Nasa scientists have explained why the weekends in Florida are so nice and sunny: because industrial pollution causes bigger rainstoms during the week.
Data from a Nasa climate satellite shows that summertime rainfall over the southeastern United States is significantly higher during the working week than at weekends, with 1.8 times more afternoon rain on a Tuesday, the wettest day, than on a Saturday, the driest.
According to the study, published in the Journal of Geophysical Research, the fine weekend weather is a side-effect of air pollution.
Scientists have long questioned whether particulate pollution from vehicles and factories, which is higher during the working week, changes weather patterns. The Nasa study found that tiny particulate matter may “seed” summer clouds in the tropical southeast, where plentiful hot, moist air tends to form more rain.
“There is a kind of a feedback loop,” said Thomas Bell, an atmospheric scientist at Nasa’s Goddard Space Flight Centre, of the mechanism where midweek storms flush smog from the atmosphere to leave the weekend weather clearer and drier.
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“Pollution dies on the weekends, so you have the tendency for less severe storms and nicer, clearer air,” Mr Bell, who was the study’s lead author, added.
Mr Bell said that when clouds are seeded by aerosol pollutants, the raindrops are carried higher into the atmosphere where they condense and freeze. “It’s the freezing process that gives the storm an extra kick, causing it to grow larger and climb higher into the atmosphere,” he explained.
The correlation between pollution and rain was less pronounced in the western half of the country, where it tends to be drier in the summer.
The Nasa study found that the “weekend effect” in the rain cycle only became strong enough to be detectable in the 1980s. Data from traditional rain gauges did suggest a weekly rain cycle as far back as the 1940s - but with peak rainfall on Sunday or Monday because of different pollution build-ups.
The most obvious implication of the Nasa study is that the weekend remains the best time to head for the beach if you live in the Sunshine State.
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But Mr Bell suggested that if weather forecasters focused more on pollution levels they could probably sharpen up their predictions, which “probably under-predict rain during the week and over-predict rain on weekends”.