Dust from the far-off Sahara desert can have some bizarre effects on weather in the United States, ranging from heavy "dirty rain" to changes in hurricane formation.
Saharan dust blown from Africa across the Atlantic Ocean can mix with moist air and prevent hurricanes from forming, according to a new paper in the journal Science Advances.
This dust is also responsible for the "dirty rain" set to drench Florida over the coming days.
Around 182 million tons of dust are blown out of the Sahara every year, according to NASA, and are swept across the Atlantic toward North and South America. Much of this dust falls to Earth over the Amazon and Caribbean, but sometimes, the plume can make it all the way to Florida and beyond.
According to the paper, this Saharan dust suppresses the formation of hurricanes over the ocean but can result in heavier rainfall from those that do form—up to a point.
"Surprisingly, the leading factor controlling hurricane precipitation is not, as traditionally thought, sea surface temperature or humidity in the atmosphere. Instead, it's Sahara dust," study co-author Yuan Wang, an assistant professor of Earth system science at the Stanford Doerr School of Sustainability, said in a statement. "A dust particle can make ice clouds form more efficiently in the core of the hurricane, which can produce more precipitation."
The researchers found that while dust makes hurricanes rain more (known as microphysical enhancement), it can also block radiation from the sun, cooling sea surface temperatures and weakening hurricane formation.
In the paper, the researchers describe how the "dust optical depth"—how much light can pass through a cloud of dust—predicts rainfall. Between dust optical depths of 0.03 and 0.06, rainfall increases, but after this point, it rapidly declines. Therefore, when dust is present in medium amounts, it increases rainfall but decreases rainfall at high concentrations.
"Normally, when dust loading is low, the microphysical enhancement effect is more pronounced. If dust loading is high, it can more efficiently shield [the ocean] surface from sunlight, and what we call the 'radiative suppression effect' will be dominant," Wang said.
Previous research had found that hurricane rainfall is expected to increase with climate change and that less dust may be blown across the Atlantic in the coming years, but none had yet delved into the interplay between hurricane formation and dust.
"Hurricanes are among the most destructive weather phenomena on Earth," said Wang. "For conventional weather predictions, especially hurricane predictions, I don't think dust has received sufficient attention to this point."
As for "dirty rain," Floridians can expect a strange film of dirt to be left on their cars and windows after rainfall due to the dust. This dust can also cause irritation to the respiratory system.
This "dirty rain" poses several challenges for Florida residents. Beyond the inconvenience of dust-coated cars and windows, there are potential health implications, particularly for those with respiratory conditions. The dust can cause eye, nose, and throat irritation due to the fine particles in the air.
"The next round of Saharan dust from Africa has arrived in Miami this morning. It will be extra hazy with lower air quality this weekend," Brandon Orr, a meteorologist at local news WPLG, posted on X, formerly Twitter, on Saturday.
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