Washington, Jan 31: Changing phases of the Moon affect the amount of rainfall on Earth, though very slightly, scientists have found for the first time. When the Moon is high in the sky, it creates bulges in the planet's atmosphere that creates imperceptible changes in the amount of rain that falls below, researchers said.
"As far as I know, this is the first study to convincingly connect the tidal force of the Moon with rainfall," said Tsubasa Kohyama, a doctoral student at the University of Washington.
Previous study by the researchers used a global grid of data to confirm that air pressure on the surface definitely varies with the phases of the Moon.
"When the Moon is overhead or underfoot, the air pressure is higher," Kohyama said.
The study is the first to show that the Moon's gravitational tug also puts a slight damper on the rain. When the Moon is overhead, its gravity causes Earth's atmosphere to bulge towards it, so the pressure or weight of the atmosphere on that side of the planet goes up.
Higher pressure increases the temperature of air parcels below. Since warmer air can hold more moisture, the same air parcels are now farther from their moisture capacity.
"It's like the container becomes larger at higher pressure," Kohyama said. The relative humidity affects rain, he said, because "lower humidity is less favourable for precipitation."
Researchers used 15 years of data collected by NASA and the Japan Aerospace Exploration Agency's Tropical Rainfall Measuring Mission satellite from 1998 to 2012 to show that the rain is indeed slightly lighter when the Moon is high.
The change is only about 1 per cent of the total rainfall variation, so not enough to affect other aspects of the weather or for people to notice the difference.
This effect could be used to test climate models, Kohyama said, to check if their physics is good enough to reproduce how the pull of the Moon eventually leads to less rain. The study was published in the journal Geophysical Research Letters.