Three hundred and one years after Edmund Halley described weather changes associated with a solar eclipse, two papers have been published confirming the phenomenon he, and many since, observed. Moreover, with the help of 4,500 citizen scientists, the wind changes that accompany these events have been explained.
On May 3, 1715, a total solar eclipse occurred over southern England. Halley, most famous for the comet that bears his name, predicted the eclipse timing almost perfectly, and took detailed observations. Among these was a description of the “Chill and Damp which attended the Darkness” of the eclipse, something Halley said caused “some sense of Horror” among those who observed it.
Since then, this cooling and increased relative humidity have been observed many times. While these are the expected effects of the absence of sunlight, reports of changes in wind direction have been more controversial. In an effort to produce a definitive account of eclipse effects, a University of Reading team led by Dr Luke Barnard had volunteers at 309 locations across the UK observe the weather during the eclipse of March 20, 2015. This was a total eclipse over the North Sea, and 83 to 98 percent across Britain.
Barnard's collection effort is published in a paper in Philosophical Transactions of the Royal Society A. This is the same journal in which Halley made his original observations.
Based on 15,606 recordings of air temperature, cloudiness, and wind speed, they concluded the approach of the Moon’s shadow produced a reduction in average wind speeds of about two knots (3.7 km/h, 2.3 mph), as well as a 20º change in direction. "As the Sun disappears behind the Moon the ground suddenly cools, just like at sunset,” Harrison said in a statement. “This means warm air stops rising from the ground, causing a drop in wind speed and a shift in its direction, as the slowing of the air by the Earth's surface changes.”
Several explanations have been offered in the past for similar, if less detailed, observations, including an “eclipse cyclone” and sea breeze effects, but Harrison argues the explanation he and Gray have provided fits the collected data better. They also note the solar eclipse of 1999 produced similar observations.
In England’s green and pleasant land, the eclipse effects were modest, reduced further by heavy cloud in many areas. However, Harrison and Gray note that in Svalbard, one of the few places where this eclipse was total over land, the cooling and lack of wind from the eclipse produced such severe fog it shut down the airport.
Time-lapse of the eclipse as seen from Svalbard. THANAKRIT SANTIKUNAPORN/Shutterstock