Insider: How humidity plays a big role in determining the heat index

You might've heard the phrase "it's not the heat, it's the humidity." However, when it comes to calculating the heat index, you need both values. As temperatures begin to climb towards the lower 90s this week, that heat index value will also experience some changes.

The heat index is a measure of how hot it feels to your body when you step outside. It takes into account the air temperature, and then accounts for the relative humidity percentage. Once those two values are combined, that's how we determine the feels-like temperature, or heat index.

A summer day might have an air temperature of only 86 degrees, but if the relative humidity is just at 80%, suddenly that bumps up our heat index to triple digits. A day like that could feel like 100 degrees! That's a noticeable difference, and that's why including heat index becomes important when forecasting.

So why does humidity make it feel warmer? Our body's natural response to cooling ourselves involves sweating. When we sweat, those droplets evaporate into the atmosphere, causing us to feel cooler. However, when the atmosphere is already fairly saturated and relative humidity is high, there's not a lot of room for your sweat to evaporate.

This causes you to feel a little sticky, and can create that "muggy" feel when you walk outdoors. Your body can't cool itself off as easily, so you might even feel overheated.

That's where the values on the heat index really start to matter. Once values reach above 90, that's when you should begin taking caution. Any value above 103 is considered dangerous, and you run the risk of heat cramps, heat exhaustion, and even heat stroke.

The ABC 17 Stormtrack Weather Team issues Weather Alert Days whenever the heat index reaches 105 degrees or above. On those days, remain hydrated and take frequent breaks.