Published online 7 December 2007 | Nature | doi:10.1038/news.2007.345

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Does it rain less on the weekend?

Surveys show no weekly pattern in rainfall — until recently.

Is there a wettest day of the week?Getty

Which day of the week is the wettest? Scientists have argued about that for decades. Two studies1,2 now suggest that there hasn't been a weekly cycle in rainfall at all during that debate. But one of the studies2 shows that a pattern of wetter midweeks and drier weekends may have emerged recently, at least in summer in the southeastern United States.

The theory that the artificial rhythm of the working week has an effect on the weather might sound strange, but there's a sensible explanation. Higher industrial activity on weekdays generates more airborne pollution particles, which can seed raindrop formation in the atmosphere.

The way that this affects rainfall isn’t simple, though — more airborne particles (or aerosols) doesn’t necessarily mean more rain. If each particle seeds a droplet, then lots of them might produce lots of droplets, distributing the water more widely and so making the droplets smaller than usual. These small droplets might never grow big enough to fall out of the clouds. These competing effects of aerosols – speeding up droplet formation, but reducing their average size – could cancel each other out.

Because of these complications, says meteorologist Tom Bell of NASA's Goddard Space Flight Center in Greenbelt, Maryland, "a weekly cycle, if present, would differ a lot in different regions and different seasons."

The simple theory of particles seeding raindrops was invoked in the 1920s when it was claimed that it rained less, on average, on Sundays. Since then some measurements have implied that Tuesdays are wettest, others pointed to Thursdays and Fridays3, and some, depressingly, suggested Saturdays.

Rain, rain, go away

Ari Laaksonen and David Schultz of the Finnish Meteorological Institute in Helsinki and their colleagues figured that the only way to settle the question was to look at lots of data, covering a wide geographical region and a long time period. They collected rainfall data measured by rain gauges (which collect and weigh rainwater) at more than 200 monitoring stations across the United States from 1951 to 1992.

They find1 that not only are there no significant overall differences in average rainfall for different days of the week, but neither are there such differences for any individual station. In other words, there has been no ‘wettest day of the week’, either locally or nationally, during all that time. If there is an effect, Laaksonen says, it is too tiny to be determined.

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Bell and his coworkers took a similar look2 at rainfall, including more recent satellite and rain-gauge data. "We also found that the weekly cycle in average gauge-measured rainfall over the southeastern United States was not statistically significant during the decades up to the 1980s," he says. But since then, "the cycle has grown in strength", he says. The most recent data shows that during summer in the southeast United States, the middle of the week is wettest, with Sundays being the driest day.

Bell admits that "we really don't know enough" to explain this change. He and his colleagues guess that it has something to do with the use of different types of fuel, and various regulations on things such as emissions from diesel engines. When all this is taken into account, he says, "the weekly cycle might actually diminish in later decades. But it will take a lot more research to validate this hypothesis."

In the meantime, it looks like at least some people will enjoy drier weekends. 

  • References

    1. Schultz, D. M., Mikkonen, S., Laaksonen, A. & Richman, M. B. Geophys. Res. Lett. 34, L22815 (2007). | Article |
    2. Bell, T. L. et al.J. Geophys. Res. (in the press).
    3. Cerveny, R. S. & Balling, R. C. Jr Nature 394, 561-563 (1998). | Article | ChemPort |
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