Ask Joe | Dew point, relative humidity differ

August 25, 2013 

Q: Are the dew point and relative humidity the same measurement?

— Doug

A: No, they are not the same measurement, but they both are measures on moisture in the air. The dew point temperature is the best measure of how much water vapor (or humidity) is in the air. It actually represents the temperature that if the current air was cooled to, condensation would occur. Sounds complicated, but it’s actually the best measure because it is only dependent on the one thing, water vapor. The lower the dew point, the less moisture and vice versa with higher dew points. During the summer, when dew points are in the 50s and below, we are usually comfortable. When the dew point is near 60, it’s noticeable, and near 70 it’s oppressive.

As for the term relative humidity, many people say they are used to this measurement; however, if I quiz those same people they really have no idea on what it means to them. In the summer, relative humidity of 50% can mean it is refreshing dry or oppressively humid. The term relative humidity can be extremely misleading and fairly useless to the common person except for when you are concerned about objects drying. The lower the relative humidity, the quicker something will dry. Relative humidity is the amount of water vapor compared to the saturation point at the current temperature. Throughout a typical day, the relative humidity will swing wildly despite no change in the amount of moisture in the air.

Let me give you an example on how relative humidity can be confusing. Let’s use an example day. If in the morning of this day, the temperature is 70 and the dew point is 60, the relative humidity would be about 72%. Now let’s say it becomes more humid by the afternoon and the dew point increases to 68 while the temperature increases to 85. The relative humidity would drop to 60% because it’s warmer, but it would feel a lot more humid to you.

I have a question for you that was brought up while I was in school. Does the radiation from nuclear facilities affect weather patterns? And if so, is it enough to steer impending storms away from the area? (Jeremy)

The radiation in these facilities is contained within the facilities, and that’s a good thing since it would be harmful for life. Therefore, the radiation would not be a factor in weather patterns. Though power facilities with cooling towers (both nuclear and non-nuclear) do add a source of heat and moisture to the atmosphere. This would create lift and can enhance clouds and there has been some link to a little added precipitation downwind to them. Everything needs to come together just right and the best chance will be during extreme cold snaps. In fact, last year, there were a couple of occasions that there was a band of snow downwind from Western Pennsylvania power plants.

If you have a question about the weather, you can write to Joe Murgo at 5000 6th Ave., Altoona, PA 16602 or email him at Murgo@wtajtv.com. Some questions will be answered here and all of the questions will be entered in a contest to be shown on WTAJ News at 5pm.

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