When the moon is high overhead at night, its gravity actually can reduce the amount of rainfall very slightly, new research reveals.
In an article for Geophysical Research Letters, University of Washington scientists Tsubasa Kohyama and John M. Wallace report that the moon causes the Earth's atmosphere to bulge toward it. That causes the pressure, or weight of the atmosphere, on that side of the planet to go up, which in turn increases the temperature of air below.
Since warmer air can hold more moisture, the same parcels of air are now farther from their maximum moisture capacity, resulting in a slight dip in rainfall.
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The researchers studied 15 years of data collected by NASA and the Japan Aerospace Exploration Agency's Tropical Rainfall Measuring Mission satellite from 1998 to 2012. It proved that the rain is reduced by an amount that is measurable, though imperceptible to humans - a change of about 1 percent in the total rainfall variation.
"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 in atmospheric sciences.
While the effect isn't going to affect agriculture or alter weather forecasts, the knowledge is potentially beneficial to climate researchers, who can use it to test the physics behind their climate models.
The effect of the moon's position on air pressure on Earth was first detected back in 1847, and researchers showed in 1932 that the moon could affect air temperature as well. A 2014 study by the same University of Washington researchers confirmed that air pressure on Earth varies with the position of the moon.
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Wallace plans to conduct future studies to see whether certain types of rain storms, such as heavy downpours, are more susceptible to the moon's position, and whether the moon has any effect on the frequency of storms.