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Why dew point matters much more than humidity

On hot and humid days it feels hotter because of the amount of water in the air. So why don't we use humidity to measure how much water there is to get a heat index?

NORFOLK, Va. — Let's talk a little bit about dew point versus humidity and how it affects the heat index, or how hot it feels to you when you go outside in the summer. 

Say you have a temperature of 80 degrees and a humidity of 78%. The heat index -- or how hot it actually feels to your body -- at that point is 84. 

Now let's look at a temperature of 86 degrees but a humidity level that's lower at 72%. Since the temperatures in both of these examples are in the 80s but the humidity level is lower in the second example, a lot of people would think that the heat index would be lower as well. Instead, with those numbers, it actually feels like it is 96 outside!

So why is that? You need to substitute humidity and use the dew point instead. In the 80-degree example, the dew point is 72 degrees, while in the 86-degree example, the dew point is also higher, at 76 degrees. That's what makes the heat index higher and why the dew point matters more than just the raw humidity percentage when accounting for how hot it actually feels like outside.

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We can look at it another way: if you're stuck in the desert and I offer you two glasses of water -- one full and one half-full -- which would you choose?  You might first say you'd want the full glass of water, but you're not accounting for the size of the cups.

If I fill the 4-ounce glass completely up, you've got four ounces of water. If I only half-fill a 16-ounce glass you'll still end up with eight ounces of water. In that case, you would want the larger container even though it didn't have as much of it filled. 

That is the same thing with dew point versus humidity. You should care about the total amount of water -- not the percentage -- of the cup that is filled. 

So when looking at the heat index, the total amount of water in the atmosphere is more important than the percentage of water, because it is the total amount of water that makes it harder for your body to evaporate your sweat and cool you off. 

Hence the use of dew point instead of humidity.

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