Lower humidity won’t stay long

Published 7:00 am Tuesday, June 5, 2018

By Skip Rigney 

After three weeks of uncomfortably muggy early summer humidities, drier air moved into our region yesterday and today giving us a slight reprieve from the stickiness that will undoubtedly be the norm for the next several months.

Drier, and for those to our north, cooler air plunged southeastward across the Canadian border on Saturday. It was a strong system with winds behind the front gusting to over 50 miles per hour in North Dakota on Saturday afternoon.

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By Monday morning the cool front had swept across the southeast U.S. into the Gulf of Mexico and Atlantic Ocean. But, as usual for this time of year, it had lost most of its coolness by the time it reached us. It’s main distinguishing feature was that the air was slightly less humid.

The more water vapor air contains, the muggier it feels. That’s because our bodies cool as perspiration evaporates from our skin, and evaporation is slower the more water vapor there is in the surrounding air. The best quantity for measuring how much water vapor is in the air, and how muggy that air feels, is the dew point temperature.

Almost everyone is comfortable when the dew point temperature is less than 60 degrees. Those of us who live near the Gulf Coast are accustomed to air that contains a lot of water vapor and therefore has a high dew point temperature. We don’t usually begin to feel sticky until the dew points rise into the upper 60s. When dew points get into the 70s, which is the case during most of the summer in the southernmost parts of the Deep South, almost everyone is feeling uncomfortable.

One of the great benefits of air conditioning is that, as it cools the air, it dehumidifies it. This is because as the air is cooled below its original dew point, water vapor condenses, and the resulting liquid water is drained away.

If your air conditioning unit is working well, the dew point temperature inside your home will likely be in the middle 50s, which is often 15 to 20 degrees less than the dew point outside during the summer in the South.

If indoor dew points are much higher than the middle 50s when the air conditioner is being used, the air feels cool but clammy. Higher dew points inside air-conditioned buildings also raise the risk for mold.

It’s little wonder that dehumidification is a major focus of the American Society of Heating, Refrigerating and Air Conditioning Engineers’ guidelines for buildings in hot and humid climates.

The last time our outdoor dew points in south Mississippi dropped below the middle 60s was back on May 10th and 11th.

The dry air mass that arrived yesterday had dew points in the lower 60s, dry enough to make afternoon highs in the 90s more bearable.

As is typical for early June, our respite from mugginess will be short-lived.

National Weather Service forecasters expect dew points to rebound into the upper 60s by Wednesday.

With the old, weakening frontal boundary nearby, and with plenty of daytime heating, there will continue to be a chance of widely scattered to scattered showers and thunderstorms each day and early evening.