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Dewpoint Vs. Relative Humidity

Updated: Wednesday, August 14 2013, 11:10 PM EDT

During weathercasts on Local 12, you've likely heard The Weather Authority use the terms "relative humidity" and "dewpoint." While both of these terms are related to the amount of moisture in the air, they mean very different things.

The "relative humidity" of the air describes a relationship between the temperature and the dewpoint. If the air temperature and dewpoint are close together, the relative humidity will be high compared to if the air temperature and dewpoint are far apart. If the air temperature and dewpoint are equal, the air is "saturated," and the relative humidity is 100%. If the air temperature cools below the dewpoint, a cloud will form. At the ground, the temperature is often equal to or less than the dewpoint during heavy rain or on a foggy morning.

An example of current conditions - including the dewpoint and relatively humidity - that is shown regularly on Local 12 News.

The dewpoint is an absolute measure of how much moisture there is in the air. A higher dewpoint means the air is more humid, while a lower dewpoint means the air is less humid. When the dewpoint is 60° or higher near the ground, it is humid outside; a dewpoint of 70° or higher is considered oppressive. The dewpoint does not necessarily change when the temperature changes.

Why is the dewpoint the best measure of how humid the air is? Suppose you watch Local 12 News First at Four and notice the temperature in Cincinnati is 93°, the dewpoint is 68°, and the relative humidity is 44%. While watching Local 12 News on the CW Cincinnati at 10pm, you notice the temperature is 72°, the dewpoint is 64°, and the relative humidity is 76%. While the relative humidity has increased between 4pm and 10pm, the air outside your house is actually less humid because the dewpoint has dropped.

Here's another example: on a foggy, spring morning, you're watching Good Morning Cincinnati on Local 12 at 6am and see the temperature and dewpoint in Cincinnati are both 40°, and the relative humidity is 100%. Later that day on Local 12 News First at Four, you notice the temperature has risen to 60°, the dewpoint has risen to 42°, and the relative humidity is 51%. Despite the relative humidity dropping through the day, the air out your front door is slightly more humid than it was at 6am because the dewpoint has increased.

An example of how dewpoint and relative humidity values changed in Cincinnati on August 12, 2013. Notice that a change in the dewpoint doesn't necessarily mean a change in the relative humidity.

Changes in relative humidity are more common than changes in the dewpoint. It is common for the dewpoint to be relatively steady in a 24 hour period, but relative humidity values regularly change through the day as temperatures rise and fall.

The relative humidity plays an important role in fire weather forecasting, including forecasts for how quickly a wildfire can spread. Despite it's limited use in meteorology and weather forecasting, relative humidity is an important measurement to airline personnel (who often keep the relative humidity of aircraft cabins under 10%), HVAC technicians (who often keep the relatively humidity high enough to avoid human discomfort but low enough to maintain building integrity), and pilots (who need an understanding of humidity to determine if an airplane's lift will be affected).

Dewpoint Vs. Relative Humidity

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