How the Moon Makes it Rain (Less)

For the first time scientists find convincing evidence that the tidal force of the moon influences rainfall, if only a little.

Humans have long granted the moon and its avatars extraordinary powers: The ancient Greeks believed that Selene drove her moon chariot across the sky each night; the ancient Egyptians thought that the moon god Khonsu possessed great healing powers; and the Maya assumed the moon to be female and the lunar phases to correspond to various stages of a woman's life.

Now, a team of scientists from the University of Washington (UW) have a new lunar attribute to add to the list, albeit one rooted more in empiricism than mysticism: The moon's ability to affect rainfall on Earth.

In a study published on Saturday in Geophysical Research Letters, the scientists found that when the moon is overhead it leads to very slight atmospheric changes resulting in lighter rainfall on Earth.

"As far as I know, this is the first study to convincingly connect the tidal force of the moon with rainfall,"said the study's corresponding author Tsubasa Kohyama, a UW doctoral student in atmospheric sciences, in a statement.

While studying atmospheric waves (disturbances in atmospheric variables such as air pressure or temperature) Kohyama noticed slight, regularly occurring oscillations in air pressure that could not be explained. Kohyama and study co-author John Wallace, a professor of atmospheric science at UW, spent two years gathering data to try to explain the phenomenon.

Their work culminated ina 2014 study which conclusively demonstrated that air pressure on the surface of Earth varies with the phases of the moon, a phenomenon which was first observed in 1847. They found that, when the moon is overhead, it causes the Earth's atmosphere to bulge towards it, so the atmospheric pressure on that side of the planet goes up.

The team isn't advocating that you start carrying an umbrella based on the phases of the moon

But as the team notes in their most recent study, this atmospheric change also accounts for "imperceptible" changes in the amount of rainfall on Earth.

The reason for this is that regions of higher air pressure increases the temperature of the air pockets below. Since warmer air can hold more moisture, these lunar-influenced air pockets need more moisture than usual to reach capacity, and are thus less likely to dump their contents.

"It's like the container becomes larger at higher pressure," Kohyama said, explaining the phenomenon. The increased moisture carrying capacity in these high pressure regions leads to lower humidity, and as Kohyama notes, "lower humidity is less favorable for precipitation."

To arrive at this conclusion the team made use of 15 years' worth of data collected by NASA and the Japan Aerospace Exploration Agency's (JAXA) Tropical Rainfall Measuring Mission satellite.

While the connection between the lunar phases and decreased rainfall is an interesting one, the team isn't advocating that you start carrying an umbrella based on the phases of the moon. The change in rainfall which results from lunar influence is roughly 1 percent of the total rainfall variation, so it isn't really enough for people to notice the difference.

Rather, this data is mainly of interest for those looking to create more accurate climate models because their physics must incorporate the atmospheric effects of the moon. In the future, the UW team hopes to push the topic further and see if certain types of rain, such as heavy downpours, are more susceptible to lunar phases and whether these phases affect the frequency of rain storms.