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.

很多人都有這樣的經驗，周一早晨上班時常常烏云密布，而到周五回家時，或圣誕節這樣的假日，才會云開日霽。

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.

兩位美國科學家的最新研究表明，雷暴和冰雹這樣的壞天氣，往往發生在一周的中間時段，而不太可能發生在周末，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.

工作日出現極端天氣的概率比周末時段高出20%。這與人類的生產活動緊密相關，人類活動排放的化學物質會在一周中段達到峰值。

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.

研究人員分析了美國東海岸夏季的氣候，發現龍卷風和冰雹多發生在一周中間時段。這一現象不是孤立事件，也不是天氣預報出現了失誤。

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.

美國航天局戈達德航天研究所的科學家在《地球物理研究雜志》上發表的研究報告說，人類工作中產生釋放的化學物質，特別是汽車尾氣等廢氣，是造成此種異常天氣現象的原因。

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.

廢氣排放會影響大氣的濕度，人們工作時，廢氣排放量會達到峰值。到周末，人們休息時，廢氣排放量就會下降。

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

人類活動對天氣產生的這種影響在美國西海岸并不適用，也許這種效應僅限于比較潮濕的氣候。

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.'

研究者說：“這項研究第一次用統計數據證明了：夏季人類活動產生的廢氣排放量在一周的中段達到峰值時，龍卷風和冰雹等異常天氣狀況發生的概率也接近一周中的最高值。”