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. The cycle over land is particularly strong for afternoon rain, with a statistical significance level near 0.1%. The National Centers for Environmental Prediction (NCEP) reanalysis wind fields and surface rainfall shows a weekly cycle that corresponds well with the TRMM observations. Rain gauge data over the southeast U.S. from the National Climatic Data Center Global Historical CLimatology Network likewise show a weekly change in rainfall during the TRMM observation period.

These variations are almost certainly caused by weekly variations in human activity. The most likely cause of the observed changes in rainfall is the well documented weekly variation in atmospheric pollution. Particulate pollution is highest in the middle of the week. Considerable observational and modeling evidence has accumulated concerning the effects of aerosols on precipitation. Most of this evidence relates to the suppression of precipitation by aerosols at low levels in deep convective clouds.  It has been argued, however, that in highly unstable moist environments--such as exist over the southeast U.S. during the summertime--the aerosol-induced delay of early rain allows more cloud water to ascend to supercooled levels of the storms, freeze there and invigorate the storm.  Some modeling studies seem to confirm this. The strong weekly cycle in rainfall observed over the southeast U.S. along with what appears to be dynamical suppression of rainfall over the nearby Atlantic, the lack of an observable cycle over the southwest U.S., and a midweek increase in storm heights observed by the TRMM radar are all consistent with this theory.

The NCEP reanalysis data and rain gauge data extend to many decades before the TRMM observation era. Evidence for past changes in the nature of the weekly cycle will be discussed.