is called the heat index. You've probably heard this term, heat index, before– but do you know how we get it? Neither do I! It's a wonderfully complicated equation that can be summed up by data points on this nifty table. So most of us meteorologists just memorize the general trend and do some quick interpolating in our heads. Heat indexes aren't really a thing until the temperature surpasses the 80 degree mark. Once temperatures climb into the 90s it doesn't take much humidity to get those heat indexes to surpass 100 degrees. Keep in mind the heat index is what YOU actually feel when you step out of your car or the front door of your house. And when it soars, it doesn't take much exposure to the heat to find yourself in a dangerous situation. For WeatherNation — Meteorologist Jeremy LaGoo