The mechanism behind how pollution is changing the weather at weekends in some of the countries in Europe has been identified by a team of researchers.
It's been known for years that man-made pollution affects weather patterns.
But, according to a report in Nature News, in the first survey of weekly weather cycles to cover Western Europe, researchers have found that pollution seems to be giving the Spanish milder winter weekends.
Interestingly, summer weekends are correspondingly colder and wetter.
Arturo Sanchez-Lorenzo, a climatologist at the University of Barcelona in Spain, and his colleagues studied 44 years of climate data from 13 weather stations across rural and urban areas of Spain, and extended their research by surveying sea level pressures for western Europe.
Scientists think that the patterns are caused by changes in the amount of particulate air pollution that build up over the week thanks to traffic and industrial activity.
Indeed, weekly cycles in weather have been reported for several areas. Parts of the United States have less rainfall on weekends, while Germany gets more rain at the end of the week.
The pollution also tends to give the United Kingdom and western France drier, cloudless weekends in winter, whereas Iceland and Greenland are also experiencing wetter winter weekends.
The emission of smoke and gases has been known to affect rainfall since the 1920s.
Airborne pollution particles can seed raindrop formation - but it can also change how rain clouds develop, potentially delaying rainfall to create fiercer summer storms as has been seen in the United States.
This complicated interaction means that scientists are unsure exactly how the cycle develops.
"We suggest that (these weekly cycles) are related to changes in atmospheric circulation over western Europe, maybe due to some indirect effect and interaction of aerosols with atmospheric dynamics," said Sanchez-Lorenzo.
David Parker, of the UK Meteorological Office in Exeter, agrees that the anthropogenic emissions of aerosol particles, through their impacts on cloud dynamics, can affect the amount of solar radiation reaching Earth's surface and therefore atmospheric circulation.
But, according to him, more detailed experiments on the effect of weekly-varying aerosols on weather and climate are needed to confirm this mechanism.