Pollution particles will have their day

Over the years, many meteorologists have amused themselves by trying to find a relationship between the weather and the day of…

Over the years, many meteorologists have amused themselves by trying to find a relationship between the weather and the day of the week.

Could it be, for example, that Tuesdays are consistently less prone to rain than Saturdays, or is Wednesday's weather warmer than that at weekends?

Prima facie, such suspicions should be groundless. Although loosely based on a quarter of a lunar month, the seven-day week, from a scientific viewpoint, is an arbitrary division of time to which nature ought to be oblivious.

Moreover, sceptical meteorologists will point out to you that if you separate the rainfall figures, for example, for any given period into seven lots corresponding to the various days, the totals will never coincide exactly; one of the seven days must inevitably turn out to be the wettest, and another to be the day on which the least amount of rain has fallen.

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Yet recent research into rainfall and wind patterns of the north Atlantic has produced some interesting results. Vast amounts of satellite data gathered between 1979 and 1995 were closely analysed and, as expected, the ocean as a whole was clearly unaffected by the weekly cycle.

But, strangely, a region just to the east of the US was found to be consistently wetter at the weekends, the rainfall for Saturdays being an average 22 per cent higher than that of Mondays.

And, even more surprising, the wind speed in Atlantic hurricanes appears to be lower at weekends than at other times by as much as 19 per cent.

The relevant link between rainfall, wind speed and the progress of the week could only be pollution. Human habits, after all, vary markedly from day to day; many of our factories close their gates at weekends and commuters keep their cars at home.

This behaviour should result in less pollution being expelled into the atmosphere on Saturdays and Sundays than on working days and, indeed, it is well know that carbon-monoxide and ozone levels in some urban areas rise as the weekend approaches.

The researchers speculate that this effect may be more widespread than we think. It is possible that pollution in the form of tiny particles in the atmosphere may provide clouds off the western seaboard of the US with abundant extra nuclei on which condensation may take place, thus bringing about a temporary increase in rainfall when the concentration is at its highest - at the end of the working week.

And in the case of the hurricanes, an increase in rainfall could reduce the amount of residual water vapour in the storms and thus rob them of some of their potential wind power.