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.

很多人都有这样的经验，周一早晨上班时常常乌云密布，而到周五回家时，或圣诞节这样的假日，才会云开日霁。

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.

两位美国科学家的研究表明，雷暴和冰雹这样的坏天气，往往发生在一周的中间时段，而不太可能发生在周末，至少在夏天如此。

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.

工作日出现极端天气的概率比周末时段高出20%。这与人类的生产活动紧密相关，人类活动排放的化学物质会在一周中段达到峰值。

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.

研究人员分析了美国东海岸夏季的气候，发现龙卷风和冰雹多发生在一周中间时段。这一现象不是孤立事件，也不是天气预报出现了失误。

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.

美国航天局戈达德航天研究所的科学家在《地球物理研究杂志》上发表的研究报告说，人类工作中产生释放的化学物质，特别是汽车尾气等废气，是造成此种异常天气现象的原因。

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.

废气排放会影响大气的湿度，人们工作时，废气排放量会达到峰值。到周末，人们休息时，废气排放量就会下降。

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

人类活动对天气产生的这种影响在美国西海岸并不适用，也许这种效应仅限于比较潮湿的气候。

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.'

研究者说：“这项研究次用统计数据证明了：夏季人类活动产生的废气排放量在一周的中段达到峰值时，龙卷风和冰雹等异常天气状况发生的概率也接近一周中的值。”