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Smog may boost storms, NASA finds

July 8, 2008
Courtesy NASA
and World Science staff

Sum­mer storms in the south­east­ern Un­ited States are worse at mid­week than on week­ends, NASA sci­en­tists say—and air pol­lu­tion is the likely cul­prit. 

Da­ta from the agen­cy’s Trop­i­cal Rain­fall Meas­ur­ing Mis­sion sat­el­lite showed mid­week storms tend to be stronger, rain­i­er and wid­er than on week­ends, the re­search­ers re­ported, while air pol­lu­tion al­so peaks mid­week.

Massive accumula­tions of rain (red) from a 2003 storm in the South­east. Data from NA­SA's TRMM sa­tel­lite has found that more rain falls mid­week. (Credit: NA­SA)

“It ap­pears that we’re mak­ing storms more vi­o­lent,” said Thom­as Bell, an at­mos­pher­ic sci­ent­ist at NASA’s God­dard Space Flight Cen­ter in Green­belt, Md., lead au­thor of the study pub­lished on­line this week in the Jour­nal of Geo­phys­i­cal Re­search. 

The find­ings are sep­a­rate from those of oth­er stud­ies that sug­gest human-induced glob­al warm­ing may ex­ac­er­bate storms in the long term. Still oth­er re­search has con­tra­dicted that idea.

Bel­l’s team used the sat­el­lite da­ta to es­ti­mate daily sum­mer­time rain­fall av­er­ages from 1998 to 2005. 

On av­er­age, they found it rained most be­tween Tues­day and Thurs­day; the in­creases were high­est in the af­ter­noons, when con­di­tions for sum­mer­time storms peak. Max­i­mum af­ter­noon rain­fall was noted on Tues­days, with 1.8 times more rain­fall than on Sat­ur­day af­ter­noons.

The team used ground-based da­ta to help con­firm the trend. To learn wheth­er pol­lu­tion could be re­spon­si­ble, they al­so an­a­lyzed na­tion­wide pol­lu­tion da­ta for the same years from the U.S. En­vi­ron­men­tal Pro­tec­tion Agen­cy. Pol­lu­tion tended to peak at mid­week, mir­ror­ing the rain­fall trend, they added.

“It does­n’t mean one caused the oth­er,” Bell said. “But it’s well known that par­tic­u­late mat­ter [pol­lu­tion] has the po­ten­tial to af­fect how clouds be­have.”

Sci­en­tists long have ques­tioned the ef­fect of work­week pol­lu­tion, such as emis­sions from traf­fic, busi­nesses and fac­to­ries, on weath­er pat­terns. Clouds are “seed­ed” by par­t­i­cles in the air. Wa­ter and ice in clouds grab hold around the par­t­i­cles, form­ing ad­di­tion­al wa­ter droplets. Some types of par­t­i­cles are pol­lu­tion.

A num­ber of sci­en­tists think more pol­lu­tion thwarts rain­fall by dis­pers­ing the same amount of wa­ter over more seeds, pre­vent­ing them from grow­ing large enough to fall as rain. Still, oth­er stud­ies sug­gest some fac­tors can override this ef­fect.

In the South­east, sum­mer­time con­di­tions for large, fre­quent storms al­ready pre­vail, which overrides the rain-thwarting dis­per­sion ef­fect, ac­cord­ing to Bel­l’s team. When con­di­tions fa­vor big storms, they ex­plained, up­drafts car­ry the smaller, pol­lu­tion-seeded rain­drops high up where they con­dense and freeze. “It’s the freez­ing pro­cess that gives the storm an ex­tra kick, caus­ing it to grow larg­er and climb high­er in­to the at­mo­sphere,” Bell said.

* * *

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Summertime storms in the southeastern United States are worse at midweek than on weekends, NASA scientists say—and air pollution is the likely culprit. Data from the agency’s Tropical Rainfall Measuring Mission satellite showed midweek storms tend to be stronger, rainier and wider than on weekends, the researchers reported, while air pollution also peaks midweek. “It appears that we’re making storms more violent,” said Thomas Bell, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., lead author of the study published online this week in the Journal of Geophysical Research. The findings are separate from those of other studies that suggest human-induced global warming may exacerbate storms in the long term. Still other research has contradicted that idea. Bell’s team used the satellite data to estimate daily summertime rainfall averages from 1998 to 2005. On average, they found it rained most between Tuesday and Thursday; the increases were highest in the afternoons, when conditions for summertime storms peak. Maximum afternoon rainfall was found on Tuesdays, with 1.8 times more rainfall than on Saturday afternoons. The team used ground-based data to help confirm the weekly trend in rainfall seen from space. To learn whether pollution could be responsible, they also analyzed nationwide pollution data for 1998 to 2005 from the Environmental Protection Agency. Pollution tended to peak at midweek, mirroring the rainfall trend, they added. “It doesn’t mean one caused the other,” Bell said. “But it’s well known that particulate matter [pollution] has the potential to affect how clouds behave.” Scientists long have questioned the effect of workweek pollution, such as emissions from traffic, businesses and factories, on weather patterns. Clouds are “seeded” by particles in the air. Water and ice in clouds grab hold around the particles, forming additional water droplets. Some types of particles are pollution. A number of scientists think more pollution thwarts rainfall by dispersing the same amount of water over more seeds, preventing them from growing large enough to fall as rain. Still, other studies suggest some factors can override this effect. In the Southeast, summertime conditions for large, frequent storms already prevail, which overrides the rain-thwarting dispersion effect, according to Bell’s team. When conditions favor big storms, they explained, updrafts carry the smaller, pollution-seeded raindrops high up where they condense and freeze. “It’s the freezing process that gives the storm an extra kick, causing it to grow larger and climb higher into the atmosphere,” Bell said.