Capturing the Calima: How Citizen Scientists Help NASA Study Saharan Dust Storms

The Canary Islands spent a few days of March 2018 shrouded in Saharan dust. Calimas, two-to three-day-long gusts of sand and warm wind named for the haze they cause, reduce visibility to a few hundred feet and leave a film of fine dust when they finally move on. They’re a seasonal hazard, so this was far from the first time it had happened. However, it was the first time NASA had put out an open call for citizen scientists to record the sky above their heads. Undeterred by the hazy view, some anonymous Canary Islanders downloaded NASA’s GLOBE Observer app and began to record. They took photos in each cardinal direction, and—crucially—upwards. They uploaded their observations, rubbed the Saharan grit out of their eyes, and went on with their day.

NASA founded GLOBE, short for Global Learning and Observation to Benefit the Environment, over two decades ago to get school children involved in the scientific process. “That’s really how GLOBE started,” said GLOBE Clouds Project Scientist Dr. Marilé Colón Robles. “How can we teach science as a hands-on, inquiry-based event, rather than just reading it from a book?” 

The project, especially GLOBE Clouds, the division led by Colón Robles and at the time, Dr. Helen Amos, was such a hit that NASA opened up participation to the general public in 2016. Their app, GLOBE Observer, makes your phone the only instrumentation you need. It has four protocols or experiments, and Clouds tops the list. Simply take a few photos of the cloud cover above you, and answer some questions about your location. When you send your observation, NASA pipes it into their database and shoots you an email thanking you for being a GLOBE Observer—one of the approximately 20,000 to 40,000 personalized emails a month to their robust citizen scientist following. 

In March 2018, the GLOBE team was in the middle of the Spring Cloud Challenge. It was a huge celebration of the GLOBE community and garnered over 60,000 observations from 15,000 locations around the world on every single continent. 

When the GLOBE team opened up the dust-draped observations from the Canary Islands, they knew they were looking at something novel. Usually, the app was for tracking static cloud formations, but this sliver of submissions was special. Citizen scientists were using the app to document an ongoing event—specifically, the storm flooding the streets with sand.

The first calima recorded with GLOBE Clouds wasn’t the worst Saharan dust storms have to offer. It reduced visibility on the islands, but an Al-Jazeera article about it called it “a weak affair,” whose haziness did more damage to the Canary Islands’ trademark sunshine than anything else. It did carry serious health risks at its peak, but it passed in a day or so. 

Dust storms like the one that put a dent in the Canaries’ sunshine start in the Sahara. The desert’s winds whip sand into massive, mobile clouds of dust. Some of that dust catapults a mile upward into Saharan Air Layer, or SAL, a 2- to 2.5-mile-thick slice of our atmosphere composed almost exclusively of hot, dry—and crucially, dusty—winds. It’s not a fun place. SAL-borne sand often persists until it reaches the Caribbean and beyond. Recently, scientists have found Saharan dust as far west as Arkansas.

The danger of dust storms comes less from the spectacle or the distance traveled but rather from how they pollute the air. The AQI, or Air Quality Index, gauges air safety by measuring a few pollutants, but with dust storms, mainly two sizes of particle: PM10 and PM2.5. PM stands for “particulate matter,” and the subscript reflects the matter’s size in micrometers. At 10 micrometers, PM10 can irritate lungs and pose a threat to people with respiratory issues. 

Clocking in at 2.5 micrometers, a fiftieth the size of a grain of sand, PM2.5 are so minuscule they can slip into the deeper structures of your lungs or even your bloodstream. They’re a danger to people with cardiovascular or respiratory conditions in small amounts or to anyone in large enough doses. Often, those large doses come after heavier particles drop into the ocean while the dust storm gains speed across the Atlantic, leaving a huge amount of PM10 and PM2.5 as the sand hits the Caribbean.

Puerto Rico is the easternmost major landmass in the Caribbean, and the first chance a dust storm has to make landfall after the Canaries. The island has a history of catching the worst of what the SAL has to offer, as it did in July of 2020. News outlets called the storm that touched down that summer “historic.” Dr. Olga Mayol-Bracero and other locals called it the “Godzilla event.”

“It was like a wave of dust,” said Mayol-Bracero, a professor at the University of Puerto Rico at Rio Piedras. “Just an enormous amount of particles.” 

Colón Robles, who is originally from Puerto Rico and has family living there, says that her aunt and uncle couldn’t step outside for more than a minute before their eyes began to water. 

For reference, the 2018 Calima in the Canaries pushed the AQI to the highest warning, Hazardous, for a day. February’s storm in Puerto Rico had the island at Hazardous levels for four days straight.

“Puerto Rico has always had Saharan dust, but not to the amount that we’ve had this year. That was unique,” said Colón Robles. There are also questions about whether the event was influenced by the changing climate. “Will that happen again? Will that happen more often? That is a serious question to be asked.”

This is not to say that every dust storm is apocalyptic—far from it. SAL-borne dust often carries nutrients that can feed Puerto Rico’s ecosystem and replace phosphorous lost to deforestation in the Amazon Basin. As dust passes over the Atlantic, it can stop solar radiation from heating up the water, deterring hurricane formation. Though every storm has health effects, and once in a blue moon, you’ll get something approaching the Godzilla event.

Colón Robles and Amos were well aware of this, and they knew a novel observation method when they saw it. Citizen scientists had used the Spring Cloud Challenge to send observations from the Canary Islands and Greece and parts of Spain, which also caught some of the dust storms. Already, the GLOBE team had a wealth of something that was often a rarity in dust event studies: data. More specifically, in-situ, on-the-ground observations. 

Before GLOBE Clouds got citizen scientists in on the action, researchers relied almost exclusively on a network of geostationary and polar-orbiting satellites to observe dust events. Those satellites are cutting-edge, and many are in NASA’s own fleet, but they can be foiled by significant cloud cover blocking an event. 

Even NASA’s CALYPSO satellite, which uses a lidar system that’s usually able to create a vertical image of a cloud, can’t pierce cloud cover that’s too tall. As a rule of thumb, Colón Robles says, if an astronaut is unable to see a dust event from space, a satellite may have trouble, too. “They’re great but not perfect, so we’re always looking for these opportunities for when the satellites might have missed something,” she said. “Particularly with dust storms.”

With GLOBE’s geotagged citizen scientist observations, the dust research community had a completely new angle to study their subject and almost immediately reached out to the GLOBE team. Colón Robles and Amos leaned into the opportunity. “To support this, GLOBE issued a call to its community of over 940,000 students, teachers, and citizen scientists to report dust events with the GLOBE Observer app,” they wrote in a retrospective article on the Spring Cloud Challenge. Now, citizen scientist observations are compared with satellite images side-by-side and used to create programs to study these dust events’ origins.

After the Challenge, the GLOBE team added a new prompt in their app to report dust storms. Any new observation of a dust storm is immediately flagged by GLOBE and sorted into a different study database.

Globe Observer App

Opportunities for citizen scientists are popping up outside the Saharan dust arena, too. There’s been an increase in dust storms globally as areas dry up due to cloud patterns disturbed by climate change. Riverbeds in Alaska and increasingly, Colón Robles says, farmlands across the US and elsewhere produce more dust for the wind to whip up into storms. Aside from concerns about harmful dust particles, these storms can give aerosol diseases a far wider reach than they’d usually have. An uptick in Valley Fever in the Southwest US has been credited to dust storms, and the spread of meningitis in North Africa has long been linked with Saharan dust events. 

The thing is, dust storms don’t usually hit Godzilla levels, and they didn’t use to send Saharan dust to Arkansas, either. Whether the changes in intensity and longevity are just flukes or climate change results are still in question. Groups like NASA’s HAQAST (Heath and Air Quality Applied Sciences Team) are already on the case and relying heavily on data from GLOBE Clouds’ citizen scientist corps. 

Though work on it has slowed due to COVID, the GLOBE team is ironing out a way to call the app’s users to action and actively request photos of important events, from dust storms to the smoke in California. “These are unique situations where citizen scientists and their observations are critical,” Colón Robles said. As always, there are plenty of good reasons for citizen scientists to keep their heads in the clouds.