Well, at least one Moon myth is true. The lunar location influences rainfall, albeit only very slightly. The discovery is not all that surprising, given the long-established links between the Moon and both tides and atmospheric pressure, but it took analysis of an awful lot of data to prove it.

We already know that the tides are determined by the gravitational tug of Earth's satellite, and in the 19th century, we learned that atmospheric pressure also rises and falls with the position of the Moon, although this effect can be swamped by much larger changes from sunlight and storms.

Since atmospheric pressure influences rainfall, it makes sense for this lunar effect to influence rain as well. However, the relationship is so subtle it can be drowned out by other forces, making it very difficult to detect.

In Geophysical Research Letters, University of Washington Ph.D. student Tsubasa Kohyama used 15 years of satellite rainfall data to demonstrate that it rains less when the Moon is overhead or directly on the other side of the Earth.

The lunar influence only accounts for about 1 percent of rainfall variation though, and as Kohyama put it in a statement: "No one should carry an umbrella just because the Moon is rising," Still less should people use the findings to support discredited claims, such as an effect of Moon phases on mental health or birth timing. 

Previous efforts have been made to find a link between lunar position and rainfall, but the paper notes, “Until recently, studies of atmospheric tides have been based exclusively on a sparse network of station data.” Satellites allowed Kohyama to draw on observations taken from the tropics and temperate zones eight times a day.

It is the Moon's location, relative to a particular point on Earth, that matters for rainfall, rather than the amount of light it is reflecting. However, the Moon is full when overhead at midnight, while first-quarter moons are closest to us around sunset. Consequently, the phases do relate, albeit weakly, to the time of day when rain is most likely.

Kohyama explains the connection by noting that when the tide is high, the atmosphere bulges slightly and air pressure increases, enabling it to hold more water vapor. "It's like the container becomes larger at higher pressure," Kohyama said. The relative humidity affects rain, he said, because "lower humidity is less favorable for precipitation."

For every 1 percent increase in relative humidity, Kohyama found 10 percent more precipitation occurs, but since lunar position is only one of many factors determining humidity, and far from the largest, the strength of this connection wasn't enough to make the Moon more than a minor influence on rain.

^{Image in text: It's not just the tide that is rising, the atmosphere bulges when the Moon is overhead. Triff/Shutterstock}