We hear it all the time around here: “it’s not the heat, it’s the humidity”! That’s true not just for how uncomfortable we feel, but also in how safe we are being outside in the summer. The humidity affects our body’s natural ability to cool off from sweat.

The way sweat works is simple: sweat absorbs some of the heat from your body so that it can evaporate. This is called evaporative cooling, and it’s really effective at keeping us cool as long as we stay hydrated. Unfortunately, this cooling mechanism starts to break down when the humidity climbs.

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When the relative humidity is high, that means there’s already a lot of moisture in the air relative to how much moisture could be in the air. So at 50% relative humidity, the air is already holding about 50% of what it could. If the relative humidity is very high, then that means the air is already ‘holding’ most of the moisture it can. This can slow down the evaporation of sweat, thus slowing down our ability to stay cool.

Once evaporative cooling starts to slow down, it starts to feel hotter than it actually is. That’s why we calculate the heat index, or the apparent temperature. To calculate the heat index we consider the actual air temperature, the relative humidity, and some assumptions based on the average human body.

The heat index will stay the same as the actual temperature up until the air temperature hits 80 and the relative humidity hits 40%. Once the temperature hits 92, the relative humidity only needs to be at 50% for the heat index to come out near 100 or above.

You can keep yourself safe this summer by helping your body regulate its temperature. That means staying hydrated, wearing light colored and loose-fitting clothing, avoiding strenuous work that will make you sweat a lot, and of course spending more time out of the heat during the afternoons.

For the latest temperature and heat index forecast, you can check our forecast discussion here: whnt.com/forecast