High-Resolution Image

The image at the top represents the average rain rate during the summers of 1998-2005 for each day of the week and each hour of the day, The averages are over the non-coastal southeast-U.S. as depicted in the map at the bottom. Rain rates are represented by colors; the key to the colors is shown by the color bar in the center. As one moves from the bottom to the top of the color plot, mean rain rates for Saturday, then Sunday, and so on, are shown for each day of the week, then repeated for a second week. Moving from left to right, mean rain rates for midnight through noon (1200 local time) and back to midnight are shown, then repeated for a second 24 hours. A paper now in press by Bell, Rosenfeld, Kim, Yoo, Lee, and Hahnenberger (2008), from which this image is taken, provides evidence that mean summertime rainfall in the SE U.S. is larger in the middle of the week than on weekends. This is believed to be occurring because of the weekly variations in particulate pollution (aerosols). It is well known that in many parts of the U.S. the amount of small particulate matter suspended in the air is larger Monday-Friday than on weekends, probably because of weekly changes in diesel vehicular traffic or power-plant operation. It is also now well established that particulates can affect the way storms develop, and, during the summertime, at least, the effect is to energize storms in this region.

The plot shows that, for all days of the week, more rain falls in the afternoons (1200-2400) than during early morning hours (0000-1200), which is typical of summertime rain over land. The plot also shows that it is the afternoon storms that are strengthened midweek, while there seems to be a compensating midweek weakening of morning rain.

Why is this important? The effects of aerosols on rainfall and storm development are still being worked out by scientists. They have found that in some environments aerosols tend to suppress rainfall, but in the hot moist environment typical of the SE U.S. in the summer the opposite seems to be the case. Weather forecast models don't yet include the effects of changes in aerosols by human activity, and so they will miss forecasting the changes observed by the satellite. Observations like these will help improve the accuracy of weather and climate models and their forecasts. It is in any case remarkable that, based on the satellite observations, humans are modulating the weather each week over large areas.

The rain data used in this plot were provided by the Tropical Rainfall Measuring Mission (TRMM) satellite, which was launched in 1997 and is now celebrating its 10th anniversary of successful operation. TRMM is a joint venture of NASA and the Japanese space agency JAXA.

(Submitted by T. L. Bell.)