Transition to summer means higher humidity

Published 7:00 am Tuesday, May 16, 2017

By Skip Rigney

The transition from spring to summer is well underway this week across the Deep South. Here in Pearl River County that means warm days and higher humidity.

No rain is expected today or Wednesday. Strong high pressure in the middle and upper levels of the atmosphere above the eastern one-third of the U.S. will cause the air above us to slowly sink. This sinking air, called subsidence by meteorologists, puts a lid on any showers that start to bubble up from the lower atmosphere.

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By Thursday the upper ridge of high pressure and its associated subsidence is predicted by the computer weather models to move to our east. That will allow a few isolated thunderstorms to pop up over south Mississippi because of daytime heating each afternoon Thursday through Saturday.

The models also predict that the surface weather maps this week for our region will resemble what we see many days during the summer. A broad ridge of high pressure will extend westward from its center over the western Atlantic into the southeastern U.S. and the Gulf of Mexico.

By this time in May, cool fronts to our north and west become very slow movers. That will be the case this week. A cool front that is in Kansas today will inch its way toward us before finally becoming stationary near us by Sunday or Monday increasing our chance of showers on those days.

Meanwhile our location on the western edge of the western Atlantic high-pressure system means that throughout the week we will have winds from the southeast as the air circulates in a clockwise direction around the high.

For us, a southeasterly breeze means that the air around us was over the Gulf of Mexico just a few hours ago.

While over the Gulf, water evaporates into the air from the sea surface. The higher the sea surface temperature the more water evaporates. The longer a blob of air stays over the ocean surface, the more water vapor will be loaded into the air, which also means the higher the dew point temperature of the air becomes.

Meteorologists often use dew point rather than relative humidity as an indicator of how muggy the air feels. This is because dew point does not change even when the temperature of the air changes.

Currently sea surface temperatures in the Gulf of Mexico are in the low 70s from the Mississippi and Alabama coasts offshore to about one hundred miles. Further offshore in deeper water the sea surface temperatures increase until they reach a maximum of near 80 degrees over the southeastern Gulf.

The dew point temperature of the air won’t be quite that high because drier air from above is continuously mixing into the air near the surface. But our dew points will climb to near 70 degrees this week. For comparison, over the weekend our dew points were in the lower to middle 60s. Many people, especially at the beginning of summer after months of lower dew points, begin noticing the humidity when dew points climb above about 65 degrees. 

Actual temperatures usually fall each night to near the dew point temperature, so expect early morning lows to be in the upper 60s this week. Afternoon highs will be in the middle to upper 80s.