African dust adds to our summer haze
Published 7:00 am Saturday, June 29, 2019
By Skip Rigney
If you thought that the sky was unusually hazy this past week, you weren’t imagining things. Dust blown across the Atlantic Ocean all the way from Africa was the culprit.
Hazy skies are quite common during summer. Stephen Corfidi of the National Weather Service explores the usual causes of this haze in an online article entitled, “Those Hazy Days of Summer: Haze over the Central and Eastern United States.” www.spc.noaa.gov/publications/corfidi/hazeintro.html
According to Corfidi, usually what we see in summer is a “wet haze,” caused by high humidity interacting with a form of air pollution, namely particles formed from sulfur dioxide.
However, most of the haze that has hung above us this week was “dry haze” caused by dust blown into the air last week in the Saharan Desert in north Africa. Prevailing easterly trade winds then blew it off the African continent eastward above the Atlantic Ocean. As the dust neared North America, the wind flow became more southeasterly pushing the dust over the Gulf states of Florida, Alabama, Mississippi, Louisiana, and Texas.
Most of the African dust that was in the atmosphere over our area this week has now either settled out of the air or spread over a large area to our north and west. However, satellite imagery shows that another large dust cloud has been exiting the west coast of Africa over the last several days. It’s not uncommon for a few of these dust clouds to stay aloft and reach the Gulf Coast each summer.
Depending on the sizes of the dust particles suspended in the air, African dust can make for some interesting colors at sunrise and sunset, although I confess I didn’t notice anything unusually pretty on Wednesday. As for Thursday, it was raining too hard at my house for me to see the sun set.
One of the benefits of Saharan dust layers is that the dry air and strong winds aloft, which are associated with these layers as they cross the Atlantic Ocean, inhibit tropical cyclone development. So, it’s not surprising that the National Hurricane Center does not expect any tropical cyclone development this weekend nor early in the coming week. Given the next blob of dust coming off of Africa, the tropical Atlantic is likely to remain quiet even longer.
A weak disturbance in the atmosphere slipped southward out of Alabama into the northern Gulf of Mexico on Friday. This disturbance will combine with the usual daytime summer heating to bubble up numerous afternoon showers and thunderstorms across southeast Mississippi this weekend. Most hours of each weekend day will be dry, but National Weather Service forecasters in Slidell say it is likely that those of us in Pearl River County will see one or more afternoon showers and thunderstorms on Saturday.
Sunday’s afternoon rain chances decrease slightly to about 50-50 as the disturbance weakens and moves off into the western Gulf. As high pressure reintensifies over the southeastern United States, forecasters expect afternoon showers will be few and far between. Most of us won’t see any from Monday through the Fourth of July. Of course, fewer showers also means less clouds and hotter afternoons with high temperatures expected to creep back into the middle 90s. In other words, typical Independence Day heat.