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Explaining Dewpoint and Relative Humidity

relative humidity dewpoint moisture heat index humidity evaporation

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#1 theweatherprediction.com

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Posted 26 July 2013 - 09:30 AM

METEOROLOGIST JEFF HABY


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The public can have a tough time understanding the difference between the "meaning" of RH (relative humidity) and dewpoint. The public has a good grasp on how the weather makes

them feel. One approach to explaining dewpoint would be to say, dewpoints above 65 F make it feel sticky and humid outside while dewpoints less than 65 F are comfortable with respect to the stickiness of the air. The higher the dewpoint is, the more moisture that is in the air. The higher the dewpoint is above 65 F, the stickier it will feel outside (feels like you have to breathe in a bunch of moisture with each breath). 75 F or above dewpoint, the air really feels sticky and humid.

RH can be more difficult to explain. The public pretty much understands that a RH of 100% means it is either foggy, very wet, or saturated outside. One misconception people have is that the RH is 100% only when it is raining. Example 1: The RH is often 100% in the early morning hours when temperature has dropped to dewpoint. Example 2: When rain first begins, it takes time for the air to saturate. RH is often much less than 100% when it is raining (it takes time and lots of evaporation to saturate air that previously has a RH of 50% for example). If the rain is not heavy enough or does not last long enough, the rain will not saturate a previously drier PBL.

RH can be explained to the public as the "closeness the air is the saturation". When the RH is less than 40%, it feels dry outside, and when the RH is greater than 80% it feels moist outside (dewpoint will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80% RH is comfortable if the temperature is also comfortable. :biggrin:



The worst combination for human comfort is a high dewpoint (65 F or above) combined with a high RH. If the dewpoint is above 65, it will generally always feel uncomfortably humid outside. Obviously, the temperature could climb to over 100 and result in a low RH, but the quantity of moisture in the air is still high and will be noticed.

The optimum combination for human comfort is a dewpoint of about 60 F and a RH of between 50 and 70% (this would put the temperature at about 75 F). The air feels dry outside when BOTH the dewpoint is below 60 F AND the RH is less than 40%.

Now the dilemma, how does the public differentiate the "meaning" between a high dewpoint and a high RH when they both indicate the air is humid??? Dewpoint is related to the quantity of moisture in the air while relative humidity is related to how close the air is to saturation. How the public is to understand this difference in meaning can be a challenge. The challenge can be overcome by describing how the weather feels and relate that information to the current dewpoint and relative humidity.



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#2 Pastor of Muppets

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Posted 26 July 2013 - 07:04 PM

Welcome to TalkWeather.  Did you only join so you can spam us with your website?

#3 metallicwx366

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Posted 26 July 2013 - 07:18 PM

Pfffftt, our dewpoint here is 79 and the relative humidity is 89% currently. It doesn't feel that bad. I guess we are used to it down here.
Very nice website though.

Edited by metallicwx366, 26 July 2013 - 07:18 PM.


#4 Taylor Campbell

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Posted 27 July 2013 - 11:47 AM

The Weather Prediction .Com is usually known as a very credible and reliable source in the weather community. Most weather enthusiast that want to take their forecasting and hobby seriously know about this website.

Edited by Taylor Campbell, 27 July 2013 - 12:31 PM.

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#5 brax

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Posted 27 July 2013 - 12:51 PM

View PostTaylor Campbell, on 27 July 2013 - 11:47 AM, said:

The Weather Prediction .Com is usually known as a very credible and reliable source in the weather community. Most weather enthusiast that want to take their forecasting and hobby seriously know about this website.

Not.... Sure if sarcasm.....
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View Postatlwx, on 10 January 2010 - 09:35 AM, said:

I love the way 1993 is the equivalent of saying "Voldemort" in the weather community!


All hail SD, harbinger of the Mega -NAO.

#6 theweatherprediction.com

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Posted 27 July 2013 - 01:24 PM

Glad to be here since talking about weather and discussing weather concepts is a passion for me.

I lived in the southern states for most of my life and I do remember how uncomfortable the heat and humidity was in the summer. Sometimes I could not get from my front door to the car without starting to sweat. Then I was greeted with a greenhouse effected super heated car. After driving and reaching my destination, even with the air conditioner on I would get a sweaty back. I don't miss the extreme heat and humidity. I am in Colorado now. Colorado has the opposite problem in being too dry at times. I have the humidifer going in winter. I do like the drier air in general and cooler temperatures outside. I no longer have to sweat getting into my car. :yes: I miss seeing the variety of severe weather in the south but I have gained being able to see winter snow storms and extremely wild temperature fluctuations in Colorado. It is fascinating how much the dewpoint and relative humidity impact the weather experienced. I do agree people in general get used to the climate they live in. I'd be in trouble though if I had to try to adjust back to the extreme heat and humidity in the south. What is the highest dewpoint you have experienced? 83 F is the highest I can remember in the south experiencing first hand.

#7 metallicwx366

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Posted 27 July 2013 - 04:31 PM

Highest dewpoint I remember was 82.
Dewpoint is 79 again currently.



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