A huge plume of dust and sand, blown by the wind from the Sahara Desert, has finally reached the U.S. mainland. It's one of the most significant Saharan dust events in decades, forecasters said.

A ‘HISTORIC’ outbreak of Saharan dust that is clogging the atmosphere from western Africa to the Caribbean and potentially posing health threats could make it as far as the U.S. Midwest.

While the phenomenon is an annual occurrence, “This dust episode has a huge spatial extent,” said Andrea Sealy, a meteorologist with the Caribbean Institute for Meteorology and Hydrology in Barbados.

And for the time being at least, it is yielding a collateral benefit to property owners on the Gulf and Atlantic Coast, and to U.S. taxpayers. All that dust suppresses storms in the subtropical Atlantic, one of the world’s most productive hurricane-spawning grounds.

This particular outbreak could well be among the largest of the last 60 years, said Frank D. Marks, director of the National Oceanic and Atmospheric Administration’s Hurricane Research Division in Miami, and who is familiar with the orange-tinted fallout that has descended upon his car in years past.

He described the outbreak that started last week as a “very opaque or dense plume.” It is formally known as the Saharan Air Layer, or SAL, but it isn’t all sand.

So what is this stuff?

The primary source is the Sahara Desert, which is roughly the size of the 48 contiguous states and as one might expect it is covered by unimaginable amounts of sand. (Think of about two million Wildwood beaches.)

If there is such a thing as a sand harvest, Sahara has one at the end of the dry winter, said Marks. In June, monsoon winds carry sand and particulate matter into the atmosphere where it is transported westward by upper-air winds, generally in three- to five-day intervals.

Police officers wearing masks as a precaution against the spread of the new coronavirus walk at dusk under a cloud of a Sahara dust hanging in the air in Havana on Wednesday,

By taking the high road, the dust can avoid the chaotic currents near the surface where we live and breathe, he said, adding that some of the larger particles won’t make the entire 4,500-mile trip, give or take a few hundred miles.

The heaviest concentrations typically are in a 2- to 2.5-mile layer that begins about a mile above the surface, said Jason Dunion, a SAL specialist with NOAA.

The aerosol ingredients can be eclectic, says Andrea Sealy, whose institute is affiliated with the World Meteorological Organization. She said studies have been looking at what else is up there making the trips, “such as various bacteria” and “camel dung; live locusts have survived the journey in the past as well.”

The bad

Sealy said the fine particles do present potential health hazards.

They can exacerbate respiratory illnesses such as asthma, and one study based on dust samples collected in Senegal measured bacteria.

Given that the dust interferes with sunlight, it can reduce the natural supplies to solar power plants.

Dust also has potential to be an aircraft hazard by reducing visibility, and by accumulating on plane surfaces and damaging engines, she said.

The good

The Saharan Air Layer is an important factor in delaying the peak hurricane season until later in the summer into September, said Marks.

While the Atlantic hurricane season, which began officially on June 1, is off to a robust start, with four named storms already, Marks pointed out that they were all short-lived and not in a league with the larger and more-potent systems that tend to develop off the coast of West Africa.

SAL’s extremely dry air deprives potential hurricanes of moisture, and its winds increase “vertical wind shear” that depress the growth of incipient storm, Sealy said.

The dust also helps to fertilize the vast Amazon rainforest and build beaches across the Caribbean, she said.

It also has an aesthetic benefit: Some folks are being treated to amazing sunsets.

(Compiled by Phildelphia Inquirer)

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