Wet weekends not just a myth

By SIMON COLLINS, science reporter

In some parts of the world, it actually is cloudy all weekend and the sun comes out just when you go back to work on Monday morning.

And the cause seems to be weekday pollution.

Scientists have found that the weekends on the east coast of the United States are wetter than during the working week, and it is colder at night.

But the pattern in the rest of the world is mixed, and there is no evidence that the urban myth holds true in New Zealand.

The "weekend effect", first identified in American rainfall patterns in 1998, has been given a boost from a study on temperature in the latest issue of the US Proceedings of the National Academy of Sciences.

As there is no known natural phenomenon that varies on a weekly cycle, the data provides some of the strongest evidence so far that human activity is directly affecting the world's climate.

Even Auckland University geographer Chris de Freitas, a leading sceptic on general global warming, says the new evidence for a weekend effect on temperature in North America and East Asia looks "credible".

"These are localised effects which could be quite regional. The aerosols [pollutants] increase night-time temperatures because the outgoing long-wave radiation at night is reduced," he said yesterday.

"But these are short-term effects that occur over a time scale of hours to days, as opposed to greenhouse gas effects, which take place on long time scales.

"These small effects could, in fact, be quite significant. They don't threaten the environment, but they certainly are impacts of human activity on the environment."

The original 1998 study found that rainfall on the US east coast built up gradually during the working week to a peak from Thursday to Saturday, apparently due to the week's accumulated pollution helping to form clouds.

As the air cleared over the weekend, rainfall declined on Sundays and through the early part of each week.

The latest study by two scientists from the US National Oceanic and Atmospheric Administration (NOAA) has found a statistically significant difference between "weekends" and "weekdays" in the gap between daily minimum and maximum temperatures at a third of US climate stations.

"Weekends" were defined as Saturday, Sunday and Monday. "Weekdays" were Wednesday, Thursday and Friday.

The "weekend effect" was limited. Two-thirds of US climate stations showed no apparent effect, and even in the one-third where it did show up, the hottest daytime temperatures were much the same throughout the week.

The coldest night-time temperatures did vary through the week in the affected areas, but they varied in opposite directions.

It was colder at night during weekends on the east and west coasts, but colder mid-week in the centre of the country.

China, Mexico and part of western Canada behaved the same as the US coasts, with colder nights at weekends.

But the rest of Canada and Japan behaved like the central US, with colder nights during the week.

The NOAA authors speculate that the buildup of pollution during the week may be increasing weekday clouds, making it warmer at night in the main centres of pollution on the east and west coasts.

But their data showed no clear pattern outside North America and East Asia.


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