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WEATHER U: Dustup in the tropics

The satellite image from June 24 of this year shows a shield of dust sweeping off the western Sahara.  It became the worst dust storm in the Caribbean in decades, changing the tropical blue skies into a hazy brown-grey colour.
The satellite image from June 24 of this year shows a shield of dust sweeping off the western Sahara. It became the worst dust storm in the Caribbean in decades, changing the tropical blue skies into a hazy brown-grey colour. - contributed

I love that so many of you are curious about the weather.

Not just the weather in your backyard, but the big picture. I found this great question in my mailbox:

"Hi Cindy, last week at this time there were half a dozen disturbances in our Atlantic area of interest. This week there are none. I noted a large cloud of sand off the west coast of Africa moving westward. Is that cloud a major factor in reduced activity? Thank you. Cheers, Stew Russell”

That's a great observation. That plume of dust that you spotted on the satellite image has a name: it’s called the"Saharan Air Layer" or SAL.  It's huge and can grow to be the size of the continental U.S. Every three to five days during the summertime, these storms roll off the African coast.

A dust storm has three main components that can suppress a hurricane. The first is very dry air. Hurricanes don’t like dry air in the middle parts of the atmosphere, and that’s exactly what the Saharan Air Layer has. It extends upward between 5,000 and 20,000 feet and is about 50 per cent drier than tropical air.

A Saharan dust storm also has a very strong surge of air embedded within it that creates vertical wind shear and can rip a developing storm apart.

And finally - dust. Researchers think the dust itself suppresses cloud formation, playing a role in preventing tropical waves from becoming more intense. Dust inhibits convection, the process of moisture rising to the higher levels of the atmosphere, and then precipitating as rain. So the Saharan dust layer has a blanketing influence on the development of convection.

In addition to the direct impacts of the Saharan Air Layer on hurricane activity, the SAL can impede storm development indirectly through its impact on SSTs or sea surface temperatures. The SAL contains large amounts of dust that reflect incoming solar radiation which causessea surface temperatures to cool, reducing the amount of energy available and impacting the likelihood for storms to develop into hurricanes.

It is difficult to anticipate when the SAL will move over the Atlantic or how extensive it will be when it does. When it moves over the ocean, however, its progress can be tracked. Infrared satellite imagery can show the current spatial location of the SAL.

So to answer the question, yes it does. Depending on the timing and the interaction with the SAL, the presence of the large plume of sand can inhibit a storm from developing.


Cindy Day is the chief meteorologist for SaltWire Network

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