The most far-reaching and potent outbreak of dust from the Sahara Desert in decades has begun to significantly affect the Lower 48 states, with air quality deteriorating markedly on the Gulf Coast on Friday. The dust, which hitched a ride along a ribbon of east-to-west winds about 5,000 miles from the Atlantic coast of Senegal and Mauritania, contains enough small particles at low altitudes to make air quality unhealthy, particularly for those with preexisting medical conditions, such as lung and heart ailments and asthma.
In Texas, which is battling a sustained increase in cases of the novel coronavirus, the simultaneous decline in air quality in highly populated areas such as Houston could add to the symptoms experienced by those who develop covid-19, the illness caused by the novel coronavirus.
The plume, which first departed Africa on June 14, is part of a phenomenon known as the Saharan Air Layer (SAL) that develops every year around this time off the coast of Africa, but the current one is unusually intense and far-reaching and is setting records.
The SAL typically forms over the Sahara Desert from the late spring into the early fall, and it moves pulses of dust cocooned in areas of dry air out into the tropical North Atlantic every few days.
Hurricane forecasters closely track these outbreaks, because such air masses can prevent incipient storms from forming and choke existing storms, starving them of moisture, until they meet an early demise.
It is common for dust blown from the Sahara in North Africa to reach the Lesser Antilles and Gulf Coast, where it can cause captivating sunsets. But it is rare for such events to dramatically lower visibility and lead to air quality problems, which have occurred with this event.
It’s even rarer still for these events to affect so many areas, from the Leeward Islands to Puerto Rico, Hispaniola, the Gulf Coast and eventually much of the Southeast and possibly even the Mid-Atlantic states.
The U.S. dust forecast
Satellite imagery and surface air quality data show that the dust is probably present in moderate to high concentrations over the western Gulf of Mexico, overspreading much of eastern Texas, parts of Louisiana and Mississippi, and even moving into southern Alabama and the Florida Panhandle.
A number of locations were beginning to report reductions in visibility commensurate with dust-induced haze, but more significant drops are likely as the dust begins to build northward.
Meanwhile, a second wave of dust scooting west over the Atlantic had already drawn its veil over the Leeward Islands early Friday, with Puerto Rico next in line and the Dominican Republic to follow by Saturday.
The wave of dust will probably cover much of the South by late Friday, meaning cities such as Houston; New Orleans; Jackson, Miss.; Birmingham and Huntsville in Alabama; Atlanta; and Jacksonville, Fla., are all likely to see impacts, if they haven’t already.
Even western and central Tennessee could be clipped by the main areas of dust.
The dust particles will pivot clockwise around an area of high pressure over the Gulf of Mexico, and this will cause the largest concentrations to be located across Florida on Saturday — although some dust could linger over East Texas and parts of the South in slightly diminished quantities.
By Sunday, the hazardous air quality threat related to this dust event will be a largely Florida event, with the dust eventually emerging off the Atlantic coastline. Smaller amounts of dust, likely at higher altitudes, may sneak north and northeast, into parts of the Midwest, Tennessee Valley and Mid-Atlantic.
After that, the forecast gets a bit more uncertain.
Although the first wave will have exited the Lower 48, models are showing that the second batch of dust — which may arrive in Puerto Rico on Friday night and by Monday encompass Cuba, the Dominican Republic and the Caribbean once again — could congeal into a sandy bowling ball that aims toward the western Gulf or even parts of South Texas and adjacent Mexico in about a week’s time.
Rain over parts of Texas and Louisiana on Friday bordered the plume of dust, which means there is an outside chance that downpours Friday afternoon could tap into some of the dust at the middle layers, with the rain scavenging it out of the atmosphere in the form of wet, sandy droplets.
It’s not out of the question that a few residents of the Gulf Coast might notice some sand on car hoods or windshields as well.
While the Saharan Air Layer will act to put a damper on the typical afternoon sea breeze storms that line the Gulf Coast like a necklace this time of year, any that manage to get going could potentially feature acutely sandy rain.
In addition, reduced visibility and a dusky, hazy overcast are possible beneath the thickest pockets of the dust cloud, which will make for gray sunrises and sunsets in most areas.
When the dust is in small concentrations and higher altitudes, it can result in spectacular sunsets. However, that’s not likely to be widespread this time, as the dust is so thick that it could even reflect and scatter enough sunlight to result in a 1- or 2-degree decrease in temperatures below what weather models are forecasting in some areas, thanks to the scattered solar radiation.
Air quality concerns
The Texas Commission on Environmental Quality was warning residents that air quality could become unhealthy for some Friday and especially Saturday, when the densest plume of Saharan dust was forecast to move overhead. “Heavy amounts of African dust will continue to expand across most of the state with the exception of Far West Texas and the Upper Panhandle,” wrote the agency.
“Overall, depending on the intensity and coverage of the intense African dust and continuing wildfire smoke, [air quality indexes are] forecast to reach the lower to middle end of the ‘unhealthy for sensitive groups’ range.”
Austin, Dallas-Fort Worth, Houston and San Antonio were all anticipated to be in this category by Saturday.
In Corpus Christi, particulate matter jumped into the unhealthy to very unhealthy categories due to dust. Unhealthy air was present over nearly the entire Houston-Galveston metro area Friday morning. In fact, poor air quality was evident along the entire Gulf Coast, with a secondary maximum of unhealthy conditions in the Florida Panhandle near Tallahassee.
At the Westlake site in Louisiana, near Lake Charles, the presence of PM25, or slightly larger aerosol particulates, spiked overnight, tripling between midnight and 8 a.m. That has bumped the air quality index into the downright unhealthy category.
Elsewhere, the air quality had degraded into the “unhealthy for sensitive groups” categories in Kenner, La., and was “moderate” in New Orleans, Baton Rouge, Lafayette and Shreveport.
The origins of the dust and historical perspective
Every year, 2 billion tons of dust enter the atmosphere, reports the World Meteorological Organization, a U.N. agency. Most of it stems from natural processes, but some is the result of poor land management.
In this case, towering thunderstorms over the Sahel during early to mid-June produced pulses of wind that surged northward into the Sahara, kicking up dust as high as 20,000 feet, according to Jason Dunion, a hurricane researcher at the National Oceanic and Atmospheric Administration.
In a year that has so far featured a devastating pandemic that’s nowhere near over; social media hysteria over an invasion of “murder hornets”; and “zombie fires” in Siberia along with a 100.4-degree temperature reading above the Arctic Circle, a huge cloud of dust arriving in the United States has been greeted on social media as being oddly in keeping with recent trends.
The ongoing dust event stands out in the context of modern record-keeping, particularly when it comes to how much it has reduced both visibility and air quality. Typically, Saharan dust events involve dust particles that are at higher altitudes, whereas with this one, many particles have blown in at altitudes of just a few thousand feet, with some near ground level.
As the first wave of dust passed through Martinique, Guadeloupe and Puerto Rico, agencies on each island classified air quality levels in the “hazardous” category with record values of particulate matter known as PM10 — which can penetrate into the lungs and cause illness, including respiratory problems.
At the peak of the event, Martinique and Guadeloupe reported PM10 concentrations higher than 400 micrograms per cubic meter (µg/m3). For reference, the U.S. Environmental Protection Agency has set a 24-hour PM10 threshold of 150µg/m3, above which is considered unhealthy.
Puerto Rico recorded PM10 concentrations higher than 500 µg/m3, the highest observed in 20 years of data, the WMO stated.
In addition, Aerosol Optical Depth, a metric that helps scientists determine how much light the dust scatters within the atmosphere, reducing solar radiation reaching the surface, was also at a record level, according to a statement released Friday by the WMO.
“This is a dust event of truly historic proportions,” said Olga L. Mayol-Bracero of the University of Puerto Rico, and Andrea Sealy, a meteorologist with the Caribbean Institute for Meteorology and Hydrology, in the WMO statement.