Rainy Days and Weekdays

“There’s no way that weather knows whether it’s Sunday or Wednesday. Certainly not over the course of years, and we see this effect year in and year out,” says researcher Tom Bell of NASA’s Goddard Space Flight Center.

Yet, at least in the Southeastern United States in the summertime, he “saw clear evidence that there was a change in the rainfall with the day of the week.” As Bell and colleagues wrote in the Journal of Geophysical Research, heavier rain during weekdays may be caused by air pollution, which is worse during weekdays than on weekends.

“It has to be due to something that human beings are doing,” Bell says. “And one thing we do know that humans are doing is that they’re polluting the atmosphere more—at least in urban areas, for certain—in the middle of the week” than on weekends.

The study also showed that rainstorms were more severe in the middle of the week. “Not only is there more rain, but the storms are more violent, and lots of things besides just rain could be affected by the increased violence of the storms,” Bell explains. “You’re going to have higher winds, it’s possible you would have more hail, more lightning...so the effects of pollution are not just on rainfall. It’s also an effect on the severity of storms. and all the effects that storms have on people.”

image courtesy NASA

The satellite Bell’s group used for its research—NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite—is special because it tracks not only rainfall, but also storm severity over the globe’s midriff rather than the less populated poles. The satellite approach, says Bell, also provides better coverage than methods “based on little buckets that are put around in various places.” With satellites, researchers can track big swaths of land, but the study was limited by TRMM’s orbit.

“This satellite is in what is called a low-inclination orbit, meaning that the tilt of the orbit with respect to the equator of the earth is not nearly as big as it is for the satellites that go over the poles. This satellite only is able to see around 40 degrees of latitude,” he says, “Which is roughly my latitude near Washington, DC. And the satellite is unable to see beyond that because the orbit never takes it north of that latitude or south of -40 degrees.”

Bell says he got interested in weekly rain patterns because of the established fact that pollution levels change with the day of the week. “I live in an urban area and I can see with my eyes that there’s more pollution in the middle of the week than on weekends,” he explains.

But he says more research is needed to figure out exactly how pollution could be causing this weekend effect. It’s “hard to doubt that the weather is changing with the day of the week. The only question is why is it changing,” he explains.

For now, the chief suspect is aerosol pollution, rather than carbon dioxide or other greenhouse gases. Aerosol pollution is the particulate matter—“the brown trail of stuff coming out of the smoke stack,” Bell says—that is known to affect the weather.

Unlike greenhouse gases, which cause the planet to heat up, aerosol pollution can have a cooling effect because the particles act like little miniature window shades, blocking out the sun. They may also make storms more severe by preventing water droplets from becoming too big. The diminutive droplets then travel higher up into the atmosphere, where they freeze and therefore have more energy to release into clouds.

“The storms just grow more vigorously, because of the extra energy released by the freezing of the droplets. The storm sucks more moist air…into the storm, and that extra moist air is, in a sense, a fuel” for the storm, explains Bell.

But if aerosol pollution can have a cooling effect, could it be useful in these warming times? Bell says no. “I still don’t like it. But it makes me appreciate how pervasive its influence is on the atmosphere,” he explains. “It still surprises me that we can change the kinds of storms we see depending on the day of the week, and just because of the fact that we drove more, or our trucks drove more in the middle of the week than on weekends.”

Bell says the findings could improve weather forecasts, which, without pollution information would tend to underestimate rainstorms during the week and overestimate them on the weekends. “If the weather forecast models don’t know about this, at their present stage of development, when they make a forecast they won’t be able to tell, 'Oh, this is a day with a high level of pollution, the storms might be particularly bad on this day,'” he says.

This research by Thomas Bell, Daniel Rosenfeld, Kyu-Myong Kim, Jung-Moon Yoo, Myong-In Lee, and Maura Hahnenberger was published in the Journal of Geophysical Research. Funding was provided by NASA and the European Community.