You’ve probably noticed it’s been a hazy summer in Austin. And you may have heard that's because of massive clouds of dust blown across the ocean from Africa. That fact alone inspires awe.
But it turns out there is much more to these dust clouds than the distance they travel.
They Contain The Remains Of Ancient Life
Around 6,000 years ago, North Africa was a wetter place. Lakes covered much of what is now the Sahara Desert, and in them lived microscopic creatures called "diatoms." When the lakes dried, the remnants of those creatures remained.
The diatoms became “this whitish powdery chalky stuff” on the desert floor, says Charlie Zender, who studies atmospheric physics at the University of California, Irvine.
Zender says that “chalky stuff” is rich in minerals and so small that it’s really good at traveling on the wind. In the Sahara, there’s plenty of wind, so every summer storms kick thousands of tons of the dust into the atmosphere.
They Can Stop Hurricanes From Forming
Once in the air, the long-deceased diatoms and everything else begins heading west toward the Americas on a wind pattern called a “tropical wave.”
“You can think of it as a cake layer between about 1 and 3 miles above the [earth’s] surface. So, it’s in a 2-mile-thick layer as it travels across the ocean,” says Jason Dunion, a research meteorologist with the National Oceanic and Atmospheric Association.
On the way, Zender says, the dust cloud drops minerals to the earth, fertilizing the ocean. It also stops hurricanes from forming, Dunion says.
“The dry air, the very strong winds and the warming that we get in these dust outbreaks … all act to suppress the hurricane activity,” he says.
They stop storms from forming on land, too. Since storms are the main thing that helps cool Texas in the summer, the dust’s presence adds to the intense heat.
They Reflect The Sunlight And Cool The Earth
The dust clouds increase the heat on land by absorbing sunlight and discouraging storms, but over the ocean, Zender says, they cool the atmosphere by reflecting sunlight back into space.
“In the net, this dust helps cool the planet a bit, just a little, but every bit of cooling we can get to offset global warming gasses is helpful,” he says.
While some researchers speculate that global warming could increase the intensity of African dust clouds in some parts of the world, Dunion says it’s still unclear how the phenomenon will be affected.
Climate change could actually decrease the dust clouds, he says, by reducing the occurrence of the “tropical waves” on which they travel.
They Make Sunsets Prettier But Cause Asthma
The shape of the dust particles has at least one benefit: They can make the sky look pretty.
“The way they’re shaped, they tend to scatter the sunlight more in the morning and in the afternoon,” Dunion says. "It’s really that sunrise/sunset time of day that you really can appreciate it overhead.”
But there's also one big drawback: These fine particles can enter your lungs and increase symptoms for people with asthma.
They Fertilize The Earth
Perhaps the most important role the dust plays happens when it’s not heading to Texas, but in the winter when it visits South America. That’s when the nutrient-rich remains of those long-dead microorganisms fertilize the Amazon rainforest, Zender says.
“Great Mother Africa still sort of feeds its former child South America with nutrients from the African desert,” he says.
That’s going to start happening soon, as our days get shorter and the sun moves the waves of dusty wind away from us and to the south.