The heat wave is about to go off the boil — just in time for the weekend, of course. It will be much fresher and wetter as the torrid heat that wafted over from Europe on the back of an anticyclone drifts away.

In its place comes our familiar cooler and rainier weather working in from the Atlantic. Temperatures will be around average for the time of year, but it will feel much cooler. It looks as though sunshine and showers will be the story for the next week, and possibly longer.

Is it just a figment of our imagination, or does the weather change at weekends? After all, why should the weather change on a seven-day cycle that has nothing to do with the natural world?

The first sign of a weekly weather cycle turned up a century ago in Rochdale, Lancashire, when it was discovered that weekdays were wetter than weekends. The reason: factories belched out coal smoke during the week and produced tiny specks of smut that seeded cloud droplets that turned into rain.

These days pollution is more likely to come from road traffic, and this too follows a weekly cycle of urban smog. German scientists looked at 14 years of weather figures in industrialised areas of Europe, including Britain, and found that Saturday tended to be the most humid and least sunny day of the week, compared with the sunniest days on Monday and Tuesday. The highest temperatures were on Tuesday.

They blamed tiny particles from traffic pollution that hung about in the air and encouraged clouds to form, as well as reflecting sunlight back into space. These so-called polluting aerosols gradually built up during the working week, but left a hangover effect at the weekend that spoilt the weather before the skies eventually cleared.

To add insult to injury, the average temperature was 0.2C (0.36F) lower at weekends than on weekdays and there were at least 15 minutes less sunshine on Saturday and Sunday than during the week, according to the study.

Perhaps the most dramatic example of a weekly cycle of weather was revealed on the US East Coast. During the hot and humid summers there, Saturdays have an average of 22 per cent more rain than Mondays — although in the southeastern US, summer thunderstorms are most common on Wednesdays and least common at weekends for reasons that are not quite clear.