The Weather Really Does Have Weekly Cycles

Main Category: Water - Air Quality / Agriculture
Article Date: 28 Aug 2008 - 6:00 PDT

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IT SEEMS to happen with depressing frequency - sunny skies turn to rain just as the weekend arrives. Now Spanish researchers say they have evidence that in some parts of Europe the weather really does follow a weekly cycle, although not in the straightforward way that the anecdote might suggest.

Evidence has been mounting over the years that the weather in certain parts of the world, including the US, Japan and China, can be driven by the weekly cycle of human activity. This is because we tend to produce more air pollution during the week and less at the weekend. Evidence that such an effect occurs in Europe is controversial and has been harder to come by.

Arturo Sanchez-Lorenzo of the University of Barcelona, Spain, and his colleagues examined data gathered between 1961 and 2004 from weather stations across Spain to see whether such a pattern existed. They claim to have found it in Spain, as well as hints of weekly changes in air circulation more broadly over western Europe (Geophysical Research Letters, DOI: 10.1029/2008GL034160).

This weekly pattern changes with the season, however. In winter there is a tendency for more rain during the week than on weekends, while in summer the effect is opposite and less pronounced.

The result is puzzling, but it is known that soot and other airborne pollutants produced by human activity can affect the weather in a variety of ways. For example, particles can be heated by absorbing sunlight, which in turn heats the air and changes air circulation patterns. Pollutant particles can also provide seeds for cloud formation. Exactly which effect has the greatest influence seems to depend on conditions that vary season by season.

They also found signs that air pressure in western Europe tends to be lower midweek than at the weekend in data from a global database. This suggests that the human influence on weather goes beyond known local effects, says team member Josep Calbó of the University of Girona in Spain.

However, it is not clear whether the team's findings are statistically significant, says Thomas Bell of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was part of a team that found a stronger weekly cycle in the US. "This whole enterprise of looking for weekly cycles is rife with possibilities for misleading oneself."

Why a weekly cycle would be less noticeable in Europe than in the US and Asia is still unknown. No weekly cycle has ever been found in the UK, probably because the weather is dominated by large systems blowing in from the Atlantic ocean. These larger systems may be harder for weekly pollution cycles to influence, points out Douglas Maraun of the University of East Anglia in Norwich, who studies UK precipitation. "I doubt that there is a weekly influence of human activity on such a large weather system," he says.

New Scientist

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