How humid is it? Check the dew point

Published 7:00 am Tuesday, May 26, 2015

Muggy. Sticky. Humid. All three words are good descriptions of the feel to the air the last several days.

Many people talk about relative humidity when discussing how humid the air feels. Relative humidity expresses how much water vapor – the invisible, gaseous form of water – is in the air compared to the maximum that could be present at that temperature. The warmer the air, the more water vapor it can contain.

When we say the relative humidity is 100 percent, it means the air cannot hold any more water vapor at that temperature. If that air is cooled even a little, some of the water vapor will begin to condense into liquid. This is how dew forms on the ground and how fog and clouds form as the water condenses onto very small particles in the air.

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 If the relative humidity is 50 percent, that air contains only half of the water that it can hold at that particular temperature. The air temperature would have to drop considerably before the water vapor would begin to condense into liquid.

Notice if the amount of water vapor in the air stays the same, the relative humidity goes down as the temperature goes up and increases as the temperature drops.

On a typical summer day our relative humidity in the morning may be near 100 percent. As the temperature goes up during the day into the 90s, the relative humidity will drop, probably down to the 50-60 percent level. But the amount of water vapor in the air may have stayed exactly the same.

That’s why relative humidity is not the best measure for how much actual water vapor is in the air. A more direct indicator is the dew point temperature.

The dew point is the temperature to which the air must be cooled in order for it to become saturated. In other words, how low must the temperature go before the relative humidity of that air becomes 100 percent? (Of course, that’s when dew would begin to form – thus, the name dew point.)

Unless drier or more moist air is blowing into the area or mixing down to the surface from above, the dew point will stay about the same even as the temperatures warm during the day and cool at night.

Early last Friday a cool front passed through. Dew points dropped from the low 70s into the upper 50s during the day, which felt very nice. But by late Friday the front had dissipated and moist air was surging back into our area from the Gulf. Dewpoints rose back to near 70. By Sunday, dew points were in the middle 70s, which is not unusual during summer in the Gulf South. In fact, our dew points hover around 80 degrees on our most uncomfortable summer days and nights.

The more moisture in the air, the less efficient our skin and perspiration are at cooling us down. Most people begin to feel uncomfortable when the dew points reach 70.

Next time you think it feels particularly sticky, go to the Internet, and check a nearby weather station. You will likely find the dew point in the 70s. You will have plenty of opportunities to try this out between now and October.

By Skip Rigney.