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Rain on Saturdays

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Air temperatures in Germany vary with a weekly cycle: wednesdays are the warmest days, saturdays the coldest. These results were produced by Bernhard Vogel and Dominique Bäumer at the University of Karlsruhe, Germany, as is reported by Die Tageszeitung. They point out that this is not a local phenomenon, since they found the same phenomenon at fairly remote stations, in the German Alps for example. Additionally they argue that such a strict weekly phenomenon could not originate in nature, and they blame little particles in the air (aerosols) that are emitted mostly during work-days from factories and traffic. Similar results were subsequently found in China and in the USA.

Saturday Rain

Rain on Saturday

Die Tageszeitung proceeds to describe how this is not unanimously accepted in the scientific community. Harrie-Jan Hendricks Franssen from the ETH Zürich. He compared Swiss data from Zürich and Lugano with the German data used by Vogel and Bäumer, trying to figure out if precipitation and temperature behaved similarly in Switzerland as in Germany as reported by Vogel and Bäumer. Lugano is south of the Alps and hence should be influenced from different weather mechanismns than Germany. Additionally, Lugano is located in the vicinity of Milano, which exhibits Smog frequently.

Both Lugano and Zurich never showed a persistent weekly cycle for precipitation and sunshine duration for the investigated period. In addition, only 4 of the calculated 28 anomalies for the period 1991 – 2005 (2 stations x 2 variables x 7 weekdays) were statistically significant (statistically 1.4 anomalies are expected). Only one of the four statistically significant anomalies had the same sign as observed by BV07. The anomalies were analyzed further in a Monte Carlo study. The stochastic simulation experiments suggest that none of the anomalies was significant; even the largest anomaly (the anomaly of Saturday precipitation in Zurich of 18.0%) occurred in 9% of the experiments due to purely random effects. In addition, for 21% of the stochastic experiments a weekly cycle in precipitation in Zurich is found due to random effects.

Vogel and Bäumer respond on Hendrick’s finding in a comment published also in the Geophysical Research Letters. Die Tageszeitung points out that there are additional studies being conducted in Spain and in the USA, all to evaluate the role that aerosols in the air play related to weather. It’s great to see that sometimes research is well published in newspapers for the public!

Written by Claus

August 23rd, 2008 at 5:53 am