En route to the continental US, the plume struck Puerto Rico on Saturday, cutting visibility down to 3 miles. It’s the worst Saharan dust event the island has seen in 15, maybe 20 years, says Olga Mayol-Bracero, an atmospheric chemist at the University of Puerto Rico. Her air-analyzing instruments were working in real time, detecting the component elements of the desert dust. “We were quite surprised, seeing such high values for all these different parameters—we had never seen that,” Mayol-Bracero says. “So it was quite shocking.”
How does Saharan dust make it all the way across an ocean? It’s a lesson in atmospheric science.Because it’s a desert, the Sahara is loaded with particulate matter, from coarse sand down to the tiniest of dirt specks, none of which is very well anchored to the ground. By contrast, the lush rainforests to the south of the Sahara have trees that both block the wind and hold on to the soil with their roots, keeping all the muck from taking to the air. The conflict between these two atmospheric regions is what births the plumes that blow clear across the Atlantic.
You might assume a film with more surface area would travel farther than a fragment, but that just hasn’t been tested.“That's one of the challenges moving forward is trying to actually model how these plastics move in 3D in the air, so we can figure out where they come from,” says environmental pollution scientist Deonie Allen of the EcoLab, part of the National Center of Scientific Research for France, coauthor on a new paper in Nature Geoscience.
The Sahara is notoriously dry and hot. But down south, around the Gulf of Guinea, it’s much cooler and wetter, on account of its proximity to the equator. “The setup between those two—the hot to the north and the cool, moist to the south—sets up a wind circulation that can become very strong, and it can actually scour the surface of the desert,” says Steven Miller, deputy director of the Cooperative Institute for Research in the Atmosphere at Colorado State University, which is monitoring the plumes. (You can watch the dust’s progress from a satellite with this neat tool. Look for the gray clouds on the map.)