Rain takes it easy on weekends, when air is cleaner

The link between rainfall and the day of the week is evident in data from NASA's Tropical Rainfall Measuring Mission satellite, known as TRMM.

Midweek storms tend to be stronger, drop more rain, and span a larger area across the Southeast compared to calmer and dryer weekends. The findings are from a study led by Thomas Bell, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Bell says the trend could be attributed to atmospheric pollution from humans, which also peaks midweek.

He and his colleagues reported their results on 31 January in the Journal of Geophysical Research – Atmospheres, a publication of the American Geophysical Union.

Rainfall measurements collected from ground-based gauges can vary from one gauge site to the next because of fickle weather patterns. So, to identify any kind of significant weekly rainfall trend, Bell and his colleagues looked at the big picture from Earth's orbit. The team collected data from instruments on the TRMM satellite, which they use to estimate daily summertime rainfall averages from 1998 to 2005, across the entire Southeast.

The team finds that, on average, it rains more between Tuesday and Thursday than from Saturday through Monday. Newly analyzed satellite data show that summer 2007 echoes the midweek trend with peak rainfall occurring late on Thursdays. However, midweek increases in rainfall are more significant in the afternoon, when the conditions for summertime storms are in place. Based on satellite data, afternoon rainfall peaks on Tuesday, with 1.8 times more rainfall than on Saturday, which experiences the least amount of afternoon rain.

The team uses ground-based data from rain gauges, along with vertical wind speed and cloud height measurements, to help confirm the weekly trend in rainfall observed from space.

To find out if pollution from humans indeed could be responsible for the midweek boost in rainfall, the team analyzes particulate matter, the concentrations of airborne particles associated with pollution, across the U. S. from 1998 to 2005.

The data, obtained from the Environmental Protection Agency, show that pollution tends to peak midweek, mirroring the trend observed in the rainfall data.

“If two things happen at the same time, it doesn't mean one caused the other,”
Bell says. “But it's well known that particulate matter has the potential to affect how clouds behave, and this kind of evidence makes the argument stronger for a link between pollution and heavier rainfall.”
Scientists have long questioned the effect of workweek pollution, such as emissions from traffic, businesses, and factories, on weekly weather patterns.

Researchers know clouds are “seeded” by particulate matter. Water and ice in clouds grab hold around the particles, forming additional water droplets. Some researchers think increased 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 dispersion effect.

In the Southeast, summertime conditions for large, frequent storms are already in place, a factor that overrides the rain-thwarting dispersion. When conditions are ripe for big storms, updrafts carry the smaller, pollution-seeded raindrops high into the atmosphere 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 says. He and his colleagues find that the radar on the TRMM satellite shows that storms climb to high altitudes more often during the middle of the week than on weekends. These invigorated midweek storms, fueled by workweek pollution, could drop measurably more rainfall.

The trend doesn't mean that it will always rain on weekday afternoons during summertime in the Southeast. Rather, “it's a tendency,” says Bell. With the help of satellites, new insights into pollution's effect on weather one day could help improve the accuracy of rainfall forecasts, which Bell says, “probably under-predict rain during the week and over-predict rain on weekends.”

Notes for Journalists

Journalists and public information officers of educational and scientific institutions (only) who have registered with AGU for direct electronic access and received a username and password, can download a PDF copy of this paper by clicking on this link http://dx.doi.org/10.1029/2007JD008623 . If you need instructions for downloading, please see http://www.agu.org/jinstructions.shtml .

Or, you may order an emailed copy of the paper by sending a message to Peter Weiss at pweiss@agu.org. Please provide your name, the name of your publication, and your phone number. Neither the paper nor this press release are under embargo.

Title:

“Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms “

Authors:

Thomas L. Bell: Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA;

Daniel Rosenfeld: Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel;

Kyu-Myong Kim and Myong-In Lee: Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA, and Goddard Earth Sciences and Technology Center, Univ. Maryland Baltimore County, Baltimore, Maryland, USA;

Jung-Moon Yoo: Department of Science Education, Ewha Womans University, Seoul, South Korea;

Maura Hahnenberger: Meteorology Department, University of Utah, Salt Lake City, Utah, USA.

Citation:

Bell, T. L., D. Rosenfeld, K.-M. Kim, J.-M. Yoo, M.-I. Lee, and M. Hahnenberger (2008), Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms, J. Geophys. Res., 113, D02209, doi:10.1029/2007JD008623.

Contact information for coauthors:

Thomas L. Bell, senior meteorologist: office +1 (301) 614-6197, Thomas.L.Bell@nasa.gov