Dew Point Versus Relative Humidity

We’ve all been sold a bill of goods over the years. We’ve been told relative humidity is what humans feel. Alas, it’s not true.

dew point explainer

I’ve gotten a few viewer emails recently about the dew point. Why show it and not the relative humidity? Here’s how I answer.

We’ve all been sold a bill of goods over the years. We’ve been told relative humidity is what humans feel. Alas, it’s not true. Relative humidity is temperature dependent. We want something that reads how moist the air is under every circumstance.

Here’s what I’m talking about. July 1 was a particularly nasty, humid day in Palm Springs. It was about as humid as our desert gets. The 4:00 PM relative humidity was 31%.

Does that help people understand? It does not.

I came here from Connecticut. On frigid January days with clear skies and snow on the ground it’s not uncommon to see a temperature near zero and relative humidity near 100%.

Does that help people understand? It does not.

Currently the relative humidity in Miami is 63%. It’s exceptionally sticky–stickier than the Coachella Valley ever gets. If offered a chance to guess their current relative humidity, Floridians would guess much higher.

I’ve been forecasting weather over thirty years. I can’t tell you how many people have told me about being in Florida when the humidity is 90%. I know what they mean, but 90% in Florida in the summer can’t happen… well, it theoretically can, but it doesn’t.

What meteorologists use (that’s me) is dew point. It’s simple and it works in every situation.

I am attaching a graphic I’ve shown on the air at least a half dozen times in the last two weeks. It explains dew point very simply. Once you understand it, you’ll never want to go back.

It’s my job to explain the weather so it’s usable for you. I have failed in that regard and will use your email to remind me and guide me in the future.

Thanks for writing.

All the best,
Geoff Fox

3 thoughts on “Dew Point Versus Relative Humidity”

  1. That’s what the major meteorologists here in CT are using now, also. And, you are right, it makes more sense. There have been days, lately, when folks are talking to me about it “being so humid” today—and I nod, but truly don’t feel it. Then there is tonight when you can see the ‘fog’ and while a bit cooler than when the sun is out—still feels like one is in a ‘croup tent’–that’s the pediatric nurse in me! According to weather bug–the present dew point is 69.8—that about says it, per your chart.

  2. Dew point temps are a MUCH better measurement of how sultry the air feels.

    However, I do think there (when applied to what people feel at certain dew point temps)…is a little geography (or maybe acclimation to it): Someone from the desert in Yuma or Palm Springs might think that a dew point of 60 F is like the Caribbean…while someone from Seattle or NYC might think it is dry. The same goes for subtropical Florida – in the winter dry season dew points are only in the 50’s to low 60’s F, but someone from an arid climate it might still feel like the air has a lot of moisture in it.

    One other note…not sure where you lived in CT Geoff (maybe up near MASS line?)…but 0 F low in Connecticut is pretty rare. I live in Branford, CT, and while the coast is of course warmer than inland, maybe 4 times in the last 35 years have I seen a low below 0 F…lows even below 10 F are rare. Maybe up in the higher elevations NW Hills, but 0 F is really rare in southern CT.

  3. Geoff, I agree completely with you, Dew Point is much more meaningful! I have a chart next to my computer that is a little different than yours, it shows ranges which pretty much match yours, but it goes one step further 76+ is “Miserable”. As you know here in CT it is not unheard of getting into that range!

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