“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.