Weather question: What in the world is the 'dew point' anyway? - WSMV News 4

Weather question: What in the world is the 'dew point' anyway?

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Being on television, we'll often get tweets, Facebook posts, phone calls, and emails from people out in "TV Land" asking questions or sharing their opinion. Some are funny, some complimentary -- most are good, but some aren't so good (I don't like your tie, why did it rain on my picnic?, etc, etc.). Trust me, I've heard them all! I received one of these such emails an hour ago I wanted to share. It's from a viewer named, George. For George's protection, we won't share his last name, town, social security #, so on and so forth….


I must ask why meteorologists are working so hard to convert us from humidity to dew point. We all know humidity. When you say the relative humidity is 905, you don't then have to explain what that means.

Notice, however, that every time you give the dew point, you then spend the next several seconds explaining what that means. You guys even take the time to put charts up to explain what dew points in the 50's, 60's, 70's feel like. Why bother?

To further prove my point that all this time spent trying to explain dew point is a waste of time, do you think there will ever be a conversation like this: "Hot, isn't it?" "Yes, but it's not the heat that bothers me, its the dew point." Not likely.

I enjoy watching your forecast and we do watch you every night. Keep up the good work, even if you must push dew point explanations.


Here's what I typed in response………

"That's me, George.....a "Dew Point Pusher" :)!! Funny!

The intent is to make it easier for all of you in TV Land.'s not about's about you! Despite what you may have heard/learned, the relative humidity is not an accurate way to assess how muggy it feels to someone. Dew point is! In support of that, when was the last time you stepped out on a cold winter day and said...."Wow, it feels so humid outdoors!"? That's because the feeling of mugginess is created by a greater number of water molecules being present in the air. Dew point is a means to quantify how many water molecules are in a given volume of air. The more there are, the more "stuffy" it feels.

To further illustrate, relative humidity can be high, while the number of water molecules in the air is low. In those situations (during winter snow storms or rain storms, for example), it doesn't feel muggy at all. It may feel damp....but not muggy. Relative humidity reflects how many water vapor molecules are present, when compared with how many COULD be present for that air mass. Hence, the adjective "relative". The warmer the air mass, the greater its potential to allow water vapor molecules to just hang out amongst the air molecules. When the amount of moisture (water molecules) increases to a certain point, they begin to condense into a liquid, forming cloud droplets. SO.....relative humidity is a good judge to how close the air is to saturation (that point at which a cloud will form). That's why many people use measurements of relative humidity to gauge when they should dehumidify their basement, for example. Who wants it to be damp there, allowing mold/mildew to form?

Was that clear as mud?

Thanks for your email and for watching, George! We DO appreciate it!!


SO, the take away here is (1) the dew point is a useful weather term……AND (2) your next email/Facebook post/tweet could end up on They're ALL fair game!

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