This article, first published, Wednesday, was updated Thursday.
An unusually thick, nearly 5,000-mile-long plume of dust that was whisked off the Sahara Desert by storm-related winds June 14 is moving ashore along the coast of the Gulf of Mexico from Texas to the Florida Panhandle. It is predicted to cause a deterioration in air quality and turn the sky a milky white, and its impacts won’t be limited to the coast.
It may even lead to a rare phenomenon of brown rain in some locations while it sweeps across parts of the United States, as water droplets within showers and thunderstorms mingle with the dust particles.
Although plumes of Sahara Desert dust are routinely ejected from Africa’s west coast during June, the ongoing event is extraordinarily rare, scientists said. This event stands out in terms of the dust layer’s thickness, its low altitude and geographic reach, causing a dramatic deterioration in air quality in Puerto Rico, Barbados, Guadeloupe and numerous other locations where records are maintained. It is simultaneously reducing the amount of solar radiation reaching the Earth’s surface.
Saharan Air Layer envelops Gulf of Mexico, reaches Gulf Coast
Since arriving in the Leeward Islands over the weekend, the dust swelled over the Caribbean, including the Virgin Islands, Puerto Rico, Hispaniola, Cuba and the Yucatán Peninsula before sweeping into the Gulf of Mexico on Wednesday.
View of the Sahara sand at 17,000 feet over Cancun, Mexico pic.twitter.com/fWNjOilONi— Jose Vergara (@Pepiny2k) June 24, 2020
On Thursday, it had oozed inland along the Gulf Coast from Texas to the Florida Panhandle before probably affecting a wide swath of the southern United States.
In Baton Rouge, La., and Biloxi, Miss., the sky had turned a milky white and visibility was substantially reduced Thursday afternoon.
Meanwhile, satellite imagery captured additional pulses of dust within the filament that remains in place across the Atlantic like an outstretched arm reaching across the ocean.
Yet another pulse of high dust concentrations which emerged off western Africa on Wednesday morning was over the central tropical Atlantic on Thursday, predicted to arrive in the Lesser Antilles on Thursday night into Friday, enveloping the area in a thick haze for the second time in less than a week. Computer model projections indicate that this trailing plume could also reach the western Gulf in about six days.
An unusual path across more than a dozen states
By Friday, the dust should begin to consolidate over a wide stretch of the Gulf Coast from the Bay of Campeche northward along the Texas shoreline and then eastward across Florida. Texas locales such as Corpus Christi, Houston, and Galveston, as well as New Orleans, Orlando and some inland areas — such as the eastern Dallas suburbs or Jackson, Miss. — could be affected.
The dust occupies even more real estate Saturday, and is forecast to cover much of the Lower Mississippi Valley, with the periphery and less dense part of the plume expanding into areas of the Midwest and Ohio Valley. The densest portion of the dust cloud may hover over the Southeast from Florida to Tennessee.
Cities that may see effects with reduced air quality and lowered visibility include Tallahassee; Jacksonville, Fla.; Nashville; Birmingham, Ala.; Little Rock; Louisville and Atlanta.
Sunday into early next week, the core of the dust is expected to expand across the southern Mid-Atlantic and portions of the Carolinas, while continuing to hover over areas to the south.
In minute concentrations, the presence of Saharan dust can actually enhance the brilliance of sunset colors. But when present in quantities this significant, the dust can have the opposite effect, graying the skies during sunrise and sunset and resulting in a dull, overcast appearance.
In Puerto Rico, forecasters noted that high temperatures in the afternoon peaked a few degrees below what was forecast — since the dust has a tendency to reflect significant amounts of sunlight back to space.
In some areas along the U.S. Gulf Coast and ahead of a cold front set to approach the Tennessee Valley, isolated instances of sandy rain are possible where storm clouds ingest a dusty layer of air. Parts of the United Kingdom experienced such oddities in April when a tendril of Saharan dust snaked its way northward. Residents of the southern and southeastern United States may even notice a bit of dust collect on the hood of their car.
Air quality forecast
Because the dust has made such an extensive journey, only the finest, most lightweight particulates have remained airborne as the mass nears North America. But some of the dust may make it close to the ground, presenting health risks if it is inhaled.
Agencies in the United States are anticipating degraded air quality associated with the dust. The Texas Commission on Environmental Quality is informing residents of the concern. “[H]eavy amounts of African dust will continue surging rapidly into and through the state from south to north, impacting most parts of the state with the exception of Far West Texas and the Panhandle,” the agency stated in an online forecast discussion. It predicted air quality levels to range from moderate to unhealthy for sensitive groups through the weekend, with the worst conditions Saturday.
The most sensitive groups to particle pollution include those with respiratory problems and older people, including those with asthma and covid-19, the disease caused by the novel coronavirus.
Rivaling the ‘most extreme events of the past’
As the core of the dust plume swept over the Caribbean, Martinique, Guadeloupe and Puerto Rico experienced concentrations of small particulate matter, known as PM10, that soared higher than 400 ug/m3, according to Olga Mayol-Bracero, a researcher at the University of Puerto Rico who is leading a research campaign on dust sampling.
Puerto Rico’s air quality had a “hazardous” rating Tuesday, with a PM10 measurement of greater than 425 ug/m3. The EPA has a 24-hour standard for PM10 of 150 ug/m3, beyond which air quality would be considered unhealthy. Visibility in Puerto Rico and other islands was reduced to three miles because of the dust, which is unusual for these plumes since the highest dust concentrations typically reside higher in the atmosphere.
Aerosol scattering values, showing how dust is scattering the sunlight, reached the highest levels since observations began in Puerto Rico more than two decades ago, Mayol-Bracero said via an email from a colleague.
According to Joe Prospero, a professor emeritus at the University of Miami, observations taken by a station at Ragged Point, Barbados, which is part of a NASA air quality observation network known as AERONET, found that aerosol optical depth values, which show how much light is scattered by the dust, spiked to as high as 1.8 in recent days.
“These values exceed by substantial margins all previous measurements made at this site,” Prospero said in an email to colleagues, noting that data at this site goes back to 1996.
The impacts associated with such readings are significant.
According to Paquita Zuidema, a professor of atmospheric sciences at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, “An optical depth of 1.5 reduces the direct sunlight reaching the surface to almost 20 percent of what it would be without the dust.” So, readings above that would block even more light from reaching the surface.
Prospero also examined data from air quality measuring stations in Martinique, located 443 miles to the southeast of Puerto Rico, and found that PM10 concentrations rose as high as 400 ug/m3. This is historically significant for that location.
“I have perused our long term filter data-base and I find that since the early 1970s the daily dust concentration exceeds 200 ug/m3 only on four days,” Prospero said in an email. “Two of these days were during the intense African drought of the early-mid 1980s. Thus, on the basis of dust concentration, the concentrations of PM10 in this event is comparable to, or exceeds, the most extreme events in the past.”
Suppressing hurricanes is the silver lining
Although the dust may present some unique challenges, it’s not all bad news — the dust may actually help to put a damper on tropical cyclone activity during the coming weeks. It’s not the dust per se, but rather the hot, dry layer of desert air with strong winds that envelopes the dust.
Such air masses can prevent tropical storms from forming and can even block existing hurricanes by choking them off from their moisture supply and inhibiting thunderstorm growth. That all means that, after a hyperactive start, hurricane season may remain suspiciously quiet for a little while. Much of July should feature unfavorable conditions and broad sinking air across the Atlantic anyway, highlighting a protracted period of potentially below-average conditions.
But that pendulum could abruptly switch by August.