Fresh evidence of 'eclipse wind'

The eclipse caused weather disturbances

By Dr David Whitehouse BBC News Online science editor

Researchers have confirmed the existence of a so-called "eclipse wind", often reported anecdotally during a solar eclipse.

The finding comes from what they say is the most comprehensive set of weather measurements ever made during such an event.

The 11 August, 1999, total eclipse, visible over Britain, the rest of Europe and the Middle East, also produced detectable atmospheric pressure fluctuations.

The effects tracked the shadow of the Moon as it raced over the Earth, covering a region of the planet's surface several thousand kilometres across.

High quality data

The August 1999 event aroused unprecedented interest in Europe and the Middle East from the public and scientists alike.

Dr Karen Aplin, from the Rutherford Appleton Laboratory near Oxford, and Dr Giles Harrison, from the University of Reading, placed accurate weather recording equipment in the path of the eclipse, and compared its recordings with data collected at Reading, which was slightly to one side of the path of totality.

"The meteorological effects of eclipses are not well understood and high quality data are unusual," Dr Aplin told BBC News Online.

"We hoped to investigate the controversy over whether there is such a thing as an eclipse wind, which is often anecdotally reported," she added.

Cold-core cyclone

According to the scientists, their measurements are the first accurate recordings of the weather changes during an eclipse.

"The instruments did detect very small changes which occurred in the cloudy conditions in Cornwall. Combining the data with that obtained in Reading enabled us to determine a structure for the wind patterns associated with the eclipse."

Wind patterns were subtly altered over a region several thousand km across by the cooling induced when the Sun's rays were blocked out.

In particular, the researchers found that a gentle "cold-cored cyclone" was induced at Reading during the eclipse because of the prevailing cloudy conditions coupled with the slight temperature change.

A cold-cored cyclone is a roughly circular pattern of winds with air at the centre sinking to feed the outward flow.

More to learn

"We found that the prevailing weather conditions were disturbed even in the cloudy area, and we found a wind circulation which moved with the Moon's shadow," says Dr Apiln.

"After the eclipse had passed the wind patterns returned to their pre-eclipse values.

"However, we also observed small fluctuations in atmospheric pressure at Reading for a few hours after the eclipse. These were probably eclipse-induced waves caused by cooling in the upper atmosphere, slowly propagating down to the surface."

While the researchers say the cold-cored cyclone model is a good one to explain what they observed, they add that it does not fully explain the wind patterns seen along the boundary between the region of the total and partial eclipse.

The research is published in the Proceedings of the Royal Society.