A TOTAL eclipse of the Sun creates a mini-cyclone that moves with the Moon鈥檚 shadow, even when the sky is cloudy. The confirmation of this phenomenon backs up common reports from eclipse chasers that the darkening sky stirs up a breeze.
During a total eclipse, the temperature drops noticeably, and this generally means that winds die down. But in 1901, American meteorologist Helm Clayton suggested that a cold-cored cyclone follows the path of the Moon鈥檚 shadow (see Diagram), explaining why some people report feeling a gust just before or after totality. But the 鈥渆clipse wind鈥� is controversial and some scientists think it鈥檚 a myth. 鈥淢easurements are quite rare because eclipses are often in remote locations,鈥� adds Karen Aplin, a physicist now at the Rutherford Appleton Laboratory in Oxfordshire.
Aplin hoped to find out more using the latest weather instruments, and the eclipse of 11 August 1999 over the south of England gave her the chance. She and meteorologist Giles Harrison of Reading University in Berkshire recorded atmospheric conditions under fairly clear skies at Reading, where the eclipse was partial, and in cloudy Cornwall, where the eclipse was total.
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They now report that at both sites, there was a slight gust at the beginning of totality and another at the end in the opposite direction, just as Clayton had suggested. At Reading they also picked up small changes in atmospheric pressure. These peaked every 35 minutes and persisted for hours after the eclipse. Such pressure waves are probably set up by the cold eclipse shadow racing across the sky at supersonic speeds.
Eclipses might sometimes even improve the weather. Another common anecdote is that clouds tend to disperse just before a total eclipse. Aplin says this could have a scientific basis. 鈥淨uite often cloud is caused by the Sun heating the air and causing it to rise,鈥� she says. Without the Sun, 鈥測ou might expect to have less cloud.鈥�
Aplin, whose report will appear in Proceedings of the Royal Society A, concludes that eclipses can cause significant changes in weather even when it鈥檚 cloudy. She hopes further observations of how the atmosphere reacts to these fleeting 鈥渘ights鈥� will help fine-tune future weather forecasts.