Full moon? Then leave your brolly at home
The Moon can influence the rainfall on Earth, scientists have found.
When the Moon is high in the sky, lunar forces create bulges in the planet's atmosphere that change air pressure to make rainfall lighter.
Rain is less likely to fall when this bulge is at its highest.
Since the full moon is always highest at midnight, that is when there is the least risk of a shower.
Tsubasa Kohyama, a doctoral student at the University of Washington in Seattle, was studying atmospheric waves when he noticed a puzzling oscillation in the air pressure.
He and his co-author John Wallace, a professor of atmospheric sciences, spent two years tracking the phenomenon.
After examining satellite data over the tropics, they discovered a dip in rainfall when the Moon was directly 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.
Since warmer air can hold more moisture, the air parcels are farther from their moisture capacity.
"It's like the container becomes larger at higher pressure," Mr Kohyama said.
The relative humidity affects rain, he explained, because "lower humidity is less favourable for precipitation".
Mr Kohyama and Prof Wallace then 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 rain is slightly lighter when the Moon is high. The change is only about 1pc of the total rainfall variation and is not enough to affect other aspects of the weather, or for people to notice the difference.
The paper, published in Geophysical Research Letters, is the "first study to convincingly connect the tidal force of the moon with rainfall," said Mr Kohyama.
The effect could be to improve climate models and weather forecasts.