JAMES WX BLOG: Stop using relative humidity

Wx Blog

Let’s face it. When we want to know how humid it is, we look at that humidity percentage and hope what we interpret makes sense. Well it doesn’t! We can have 100% relative humidity (RH) and it’s snowing like the dickens outside (whatever that means). We could have 40% relative humidity and it feels like the tropics. I’ll get into why this is later, but first let’s look at WHAT YOU SHOULD be using to judge the humidity of how it feels: The Dewpoint!

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Us meteorologists like to talk numbers and the dewpoint is a big one when it comes to humidity. But what IS IT?

Put simply, the dewpoint is the temperature at which dew forms. We know there’s always some amount of moisture (water vapor is a fancier way to say it) in the air, right? Let’s say the dewpoint is 56° and the air temperature is 70°. That means after the sun sets and temperatures drop, if the air right above the ground gets all the way down to 56° then you can be confident in saying there will be dew on the grass, or fog in the air, or maybe even some rain showers… You get the idea. 

We often refer to how muggy it is based on where the dew point is. If the dewpoint is 50° or lower, it’s comfortable and there is no noticeable mugginess. 50-60° is still, at least for Western New York, pretty manageable from a body temperature perspective. 60-65° is when we’re starting to talk about how sticky it is out there and as temperatures climb into the 80s with this humidity, you’ll feel it. 65-70° starts to get really uncomfortable and soupy. Anything over 70° and it’s a swampland out there. We’re talking southern Mississippi down in the bayou type humidity. I’ve heard some call it “Air you can wear.”

A good rule of thumb is if the dewpoint is below 65°F the air is “comfortable”. If it’s above 65°F, the air is “sticky”.

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So back to relative humidity. The percentage you see in current conditions shows how close the air is to saturation. Full stop.

Want to know more? Want to have your eyes glaze over? Okay here we go. Relative humidity is the amount of moisture in the air divided by the maximum amount of moisture that could exist at the current temperature. This is used by calculating the moisture mass or vapor pressure and calculating the ratio. RH is not dewpoint/temperature. Although you can find the RH using the dewpoint and temperature. One method is to calculate the vapor pressure (E) and saturation vapor pressure (Es), dividing them and multiplying by 100. So RH=E/Es*100%

To calculate vapor pressure and saturation vapor pressure, you’ll need the Clausius-Clapeyron equation.

Ln(Es/6.11) = (L/Rv )(1/273 – 1/T)
Es = Saturation vapor pressure
L = Latent heat of vaporization = 2.453 × 10^6 J/kg
Rv = Gas constant for moist air = 461 J/kg
T = Temperature in Kelvins

See what I mean? How about we just start using dewpoint more. Thanks for reading! You can find more information here.

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