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Dew Point vs Humidity

By Staff | Jul 10, 2012

Like Dave’s cartoon says:?”It was hot enough for me!” We’re finally getting a reprieve from a “sticky” summer.

We were talking recently about the difference between dewpoint and humidity. When we have those really sticky days, I always look at the dewpoint. The higher it gets, the more sticky it feels outdoors. Seems like a dewpoint over 60 is pretty awful. Of course, the humidity is also right up there too.

Here is one explanation of the difference between dew point and humidity. You can Google the question and get a whole bunch of explanations, but here’s what Meteorologist Jeff Haby has to say:

The public can have a tough time understanding the difference between the “meaning” of Relative Humidity (RH) 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 makes 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-percent means it is either foggy, very wet, or saturated outside. One misconception people have is that the RH is 100-percent only when it is raining. Example 1:?The RH is often 100-percent 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-percent when it is raining (it takes time and lots of evaporation to saturate air that previously has RH of 50-percent for example).

RH can be explained as the “closeness the air is the saturation.” When the RH is less than 40-percent, it feels dry outside, and when the RH?is greater than 80-percent it feels moist outside (dewpoint will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80-percent is comfortable if the temperature is also comfortable.

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-percent (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-percent.

With all that, here’s the bottom line from Meteorologist Jeff Haby: Dewpoint is related to the quantity of moisture in the air while relative humidity is related to how close the air is to saturation.

Got it? Or is the distinction between the two still about as clear as molasses?