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Weekend does get more rain

The common feeling that it rains more on weekends could be true, at least for the eastern USA. An analysis of Eastern Seaboard weather shows it really does rain more frequently on the weekend than during the week. Saturdays receive an average of 22% more precipitation than Mondays, climatologists at Arizona State University report. What's more, the clouds are, to some extent, the product of the very jobs people are trying to escape. The ASU researchers, who analyzed weather data dating back to 1946, say weekend storms probably are enhanced by air pollution spewed by millions of cars and trucks during weekday commutes. The pollution generates tons of tiny airborne particles, called aerosols, that become the microscopic seeds around which rain drops develop. The study appeared in Nature magazine in August 1998.

"We were quite surprised to see weekends are substantially wetter than weekdays," said climate geographer Randall Cerveny, one of the study's principal authors. "We've run over 45 statistical analyses to make sure the cycles we see are indeed there."
Other researchers said they were startled by the suggestion that human activities influence weather patterns on a regional scale, and said the next step will be to develop a computer model simulating Eastern Seaboard weather to validate the researchers' statistical conclusions.

"It's really surprising that pollution could have such far-reaching effects on meteorology," said David Parrish, an atmospheric scientist at the National Oceanic and Atmospheric Administration. "I would've thought it would be undetectable."
Cerveny and Robert Balling of ASU analyzed rainfall records along the Atlantic coast from Florida to Nova Scotia and extending about 200 miles out to sea.

They determined that the rain falls in a seven-day cycle, peaking on Saturday with an annual average of 658 millimeters, about 26.3 inches. Monday rainfall is the lowest, with an average of 538 millimeters a year. To link the rainfall with pollution data, the researchers looked to a research station on Sable Island, a pristine speck about 200 miles east of Halifax, Nova Scotia.
Monitors there continuously sniff for carbon monoxide, ozone and other urban pollutants that spread northeastward from Boston, New York and other coastal cities. Vehicle traffic is the primary source of the pollution, along with power plants and industry.

The ASU study found that weekly pollution highs and lows match the weekly precipitation cycle, with pollution peaking by Saturday and dipping noticeably by Monday. Other scientists said they would like to see a stronger cause-and-effect relationship, something the ASU team says it is working on.

"It's a striking correlation," Parrish said. "But I'm not sure that the connection to the pollution is entirely clear to me." The researchers also examined Atlantic hurricane records since 1946 and found that weekend storms featured wind speeds as much as 12 mph slower than midweek storms. They suggest air pollution aerosols diminish the fury of weekend storms by triggering more rainfall.

On remote Sable Island, the ASU study came as no surprise. Meteorologists there frequently have to wipe sooty crud from their instruments, and director Gerry Forbes said his staff members complain much like urban beachcombers. "People here claim it always seems to be raining on their days off," Forbes said.

By the Associated Press