For many of us, it might seem that a dark cloud descends on Monday morning and only lifts when it's going-home time on Friday - or a holiday such as Christmas.

But new research by two American scientists has found that bad weather - or at least thunderstorms and hail - does seem to happen midweek far more often than it does at weekends, at least in summer.

There were 20 per cent more violent weather events during the week, and 20 per cent fewer at weekends. The effect is related to the hum of human activity - and thus chemical releases - and peaks mid-week.

The study analysed weather along the East Coast of America during summer, and found that there is a blip in the number of tornadoes and hailstorms mid-week - and it can't be explained away by isolated events, or by improvements in weather reporting.

The study, by Nasa Goddard scientists and published in the Journal of Geophysical Research says that the peak in storms is caused by chemicals released by humans as they work, particularly 'aerosols' such as exhaust smoke.

Aerosol release has known effects on moisture in the air - and releases tend to peak when people are at work. When people relax at weekends, releases drop.

The effect doesn't seem to be reproduced on the West Coast of America - so it may be limited to moister climates.

The researchers said, 'This study shows for the first time statistical evidence that when man-made aerosol releases during summer time are at their weekly mid-week peak, tornado and hailstorm activity there is also near its weekly maximum.'