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# Meteorology (Weather)/relative humidity vs. dewpoint

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QUESTION: The weather website I use for current reports and forecasts lists both the relative humidity and the dew point. Is there any reason to list both? What will the dew point tell me that I can't discern from the relative humidity?

ANSWER: Hi Richard

Relative humidity is a measure of moisture in the air.

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/relative_humidity.rxml

The dew point is the temperature at which the relative humidity reaches 100% and dew will form on colder objects.

For more info, see:

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/dwp.rxml

http://www.theweatherprediction.com/habyhints/190/

http://weatherknowledge.com/learn.html

http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-86-2-225

---------- FOLLOW-UP ----------

QUESTION: Thank your for your reply, but it is not helpful.

I do know the definitions of these terms. My question, as I attempted (unsuccessfully, I guess) to make clear, was the use of these measures to a person who consults a weather site for information about the weather as it affects him or her. In that context both these measures will indicate how humid the air is. Since the relative humidity is the simpler of the two, my thought was that the relative humidity measure would be all the user needed to know. Yet the website in question provides both measures. I was trying to get your view of what I might be overlooking: what use the second measure would serve.

Thank you for the references, but what I'm after is your view on my question.

Answer
To be more specific to your question:

There are about three main audiences for basic weather info: general public, department of public works type people, and weather hobbyists.

Relative humidity basically tells you whether you will feel humid or not.

Dewpoint will let you know when fog will form, black ice may be on the roadway and other miscellaneous more technical things.

So, the use of relative humidity and or dewpoint may serve to give some people more information than others.

Meteorology (Weather)

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#### Donald Rosenfeld

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Any questions (except private) answered from the 1st grade level on up pertaining to any aspect of Weather. I am a 20 year member of the American Meteorological society and a long time forecaster of eastern United States snow storms and Hurricanes.

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