Summer thunderstorms are getting more violent. Is pollution to blame?


Summer thunderstorms are getting more violent. Is something supercharging them?

HOUSTON — At a small municipal airport amid a sea of oil facilities and petrochemical plants east of downtown, Katia Lamer prepared to release a weather balloon. A light rain fell as the Brookhaven National Laboratory research scientist let the large white balloon go, one of four sent up from the site every day — unless there are thunderstorms in the area, when several more are sent skyward.

The balloon carried a radiosonde, a small device that collects data on temperature, humidity, pressure and more on its way toward the stratosphere. Its launch is just one part of a wide-ranging, yearlong study — the Tracking Aerosol Convection Interactions Experiment (TRACER). The project is designed to help scientists better understand how violent convective thunderstorms form — and whether the pollution emitted from fossil fuel and chemical infrastructure, omnipresent in the Houston area, might be making them stronger.

Even though thunderstorms are common, climate scientists and meteorologists still struggle to understand their inner workings, Lamer said. “Convection has the power to dump a lot of water in one place,” she added — and major downpours that can cause flooding have already increased in frequency over the last few decades — so filling in those gaps is key.

One crucial question is just how important aerosols — like the particles emitted from oil and gas refineries and factories, cars and trucks, along with other human and natural sources — really are in determining when a thunderstorm forms or how strong it is. The TRACER project, which began last summer and will run through September, hopes to provide some answers and, in doing so, improve weather forecasts and climate models. Eventually, the data gathered in Houston could improve our overall understanding of the climate, aid in managing increasingly difficult issues like water usage and make people safer when extreme weather hits.


“We have trouble representing those clouds in models,” said Michael Jensen, a meteorologist at Brookhaven National Laboratory and TRACER’s principal investigator. “They’re super important for lots of the things that impact us, from safety in daily thunderstorms or flooding or any number of things.”


Juiced-up storms

On the wet ground beneath the rapidly disappearing weather balloon, a few hundred feet from a runway, a collection of about 20 shipping containers housed an impressive array of scientific instruments. These radars, lidars, radiometers and particle counters all connect back to a data collection system processing terabytes of information every day. And this primary TRACER site at the La Porte Municipal Airport is a perfect spot to measure pollution’s impact on Houston’s stormy subtropical weather.

“Some days we have to change the [air] filters twice a day,” said David Oaks, a technician with the Energy Department, which runs this Atmospheric Radiation Measurement mobile facility. The instruments pick up spikes in various pollutants — ozone, sulfur dioxide and particulate matter. “Some days we get a smell,” Oaks said.

Houston is home to hundreds of energy and related companies with some of the biggest oil refineries in the country nearby. It also accounts for nearly one-half of the country’s petrochemical manufacturing capability. All that industry and the city’s extensive sprawl and traffic-heavy layout produces aerosol particles — lots of them.

All those swirling aerosols may offer up a secret sauce for stronger thunderclouds to take shape.


“Cloud droplets need to form on aerosols,” Jensen said. “They generally form on some particle in the atmosphere” — that could be dust, pollution particles, sea salt or others. The size of the particles that dominate a given cloud play a role in how the droplets move and thus how the cloud evolves.

An image of a truck in front of a sunset.

If a cloud forms around a lot of smaller particles like those from human sources of pollution — rather than, say, sea salt, a much bigger particle — then the water droplets are small and spread out, and don’t collide as much inside the cloud, Jensen said. If they don’t collide, they don’t get big enough to start falling as precipitation, and instead all rise together in the cloud’s updraft. As they get higher, they freeze, a process that releases energy as heat which strengthens the updraft and the power of the storm cloud. And when the precipitation finally does fall, it could release more water, more quickly than it otherwise would.

In other words: A more polluted environment could mean they get even stronger.

“Where the debate comes in is — how important is that process really?” Jensen said. Some previous studies have suggested the answer is very important. For example, one 2018 study of convection in the Amazon published in the journal Science found that small aerosol particles “invigorate” the storms. The authors wrote that this could lead to stronger thunderstorms in “previously pristine regions of the world.”

Other research though hasn’t borne this out — hence TRACER. In La Porte, Texas, after ducking into an unused shipping container to avoid the rain, Chris Cappa, a professor of environmental engineering at the University of California at Davis, reiterated that the TRACER team had picked a perfect spot to study the question. “We use the atmosphere here as a laboratory,” he said.

Jensen thinks the signals will likely emerge to settle the debate, and that some of the wide-ranging TRACER and related observations have already started to bear fruit. “I think we’ll wind up seeing that indeed it is important,” he said.

Flight of the RAAVEN

Two days before the rainy balloon launch, on a hazy late-June morning in a quiet field across from a rice paddy near the Texas Gulf Coast, a raven took flight. Or, more accurately, a RAAVEN.

“Everything’s an acronym,” said Justin Buchli, a research assistant for the University of Colorado Boulder’s Integrated Remote and In-Situ Sensing initiative. The RAAVEN or Robust Autonomous Aerial Vehicle — Endurant Nimble is a fixed-wing unmanned aerial vehicle (UAV) — a drone. More than seven feet from wingtip to wingtip and weighing around 15 pounds with its battery pack and variety of sensors on board, the RAAVEN sat atop a University of Colorado Boulder SUV, attached to a pneumatically powered catapult.

With lab manager Michael Rhodes standing nearby holding a controller and Buchli in the SUV’s back seat eyeing several laptop monitors, postdoctoral researcher Radiance Calmer reached up to a switch on the catapult’s mechanism.

“Three. Two. One. Launch.”


The RAAVEN shot forward releasing from the catapult at the high end and only briefly dipping toward the ground before its propeller and wing flaps angled it skyward. The drone started circling based on its preprogrammed flight path, with Buchli adjusting its altitude from a laptop, per the scientific plan — first circling from 40 meters up to 600 meters (around 2,000 feet, the limit of their Federal Aviation Administration-approved flights), then in “racetracks” back and forth across the fields, eight minutes at 600 meters, then 400, then 250, and so on down to 40 meters again. Then more circling — sampling a vertical “profile” of the air — up to 600 meters and back down. In total, an hour and 40 minutes or so of flying, three times per day.

Image of two people holding a device in a field beneath a canopy

With the RAAVEN aloft, egrets and anhingas strutted and flitted about the field, as dragonflies buzzed around the crew and fire ants wandering the ground threatened to ruin someone’s day. The team was constantly on the lookout for other aircraft that might intrude on their patch of sky. “Do you hear a helicopter?” was a common question. The team monitored flight radar sites as well, correlating any sound of engines with the planes on the screen. At one point, a small plane flew by just to the north executing multiple barrel rolls. Rhodes called the control tower at the George Bush Intercontinental Airport in Houston before and after every flight, as required by the FAA, though he said they never seemed all that interested in the drone’s activities.

The RAAVEN had a sensor on board known by yet another acronym, POPS — portable optical particle counter, which can help characterize the air’s aerosol abundance during each flight. The site worked well to help differentiate the influence of different types of aerosols, Calmer explained. The sea breeze coming in off the gulf introduces one sort of particle, while the industrial sites to the north offer another. “The particles could be sea salt on one day but pollution particles on another,” she said, as the sound of gunshots drifted over from a nearby shooting range.

The drone also sent back detailed atmospheric data. At one point, Buchli called out that it had registered a temperature inversion at around 400 meters, where the temperature rose as the RAAVEN climbed — the opposite of the standard gradient seen as altitudes rise. “We found the boundary layer!” an excited Calmer cried.

Later, Jensen explained that while some parts of TRACER involved actively seeking out thunderstorms using both high-tech, instrument-laden trucks as well as special targeted radar systems that could closely monitor and measure short-lived convective clouds, characterizing what’s happening in the local atmosphere when storms are not present is also important. Once all the data have been collected and the forecasting and climate models adjusted accordingly, the researchers will be able to add or subtract aerosols to those models and see what happens to the formation of storms.


“You can tune, you can change these knobs and play games with it,” Jensen said. Those “games” include changing the particle sizes, the specific chemical composition and more. Then, with a whole year’s worth of data collection behind them, the modeling can be matched up with actual observations. “That’s how we’re going to go about trying to solve that question.”

The thoroughness of the TRACER data collection marks it as unique — this degree of intense scrutiny of the atmosphere in this sort of environment simply hasn’t been undertaken before. In fact, the Energy Department’s Atmospheric Radiation Measurement mobile facility had never been deployed in an urban setting, instead it was sent to places like the Alaska North Slope. Along with the main La Porte Municipal Airport site, the RAAVEN flights and the mobile truck units, other data are collected at an ancillary site in Guy, Texas, to Houston’s southwest, where there is less pollution nearby. The site is on a working ranch, and Jensen said they had to navigate some particularly ornery cows to act as a sort of control experiment. The full team includes scientists from dozens of universities and labs from Columbia University and Rice University to the University of Oxford, NASA and Leipzig University.

“I think it’s fair to say we’ve never had this much detailed information focused on this type of problem,” Jensen said.

Saving lives through forecasting

Yet more of that data arrive from weather balloons released by a team mostly made up of Texas A&M University students, who on one hot day in late June set up shop in the northeast corner of a massive Buc-ee’s parking lot in Waller, Texas, to Houston’s northwest. As the students, led by postdoctoral researcher Montana Etten-Bohm, worked to inflate the balloon and attach its instrumentation, patchy clouds cruised by overhead while a few towering convection clouds bubbled upward in the distance.

The student team — seniors Peyton Langford and Sam Gardner and junior Michelle Mancilla, all meteorology majors — handled the balloon wearing blue latex gloves to avoid transferring oils from their hands to its surface. As the balloon rises past 50,000 feet and higher, the reducing atmospheric pressure causes it to expand, so any weak points on its surface will get thinner and thinner. And if it pops before reaching its target altitude, it’s something of a wasted effort. But as long as they can avoid such disappointments, the balloons will further help clarify the thunderstorm picture.


An image of three people standing on a concrete surface holding a large white weather balloon.

“Isolated convection is really, really hard to predict,” Etten-Bohm said. “The improvement in the models is the biggest thing.”

Once the balloon was airborne, the crew mostly just waited, sitting in folding chairs underneath a tent to avoid the Texas sun. Gardner monitored the data coming back on a laptop, while a man ambled over from his car to ask what was going on. “We get one or two of those most days we’re out here,” Langford said.

Later that day, around 15 TRACER scientists on the group’s daily forecasting Zoom call eyed up a slightly menacing tropical system idling just off the Gulf Coast. The National Hurricane Center gave the system a 40 percent chance of becoming an organized tropical depression or storm within the next two days, and it was likely to dump quite a bit of rain on the region regardless. That meant that some of the team’s activities — a tethered weather balloon at the Guy, Texas, site, among others — would likely be grounded for a few days. Though Houston offered plenty of opportunity for relevant study, it also had its weather downsides.

Along with its propensity toward industrial pollution, the area is remarkably flood prone, as Hurricane Harvey demonstrated to the tune of $125 billion in damage in 2017. Hurricanes, though, remain relatively easy to predict, compared with the isolated and brief violence of a strong convective thunderstorm. Jensen said one result of TRACER will be some potentially useful information about Houston itself, and how better to prepare for dangerous storms and flash flooding. But the real value will come in extrapolating beyond Houston’s swampy, polluted borders.

“How an aerosol that’s ingested into a cloud and impacts the cloud microphysics isn’t going to change from Houston to New York or to Asia or South America,” he said. “Those processes are going to be the same wherever you measure them.”


Back at the Buc-ee’s parking lot, the Texas A&M University students mused about their role in the research and its overall importance. “The better the models get, the better we can communicate to the public,” Langford said. “It could help save lives.”

Thanks to Alicia Benjamin for copy editing this article.

  • Dave Levitan
    Dave Levitan

    Climate Reporter

    Dave Levitan is a climate reporter for Grid where he focuses on interconnected stories about climate and science, and politics shaping action around both.