High-Resolution Image

Another summer has passed. Four years ago the rainfall estimates made by the Tropical Rainfall Measuring Mission (TRMM) over the southeast U.S. (see this earlier image for the precise location of the area examined) were found to exhibit a strong tendency to be highest during the middle of the week. This was documented in a paper by Bell et al. in 2008. They showed in fact that atmospheric behavior changed in a number of different ways with the day of the week. It all suggested that storms get bigger and are more numerous during the middle of the week. The paper proposed a theory for this, building on research by Danny Rosenfeld. The theory proposes that the changes are due to the effects of the
well-known midweek increase in pollution that occurs regularly over much of the U.S.

Lightning activity is also expected to increase during the middle of the week according to this theory, and, indeed, data for lightning covering the years that TRMM has been orbiting, but collected by ground-based stations [the National Lightning Detection Network (NLDN)], show a similar, and perhaps even stronger tendency for midweek increases. A paper documenting this has been submitted to the journal Geophysical Research Letters. These ideas can be tested as each summer comes around, simply by checking what day of the week has seen the greatest lightning activity for that summer. With bated breath, then, we analyzed the data for this summer's lightning to see how the pattern is holding up. The accompanying figure follows the same scheme as was used in an earlier Image of the Week, where it was shown that in each summer for 1998 to 2008 lightning activity tended to peak during the middle of the week, although the strength of the signal and the exact day of the week varies from year to year. The figure shows balloons with the last two digits of each year inscribed, positioned on the "clock" diagram depending on the exact day of the week when activity peaks (actually, when a smoothed version of a graph of the activity peaks). The distance from the center shows how strong the signal is relative to the estimated amount of noisiness one would see in such plots if there were no real weekly cycle. Only data for afternoons is included in the analysis, because the theory predicts that the pollution effect should be largest then.

For yet another summer, lightning activity has tended to be highest during the middle of the week (somewhere between Tuesday and Wednesday this time). For 12 summers in a row, lightning activity has avoided Sundays! If the theory is correct (for which evidence is mounting), this pattern should persist until pollution levels change substantially.

NLDN data were provided by the NASA Lightning Imaging Sensor (LIS) instrument team and the LIS data center via the Global Hydrology Resource Center (GHRC) located at the Global Hydrology and Climate Center (GHCC), Huntsville, Alabama through a license agreement with Global Atmospherics, Inc (GAI). (The data available from the GHRC are restricted to LIS science team collaborators and to NASA EOS and TRMM investigators.). We are indebted to them for their assistance, and to Ramesh Kakar of the Science Mission Directorate of the National Aeronautics and Space Administration for research support.

(Submitted by T. L. Bell)