What is a heat index? We’re glad you asked

It’s a term we use a lot this time of year, so meteorologist Amanda Curran breaks down how it impacts you

Alabama's heat index explained

MONTGOMERY, Ala. (WSFA) - “It’s not the heat, it’s the humidity” is likely a phrase you’ve either said or heard several times in your life, especially when it comes to describing Alabama’s weather during the summer. But did you know it’s actually both the heat AND the humidity that make it feel so hot this time of year?

The “feels like” temperature - also known as the Heat Index - is a value that is calculated based off of current air temperatures and relative humidity levels to better explain how it feels outside. It’s not only important when considering is it comfortable of uncomfortable outside, but it also impacts how we as humans control our internal temperature.

When the body gets too hot, it begins to perspire - or sweat - to cool itself off.

Sweat evaporates easier when the atmosphere is drier.
Sweat evaporates easier when the atmosphere is drier. (Source: WSFA 12 News)

If the perspiration is not able to evaporate, the body cannot regulate its temperature. When perspiration is evaporated off the body, it effectively reduces our temperature; when the atmospheric moisture content, or relative humidity, is high, the rate of evaporation from the body decreases.

Simple put: Evaporation is a cooling process and the human body feels warmer in humid conditions.

Excess moisture in the atmosphere makes it harder for your body to cool itself.
Excess moisture in the atmosphere makes it harder for your body to cool itself. (Source: WSFA 12 News)

There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase, the heat index increases.

The heat index is not just a made up number, it’s actually carefully calculated using the mathematics formula:

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2

Where T equals air temperature (° F) and variable R stands for relative humidity (% of moisture in the atmosphere).

However, this equation was obtained using a multiple regression analysis, and therefore, it has an error of ±1.3°F.

This chart shows what a 92° air temperature feels like with differing levels of humidity.
This chart shows what a 92° air temperature feels like with differing levels of humidity. (Source: WSFA 12 News)

Also important to remember: these are values for a shady location! If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F. As shown in this table, heat indices meeting or exceeding 103°F can lead to dangerous heat disorders.

So while high temps and muggy air in Alabama are nothing new, it never hurts to be reminded that prolonged exposure and/or physical activity in the heat isn’t something to mess with, especially this time of year.

Copyright 2019 WSFA 12 News. All rights reserved.