Unlike the day or year, the week isn t tied to any naturally recurring phenomenon. In 1998, researchers at Arizona State University in Tempe reported that daily oceanic rainfall shows a weekly precipitation cycle across a stretch of water to the lee of the coastal metropolitan area in the eastern United States. This extreme western part of the North Atlantic Ocean receives significantly more of its precipitation on weekends, they observed. In 1994, an atmospheric scientist noted that daily satellite measurements suggest that, in the Northern Hemisphere, the temperature in the lower troposphere peaks on Wednesdays. However, there appeared to be no significant effect in the Southern Hemisphere.
Statistics is a tricky business, however.
Statistician Kevin J. Coakley of the National Institute of Standards and Technology in Boulder, Colo., has analyzed temperature data collected at the San Francisco International Airport from 1949 to 1994. For each week, he determined the day of the week on which the weekly maximum temperature occurs. Coakley discovered a surprising effect. In other words, if Coakley chose Sunday as the first day of the week, then Saturdays and Sundays tended to be the warmest days of the weeks. However, if he arbitrarily chose Thursday as his starting point, Wednesdays and Thursdays would emerge as the balmiest.