During a solar eclipse, when the Moon moves between the Earth and the Sun, the astonishing spectacle momentarily alters our perception of the world. Yet, the impact on our planet extends beyond mere moments of daylight darkness.
One surprising effect is the rapid dissipation of clouds when only 15 percent of the Sun is obscured by the Moon. Of course, not all cloud types vanish, or else complaints about eclipses ruined by overcast skies wouldn’t exist.
A research team led by Victor Trees from the Royal Netherlands Meteorological Institute and Delft University of Technology discovered that shallow cumulus clouds over land swiftly dissipate during an eclipse.
Trees suggests this finding holds significance for potential climate engineering endeavors. “If we employ technological solutions to eclipse the Sun in the future, it could influence cloud formation,” he explains. “Reduced cloud cover might hinder climate engineering’s intended purpose since clouds reflect sunlight and aid in cooling the Earth.”
Understanding how clouds behave during an eclipse from our viewpoint on Earth’s surface poses considerable challenges. However, as Trees emphasizes, it’s crucial. One proposed solution to combat climate change involves obstructing some of the Sun’s rays from reaching the Earth’s lower atmosphere. While modeling suggests this could effectively reduce temperatures, the full scope of its consequences remains uncertain.
Counting clouds from Earth is impractical due to the complexity of cloud layers and their constant motion. Moreover, previous satellite studies failed to factor in the Moon’s shadow during eclipses when calculating cloudtop reflectivity, leading to biases in measurements of cloud cover and thickness.
To address this, Trees and his team devised a method to correct for the lunar shadow. They accounted for the proportion of the Sun obscured at any given time and location on Earth’s surface.
“Most of the solar eclipse comprises a partial eclipse phase, where sufficient light remains outside,” Trees explains. “During this partial eclipse, satellites receive ample reflected sunlight, allowing for accurate cloud measurements after adjusting for obscuration.”
Continuing their research, the team applied their methods to data gathered during three previous solar eclipses across the African continent spanning from 2005 to 2016. They were astonished to observe cumulus clouds vanishing in significant numbers with just a 15 percent coverage of the Sun, continuing until the eclipse concluded.
To understand this phenomenon, the team conducted simulations using cloud modeling software, DALES. These simulations revealed that when sunlight is obstructed, the surface undergoes cooling, diminishing the warm air updrafts from below.
These updrafts play a crucial role in cumulus cloud formation by carrying water vapor, which condenses into droplets at cooler altitudes, thus forming clouds.
As the ground cools and the updrafts diminish, cumulus clouds cannot be sustained, only reappearing once the Sun re-emerges to warm the surface again. This effect is specific to land areas, as the ocean’s slower cooling prevents this phenomenon.
While cumulus clouds themselves are not rain clouds, they can evolve into rain clouds. The team’s discovery implies that climate geoengineering methods involving sunlight blockage could significantly disrupt weather patterns.
Given that this outcome contradicts scientific objectives, the researchers stress the necessity for further investigation into this phenomenon.
The research has been published in Communications Earth & Environment.
