Raining on the Super Tuesday Parade

If the band of showers stretching from the Midwest to New England dampens voter turnout today, we’ll know what to blame.

Cars. Factories, too.

Meteorologists have long known that particles “seed” rainclouds, a process in which water and ice in the clouds grab hold of the particles, forming additional (and larger) droplets that are more likely to fall as rain. That led to the suggestion that particulate pollution emitted by traffic, businesses and factories, all of which are greater during the workweek than on the weekend (traffic to malls notwithstanding), should make for greater rainfall Monday through Friday. A competing theory, however, held that the increased pollution might instead thwart rainfall, by dispersing the water in clouds over more seeds; that would prevent the droplets from growing large enough to fall as rain.

At least for summertime rainfall in the southeastern United States, the verdict is in: weekday pollution is causing more rainfall midweek than on weekends.

So conclude scientists analyzing data from NASA’s Tropical Rainfall Measuring Mission satellite, known as TRMM. As they report in the Journal of Geophysical Research-Atmospheres, those data show that midweek storms tend to be more intense than those during the weekend, dropping more rain, compared to calmer and drier weekends. Human-caused atmospheric pollution also peaks midweek, as the emissions from tailpipes and smokestacks build up before falling off as Friday approaches.

The scientists find that, on average, it rains more between Tuesday and Thursday than from Saturday through Monday. Tuesday has 1.8 times more rainfall than Saturday, which has the least amount of afternoon rain. Now I understand why, on this Super Tuesday, I am wringing out my drenched socks after venturing out into the storm to vote.