The Moon’s Tidal Forces May Affect How Much It Rains

The higher the moon, the lower the chance of rain

Moon Rising
The moon rises over the Sahara Desert. New research links a rising moon to lower humidity and a decreased chance of rain. roberto cattini/PhotoFVG/Corbis

The moon has long been linked to the ebb and flow of ocean waters—as the gravity of the moon pulls on Earth, the oceans bulge toward it ever so slightly and water levels fluctuate. Now, scientists have discovered another way that silvery body in the sky affects its closest neighbor’s water. A new study suggests that the phase of the moon changes how much it rains on Earth.

Scientists spent two years tracking and verifying the phenomenon, they write in a release. It all started when a doctoral student at the University of Washington spotted a very slight oscillation Earth’s air pressure that corresponded with different moon phases. His research team then used 15 years of weather data to tie that oscillation to rainfall back on Earth.

The same forces that cause the tidal bulge—or lumps in the Earth's oceans—are behind the phenomenon. At any given time, there are two opposing forces that make these bulges of water: one caused by gravity, the other caused by inertia. The side of the planet closest to the moon is sucked in by the orb's gravity, overcoming inertia that pulls in the opposite direction. On the other side, further away from the moon’s tug, the pull of inertia is greater than the pull of gravity and another bulge is formed. (Fun fact: The moon has its own bulges, too, caused by the gravitational pull of Earth.) 

Previous research has shown that these same forces pull on Earth's atmosphere, causing changes in pressure. But this latest research links these bulges with rainfall.

Their observations showed that when the moon is directly overhead, atmospheric pressure mounts along with the atmospheric bulge. High pressures are linked to high air temperatures. When air molecules heat up, they can hold more moisture, which means lower humidity and a lower chance of rain. As the moon sets, its tug weakens, causing lower air pressure and colder air molecules that can’t hold as much moisture and shed rain.

It’s probably not a great idea to reach for your rain boots only when the moon is rising—researchers found that the moon only causes rain levels to vary by about one percent. But they hope to use this data to create more accurate climate models. Meanwhile, as the moon rises and falls it exerts its subtle, almost secretive pressure on our planet’s atmosphere and continues to dazzle its closest neighbors.

Update February 1, 2015: The title of this article has been corrected to show that the moon's tidal forces, not necessarily the phases, influence rainfall.