Why spring breezes disappear during summer

Published 7:00 am Saturday, May 16, 2020

By Skip Rigney

There’s nothing I find more relaxing than grabbing a good book in the late afternoon on a sunny, mild spring day, dragging my lawn chair into a shady spot, and reading while the breeze blows through the oak trees overhead.
Over the past month the weather has cooperated to give me plenty of opportunities for just that kind of relaxation. But, those days will be coming to an end over the next few weeks.
We usually associate the transition from spring to summer with rising temperature and humidity levels. Less noticed, but just as real, is the decrease in wind speeds in south Mississippi as we enter the last half of May and move into June.
Winds are caused as air moves from areas of high atmospheric pressure to areas of lower pressure. In the northern hemisphere, the earth’s rotation causes the winds to curve in a counterclockwise direction around centers of low pressure, clockwise around highs. Differences in atmospheric pressure over the United States tend to be greatest during winter and spring as low pressure storm systems tend to be stronger and track much further south than they do in summer and early fall.
This past Wednesday a strong high pressure system was centered just off the North Carolina coast. At the same time, a center of low pressure was developing over Kansas.
Measured in units of pressure called millibars, the difference in pressure between the two locations was nearly 30 millibars. South Mississippi was about halfway between the high and the low in a moderate breeze as winds blew from the south at 10 to 20 miles per hour.
Contrast that to just a few days earlier on Sunday. The pressure for 500 miles in any direction from us changed less than 10 millibars.
That made for a nearly calm day last Sunday with wind speeds in our area generally five miles per hour or less.
As we enter summer, the tracks of strong low pressure systems will retreat northward into Canada. Our weather will tend to be dominated by sprawling, gigantic high pressure systems in which the pressure changes are quite small over large distances.
That change in pressure patterns explains why we essentially have two seasons here in south Mississippi when it comes to wind speed. The breezy season usually lasts from November into May. A much lighter wind regime takes hold in May or early June and lasts through most of September.
However, we’re not quite there yet. A low pressure system is developing over Texas today and moving toward us on Sunday. The pressure differences across our area will make for a pretty breezy weekend. Behind the low, pressures will rise, and northerly winds will bring slightly cooler, drier air into our area Tuesday and Wednesday.
So, this week get outside and enjoy some of the last mild breezes of spring.
It won’t be long until your best chances of hearing the wind blowing through the leaves will come from the outflow of a summer afternoon thunderstorm rather than the pressure differences between high and low pressure systems.

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