In an article published in January 2008 in the Journal of Geophysical Research, NASA atmospheric scientists claim that the average summertime rainfall in the southeast US is significantly higher during the middle of the week than on weekends. These findings come from data measurements taken by the Tropical Rainfall Measuring Mission (TRMM) satellite from 1998-2005.
According to Thomas Bell and his team, midweek storms tend to have more rainfall and cover larger areas than weekend storms, and these variations are likely due to air pollution from humans, as the timing of fiercer storms coincides with peaks in atmospheric pollution.
It is well-known that particulate matter can potentially affect how clouds behave. However, previous studies have concluded that an increase in airborn particulates could reduce rainfall.
Clouds are "seeded" by particulate matter, meaning that water and ice in clouds grab hold around the particles and thus form additional water droplets. Some researchers think that increased pollution decreases rainfall by dispersing the same amount of water over more seeds, preventing them from growing large enough to fall as rain.
Bell's team, however, suggests that this process can be thwarted when conditions are favourable to large storms. In these cases, updrafts carry the pollution-seeded raindrops higher into the atmosphere where they condense and freeze rather than dissipate. Indeed, the TRMM satellite showed that storms climb to higher altitudes more often during the midweek than on weekends.
Satellites such as the TRMM are giving scientists new insights into the effects of pollution on weather patterns and will hopefully help to improve the accuracy of rainfall forecasts. In the meantime, don't forget your umbrella!
See more about this paper in the Journal of Geophysical Research, or at the NASA website.