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The summer weekday effect: industry heats up Tokyo, moistens U.S. Southeast


Two recent studies have shed new light on how the Monday-to-Friday focus of urban work life helps shape local and regional climate.

In Japan, a group of university and federal scientists has found that waste heat from air conditioning appears to be raising air temperatures by at least 1–2°C on summer weekdays in office-dense parts of Tokyo. Led by Yukitaka Ohashia (Okayama University of Science), the team employed a multilayer model of urban areas—including buildings and the “canyons” between them—and a building energy-analysis model. This work was supplemented by four days of observations from central Tokyo. Although the model simulated nighttime temperatures well, it was unable to replicate the observed weekday readings at ground level unless it factored in the heat generated by air conditioners at rooftop level (about 36 meters or 120 feet).

For the southeast United States and nearby waters, Thomas Bell (NASA) and colleagues found a distinct weekly cycle in summer precipitation as detected by the Tropical Rainfall Measuring Mission satellite from 1998 to 2005. The average coverage and duration of afternoon showers and thunderstorms were more than 40% greater over land areas on weekdays than on weekends, and more than 40% more of the land-based thunderstorms grew to altitudes greater than 9 kilometers (5.6 miles) from Tuesday to Thursday than from Saturday to Monday.

Bell and colleagues are now examining longer-term rainfall records prior to 1998. Daniel Rosenfeld (Hebrew University of Jerusalem) hypothesizes that the increased aerosol pollution on weekdays produces a larger ­number of smaller droplets that rise to greater heights, stimulating more ice-phase activity and more energetic thunderstorms. “It will take ­modeling to sort all this out,” said Bell at the American Meteorological Society’s annual meeting in San Antonio, where he presented the work on 18 January.

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