First heat advisory of summer issued

Published 7:00 am Saturday, June 22, 2019

By Skip Rigney

In case you needed proof that summer is in full swing, this past Thursday saw the first heat advisory of 2019 issued by the National Weather Service Forecast Office in Slidell. The advisory began, “Oppressive heat developing. A very humid environment is in place and this will combine with temperatures approaching the lower and possibly even mid 90s. The combination of the warm temperatures and humid air will lead to heat index readings near or greater than 105 degrees in some locations.”

Thursday’s advisory covered not only Pearl River County, but also the three coastal Mississippi counties and all of south Louisiana. It connected geographically to heat advisories issued by National Weather Service Forecast Offices in Texas and Oklahoma.

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The heat advisory might be dropped this weekend if afternoon temperatures drop the degree or two needed to fall underneath the 105 degree heat index threshold. Even so, expect generally sweltering conditions to persist due to the continuation of high pressure in the atmosphere above us and humid southerly flow from the Gulf of Mexico.

The heat index is a combined measure of air temperature and humidity. Humidity is the general term for how much water vapor is in the air. The more water vapor, the less quickly and efficiently perspiration evaporates from our skin, and the more likely we are to feel hot and sticky.

Relative humidity is not the best measure of how muggy the air feels. Relative humidity is a measure of how much water vapor is in the air “relative” to the maximum amount of water vapor that air at that temperature can hold.

Warm air can hold more water vapor than cool air. For example, air with a temperature of 90 degrees and relative humidity of 50 percent actually contains over double the amount of water vapor contained in air with a temperature of 65 degrees and 50 percent relative humidity.

That’s why meteorologists prefer to use something called the dew point temperature to convey how moist the air is and how muggy it feels. The dew point is the temperature to which the air would have to be cooled in order for that air to become saturated.

The higher the dew point, the more water vapor is in the air.

What is a comfortable dew point? That depends on what you’re used to, but when dew points rise above 65 degrees many folks, especially in generally less humid parts of the country, begin to complain about the humidity.

Once dew points get into the 70s, you find words like “extremely uncomfortable” and “oppressive.”

If you have spent much time during the summer elsewhere, you soon realize that we are indeed in the major leagues of humidity. Over the past several days dew point temperatures across the Gulf Coast region have been in the middle 70s, very typical readings for the summer here.

How wonderful that air conditioning lowers both the temperature and dew point of the air. How ironic that Willis Carrier invented air conditioning in Buffalo, New York, where the typical June dew points are in the comfortably dry 50s.