“Waves of Saharan dust will continue to fly over Florida, but will it bring ‘dirty rain’?” That’s the lead-in question in a USA TODAY article titled “What is ‘dirty rain’? How Saharan dust is impacting the weather in Florida.”
In that July 24 story, author Julia Gomez explained that Florida was seeing plumes of Saharan dust and would continue to see them every three to five days. Such dust brought the possibility of “dirty rain.” Some Floridians expected to see dirty rain at July’s end. South Florida, including Miami and Fort Lauderdale, already saw dirty rain showers, and areas in the panhandle could be next.
“It’s just dust,” said Alex DaSilva, AccuWeather’s lead hurricane forecaster. “It won’t hurt you.”
Saharan dust is just dust (and sand) from Africa, transported via wind around this time every year.
Trade winds pick up Saharan dust, lift it into the atmosphere and bring it across the Atlantic. June and July are the months that bring the most dust. It’s not a new phenomenon, but people talk about it more because meteorologists can better predict when the dust will arrive.
Saharan dust can influence how storms form in the tropics, according to DaSilva. He said a warm, moist environment is crucial for creating storms. But the dust acts like a silica packet found in food packages and sucks out any moisture that might be in the atmosphere.
“What happens with the dust is it cuts off the moisture,” he said. “It basically can choke off these systems because, again, they want plenty of moisture, and when you’re taking the moisture away, it makes it harder for thunderstorms to develop.”
Coincidentally, such dust most likely appears during a period when, normally, hurricane season is its calmest. An overpowering exception to that statement manifested itself as Hurricane Beryl, which formed in June and proved to be is the earliest Category 5 hurricane ever recorded in the Atlantic, according to NOAA (National Oceanic & Atmospheric Administration).
Two terms, conspicuous by their absence in the dialogue of these Florida forecasters, were “Saharan air layer” (SAL) and “fertilizer.” I first learned about SAL about 3.5 years ago when one version of SAL blanketed much of Europe when sand was blown their way from the Sahara Desert across the Mediterranean.
On Feb. 8, 2021, BBC journalist Steve Brown wrote, “Skies became orange in certain parts of southern Europe after winds carrying Saharan sands swept through. Though the phenomenon happens every year, regional meteorologists said the winds were stronger that time, with a greater volume of sand particles in the air.”
Clouds of dust from the Sahara had blown over to Europe late the previous week, leaving skies and ski slopes dark tan in several countries. When the dust got wet, it was no longer orange.
Brown further explained that it was common for strong southerly winds to carry dust from the vast African desert during winter. But such a climate event taking place then was unusually intense, as these dust clouds changed central Spain’s ski slopes from white to dark tan.
Now let me work into a blessing accompanying this Saharan fallout. Parts of the U.S. got a dose of it a few months before Switzerland got dusted. In late June 2020, a Saharan dust plume reached North Carolina and Richmond, VA. Skies were occasionally hazy then, with airborne dust particles irritating eyes and respiratory systems, according to the National Weather Service. NWS urged folks with respiratory and/or cardiac issues, older adults and children to reduce prolonged or heavy outdoor exertion. That was bad news.
Here’s good news: Few crop growers know about a soil fertility source called SAL. SAL is a layer of dry, dusty Saharan air that billows up over low-level moist air blanketing the tropical Atlantic Ocean. According to NOAA, at the boundary between the SAL and the low-level moist air – where the trade winds blow – is what climatologists call the trade wind inversion. This is a region of the atmosphere where the air temperature increases with altitude.
Normally, atmospheric temperature decreases with height; this inversion effectively puts brakes on thunderstorms trying to grow vertically. Braking action occurs because air in a thunderstorm’s updraft encounters a zone where updraft air is cooler and thus less buoyant than surrounding air, where trade winds blow. This nips fledgling thunderstorms in the bud.
When the SAL-borne cloud of dust takes a right (northwesterly) turn after departing Africa, it shades the southern regions of the North Atlantic.
These dust shadows reduce solar radiation hitting the ocean surface, thus reducing sea water warming, robbing energy required to form hurricanes. If this steady, roughly 3,000-mile-long dust cloud maintains its heading — now west-by-southwest – it ends up in Brazil’s Amazon Basin. Occasionally this dusty plume deviates far enough north to coat parts of Southeast U.S. with African fines. Borne by precipitation common in the tropics and subtropics, nutrient-laden dust settles in the jungle, on cars in Miami or even as far north as Richmond.
The most significant SAL-stowed away nutrient is phosphorus (P). Such dust, airlifted from the Sahara to the Americas, is visible in satellite images. Such dust feeds Brazil’s rainforests – often referred to as the lungs of our planet – just enough to replace many nutrients lost there. Scientists are able to determine how much P gets wafted from one of Earth’s most desolate places to one of its most fertile. Phosphorus updrafted from Saharan sands originated from the desert’s history as a lakebed.
Quoting scientists at NASA’s Goddard Space Center, “This transcontinental journey of dust is important because of what is in the dust picked up from the Bodélé Depression in Chad, an ancient lakebed where rock minerals composed of dead microorganisms are loaded with phosphorus.”
Funny that the Floridian weather reporters didn’t mention that at all in their July 2024 “dirty rain” coverage.
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