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Dew Point versus RELATIVE Humidity

(KWQC)
Published: Jul. 17, 2019 at 8:23 PM CDT
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I took a phone call the other day asking why I don’t use the “Relative” Humidity number on my weathercasts like others, but use the Dew Point, instead. Then I wondered if the person on the other end was using a cell phone or a rotary-dialer. Why? Because, it hit me that using the relative humidity is like still using a dial phone in the age of smart phones. A simple adage to remember is the title of a book once written by a TV meteorologist who I really looked up to as a kid: “Smile when the Dew Point drops”. And that’s EXACTLY how simple the concept of dew point is! Can you tell me how humid the air is if the “RH” is 73%? Did you know that that same amount of moisture in the air could be expressed as 28% at another time? That’s the problem with “relative” humidity, it’s relative to the air temperature. Since air temperature changes all day long, so does the “relative” humidity!!! What DOESN’T change during the day, or at least till a front comes through and changes the air mass, is the dew point! Once I heard my Sister-In-Law in Minnesota mention what the dew point was, one morning, and she went on to say it was going to be a sticky day. I was SO proud!!! The dew point is expressed as a temperature. And, there are certain things we can be certain about when it comes to the dew point. If it’s in the 40s or 50s, the air will be comfortable. 60s will start to feel muggy. If the dew point is in the 70s, or even the 80s, the air will feel tropical and borderline oppressive. The higher the dew point the more moisture is in the air, and that’s really all you need to know. Look at today. In the morning we had a temperature of 74 degrees, a dew point of 70, and a “Relative” humidity of 87%. In the heat of the afternoon it was 91 degrees, with a dew point of 70, and a “Relative” humidity of 49%. Do you see how the “Relative” humidity changed? It was high in the morning (typical) and LOWER in the afternoon heat! So, does it make sense that it’s a useless variable in figuring out how sticky it might be? And, since we were in the same sultry air mass ALL DAY, the dew point was a constant – in the 70s, meaning very stuffy! So, don’t you think it’s time to dial a big fat ZERO on the “relative” humidity?