Why low clouds disappear during a solar eclipse

The magnificence of a solar eclipse is unique to our world; Nowhere else in the solar system does a planet’s moon block the sun’s light so perfectly. The rapid and transient darkness of these events affects many things on Earth, including animal behavior and waves in the ionosphere.

Researchers have now found that cumulus cloud cover dropped by more than a factor of 4 on average as the moon’s shadow passed over Earth during a recent annular solar eclipse.

This little-studied aspect of solar eclipses offers important lessons for geoengineering efforts aimed at blocking sunlight, the team suggested.

Related: What happens if it’s cloudy during the April 8 solar eclipse?

Experiments in the air

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Solar eclipses occur two to five times a year, and these events provide great opportunities for scientific research, says Victor JH 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 quickly dimmed, he said. “They are very different from the normal day-night cycle.”

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

The researchers focused on a square area 5° in both latitude and longitude, centered over South Sudan. From their bird’s eye view, they followed the evolution of the clouds for several hours leading up to the eclipse, during the eclipse and for several hours afterwards.

Goodbye, sun; Goodbye, clouds

Low cumulus clouds – which tend to reach their tops at an altitude of about 2 kilometers (1.2 miles) – were strongly influenced by the degree of solar eclipse. Cloud cover began to decrease when about 15% of the sun was covered, about 30 minutes after the eclipse began. It wasn’t until about 50 minutes after the maximum eclipse that clouds began to return. And while typical cloud cover under non-eclipse conditions hovered around 40%, less than 10% of the sky was covered in clouds during maximum eclipse, the team noted.

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

To investigate the physics behind their observations, Trees and his colleagues collected land surface temperature measurements from the same two geostationary satellites. The temperature of the ground matters when it comes to cumulus clouds, Trees said, because they are low enough to be significantly affected by what happens on the Earth’s surface.

Not surprisingly, land surface temperatures dropped as the moon increasingly blocked the sun’s light. “We knew that even small changes in solar radiation have an effect on land surface temperatures,” said Virendra Ghate, an atmospheric scientist at Argonne National Laboratory in Lemont, Illinois, who was not involved in the study.

The researchers estimated a maximum change in land surface temperature of almost 6°C for the 2005 solar eclipse. They also found that surface temperatures decreased in parallel with the eclipse fraction, with no significant time difference. This is consistent with observations made during other solar eclipses.

Follow the heat

The pronounced drop in land surface temperatures during an eclipse is the cause of changes in cumulus cloud cover, the researchers concluded. That makes sense, Ghate said, because cumulus clouds form when relatively warm and moist air rises from the Earth’s surface, cools and eventually condenses into cloud droplets. When temperatures at the land surface drop, there is a smaller temperature gradient near the Earth’s surface and therefore a smaller force pushing cloud-forming air upward, he said. “You don’t have the source of buoyancy.”

The delays that Trees and his colleagues observed – between the start of the eclipse and the moment the clouds began to disappear and also between the time of maximum eclipse and the moment the clouds began to return – also reveal something about the so-called boundary layer, the lowest level of the Earth’s surface. atmosphere. Each of these delays has a physical meaning, Trees said. “It tells us how fast the air is rising.”


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These new findings not only shed light on the physics of cloud dissipation during solar eclipses, but also have implications for future geoengineering efforts, Trees and his collaborators suggested. Discussions are underway to mitigate the effects of climate change, for example by seeding the atmosphere with aerosols or sending solar reflectors into space to prevent some of the sunlight from reaching Earth. Such geoengineering holds promise for cooling our planet, researchers agree, but its consequences are largely unexplored and could be widespread and irreversible.

These new results suggest cloud cover could decrease as a result of geoengineering efforts involving solar eclipses. And because clouds reflect sunlight, the effectiveness of any effort could decrease accordingly, Trees said. That’s an effect that should be taken into account when considering different options, the researchers concluded.

This article was originally published on Eos.org.

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