Why low-level clouds vanish during a solar eclipse

The splendor of a solar eclipse is unique to our world — nowhere else in the solar system does a planet's moon so perfectly block the light of the sun. The fast and fleeting darkness of those events affects many things on Earth, including animal behavior and waves in the ionosphere. Researchers have now found that cumulus cloud cover fell by more than a factor of 4, on average, as the moon's shadow passed over Earth during a recent annular eclipse. This little-studied aspect of solar eclipses has important lessons for geoengineering efforts aimed at blocking sunlight, the team proposed.

Experiments in the sky

Solar eclipses occur anywhere from 2 to 5 times per year, and these events confer neat opportunities for scientific investigations, said Victor J. H. Trees, a geoscientist at Delft University of Technology in the Netherlands. "Solar eclipses are unique experiments." They allow researchers to study what happens when sunlight is rapidly obscured, he said. "They're very different than the normal day-night cycle."

Trees and his colleagues recently analyzed cloud cover data obtained during an annular eclipse in 2005, visible in parts of Europe and Africa. They mined visible and infrared imagery collected by two geostationary satellites operated by the European Organisation for the Exploitation of Meteorological Satellites. Going to space was key, Trees said. "If you really want to quantify how clouds behave and how they react to a solar eclipse, it helps to study a large area. That's why we want to look from space."

The researchers focused on a square region spanning 5° in both latitude and longitude centered above South Sudan. With their bird's-eye perspective, they tracked cloud evolution for several hours leading up to the eclipse, during the eclipse, and for several hours afterward.

Related: Places with the best weather to watch the April 8 solar eclipse (and what happens if it's cloudy where you are)

Goodbye, sun; Goodbye, clouds

Low-level cumulus clouds — which tend to top out at altitudes around 2 kilometers (1.2 miles) — were strongly affected by the degree of solar obscuration. Cloud cover started to decrease when about 15% of the sun's face was covered, about 30 minutes after the start of the eclipse. The clouds started to return only about 50 minutes after maximum obscuration. And whereas typical cloud cover hovered around 40% in noneclipse conditions, less than 10% of the sky was covered with clouds during maximum obscuration, the team noted.

"On a large scale, the cumulus clouds started to disappear," Trees said.