Q: “What is the difference between relative humidity and dew point?”

May 28th, 2012 by cathay
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Dew shown on blades of grass

"Dew on Grass" courtesy of Encyclopedia.com

Question:  “What is the difference between relative humidity and dew point?  I hear this on my local weather broadcast but don’t know what they mean.”

This is right in time for the start of the summer season!  Whether it’s offered as ‘dewpoint’ or ‘dew point’, the confusion of these terms it seems to be one that is fairly common.  And it’s so commonly misunderstood some of those who give us daily weather forecasts discuss how to describe this to the public as well as providing a thorough explanation, as shown on Meteorologist  Jeff Haby’s site “Weather Prediction” at:

http://www.theweatherprediction.com/

“Explaining Dewpoint and Relative Humidity to the Public” is specifically found at:

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

The U.S. Government Department of Energy’s site also has information, as presented in a more informal question-answering style:

http://www.newton.dep.anl.gov/askasci/wea00/wea00130.htm

As stated there, “The first thing you need to realize is that warm air can hold more water than cold air. Then dew point makes sense.”   The explanation then goes on to tell us that “dew point is described in degrees of temperature,” while “relative humidity is given as a percent.”

The NOAA’s National Weather Service gives thorough definitions, too.  For dew point, the site states:  “A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air.”  (see:  http://weather.gov/glossary/index.php?word=dew+point)

Relative humidity is “dependent upon temperature,” according to NOAA’s definition.  They further explain that it is:  “A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.”  (see:  http://weather.gov/glossary/index.php?word=relative+humidity)

In case you are interested, there is a humidity calculator you can freely use online, offered through Vaisala:

http://www.vaisala.com/humiditycalculator/?SectionUri=%2finstruments%2frhcalc

Just click on “start the humidity calculator” and give it a try!  It just may help give you a view of how dew point is affected by humidity and overall how temperature plays a role.  (Tip:  You can change the temperature calculator from Celsius to Fahrenheit if desired.)

All this talk of rising temperatures and humidity may get you longing to head to a beach or community pool but wondering where to go.  Feel free to ask your librarians, anytime you have questions!  Just log onto our library chat service, Ask a Librarian Delaware:

http://lib.de.us/askalibrarian

We’re glad to be of assistance and we’re 247!

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