Every week the U.S. population carries out a climate-change experiment by varying their activities with the day of the week. Measured pollution levels vary on a weekly basis. Particulate aerosol pollution is generally a maximum in the middle of the week and a minimum on weekends. It is well known that aerosols can affect cloud development and precipitation, although whether they suppress or enhance storm development depends on many factors. The Tropical Rainfall Measuring Mission (TRMM) satellite has provided evidence that rain statistics change with the day of the week over the southeast U.S. and neighboring waters during the summer months (JJA) of 1998-2005. There is a midweek increase in both rain area and intensity over land, and a midweek decrease over the nearby Atlantic and perhaps the Gulf of Mexico. Statistical tests suggest that the weekly variations are very unlikely to be due to the random behavior of weather.

We will review the TRMM evidence and analyses of wind data from model reanalysis, rain-gauge data, and TRMM radar data. Analyses of TRMM data for the weekly cycle in 2006-07 will be presented, as well as analyses of lightning data and MODIS cloud cover and cloud-top temperatures. All appear to be consistent with the physical picture that aerosols reduce droplet size and, in intensely convective situations, the smaller droplets are carried high enough to freeze and release additional heat (of fusion), causing summertime storms to grow more vigorously, reach higher altitudes, and produce more rainfall.