Over the past week or so, the First Alert Weather Team has been pretty focused on the temperatures, specifically how hot they are. During the majority of the year, we can look at the thermometer and get a fairly accurate description of what it feels like outside. However, now that we're into the first days of summer and the humidity has settled in, we have to give focus to a second temperature: the heat index, or "feels like" temperature.

So what exactly is the heat index? Simply put, it's the temperature that takes humidity into account. The idea here is that when it's hot out we start to sweat, and that sweat evaporates off of our skin to cool us off. However, when the air is already saturated with water vapor (which we perceive as humidity) the sweat evaporates more slowly. Slower sweat evaporation isn't as effective as cooling us off, thus it winds up feeling hotter with more humidity.

At least, that's the easy explanation. It actually gets quite a bit more complicated than that. For one thing, every person is different, and therefore every person perceives temperature slightly differently. In order to come up with an equation to calculate the heat index, a few assumptions need to be made. Here are a few of them:

- You are 5'7" tall and weigh 147 lbs
- You are wearing long trousers and a short sleeve shirt
- Your body temperature is normal (98.6°) and you aren't running a fever
- You are walking outside at a rate of 3.1 mph
- You are sweating uniformly all across your body, and it is not dripping off of you
- The wind is blowing at around 6 mph

Those are a lot of assumptions to make, and as a result, the equation for calculating the heat index could never truly be accurate because it will never be customized for each and every individual! That's important to note, because some people could become more susceptible to heat related illnesses as a result. On top of that, the heat index equation isn't a "perfect" equation. It was derived from a regression based on a mathematical model, and therefore contains some flaws.

The long and short of it here is that regardless of the assumptions, it's hot outside! We want you to take precaution and stay safe. The best thing you can do in this weather is to avoid going outdoors and stay inside in the air conditioning. If you do have to go outside, drink plenty of water and wear loose fitting, light colored clothing. Remember, children and senior citizens are more susceptible to the nasty effects of heat, and pets need to keep cool too!

For the math nerds out there who have been wondering what the heat index equation actually looks like, here it is:

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T 2 - 5.481717x10-2R 2 + 1.22874x10-3T 2R + 8.5282x10-4TR2 - 1.99x10-6T 2R 2

Where R = the relative humidity, and T = the temperature.

Want to read more about the heat index? Check out the links I used as references for this blog! In the meantime, keep cool.

Meteorologist Julia Weiden

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