Whether you're planning your next vacation or you just want to know how many layers you will need to put on in the morning, you probably check the weather forecast pretty often.

But you should know before you go out with that extra jacket that the temperature might not be the best measure of what it actually feels like outside.

Humidity is important too. Which is why you might sometimes think the forecasters must have been wrong, because it's definitely about 10 degrees hotter right now and you're covered in sweaty grossness.

Your body's built-in thermostat likes to keep your internal temperature steady at 37 degrees Celsius so that all your enzymes can do what they're supposed to do. You know, keeping you alive. That's the temperature they like.

When you need to cool off, your blood vessels open up, letting blood flow to your arms and legs so heat can escape through your skin. Your sweat glands also start to produce moisture, which takes a bunch of heat with it when it evaporates.

And it's this sweat evaporation that runs into a bit of a problem when it's humid out. When there's more moisture already in the air, your sweat doesn't evaporate as quickly. So you end up holding on to the extra heat. You just get hotter and damp. Awesome.

You usually see what's known as relative humidity in the forecast, which tells you how close the air is to saturation, where there's as much water vapor in the air as possible. 

But it turns out, the relative humidity forecast also isn't super helpful for figuring out how hot it'll feel, because the saturation point increases with the temperature. The hotter it gets, the more energy water molecules have, which means more of them can zoom around in the gas phase.

So you might have two days where the relative humidity is 50%, but if it's 30 degrees Celsius the first day and 35 degrees Celsius the second day, there's way more moisture in the air on the second day because it's 50% of a much higher saturation point.

Which means that the humidity on the second day is gonna feel way worse. 

So even though people talk about it a lot, the relative humidity forecast isn't a great way to tell how hot it feels. The Dew Point is a lot more useful.

That's the temperature where, given the amount of moisture in the air, it would reach 100% humidity and the moisture would condense into dew or fog.

What's helpful about the Dew Point is that it tells you how much moisture is in the air in absolute terms. So the higher the Dew Point, the grosser you'll feel, because there's more moisture in the air.

So on that first day, for example, when it was 35 degrees out, with 50% relative humidity, the Dew Point would be 18 degrees. On that second day, though, the Dew Point would be 23 degrees. Which tells you that it would feel a lot muggier, even though the relative humidity would still be 50%.

According to meteorologists, a good rule of thumb is that if the Dew Point is above 18 degrees, it's going to start to feel sticky and gross. So if you see a higher Dew Point in the forecast, you might want to spend the day somewhere there's a lot of good air conditioning. Go see a movie or hang out at Barnes and Noble and read books.

Thanks to Patreon Patron Miranda McCarthy for asking this question. I actually have been curious, so thank you. And thanks to all of our patrons who keep these answers coming.

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