When you see a bad moon rising, expect an ever-so-slightly wetter day. The lunar gravitational pull imperceptibly boosts rainfall when the moon is on the horizon and somewhat reduces rainfall when the moon is overhead or on the opposite side of the Earth, a new analysis of global rainfall concludes.
The cause is the atmospheric equivalent of ocean tides, researchers propose in a paper to be published in Geophysical Research Letters. Air gathers on Earth’s moon-facing side and on the opposite end of the globe. Scientists noticed that this pileup increases atmospheric pressure and predicted that atmospheric tides could alter precipitation rates as well. Scouring 15 years of global precipitation data, the researchers have discovered that the effect is present, but tiny: an approximately one micrometer per hour change in rainfall rate.
“No one should carry an umbrella just because the moon is rising,” says lead author Tsubasa Kohyama, an atmospheric scientist at the University of Washington in Seattle. The effect’s meager size may be an asset, however: The relationship between the moon’s tug and rainfall could serve as a useful test of how accurately weather simulations handle small external forces, Kohyama says.
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The moon’s gravitational pull, which is responsible for ocean tides, also creates atmospheric tides. As more air gathers during atmospheric high tide, atmospheric pressure increases. In 1969, scientists proposed that these rises and falls in atmospheric pressure could also cause changes in relative humidity and rainfall. A lack of global weather data at the time meant that this prediction would go unconfirmed for nearly half a century.
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Satellites now offer global coverage of where and when rain falls. Kohyama and climate scientist John M. Wallace, also of the University of Washington, compiled 15 years of weather data from across the tropics collected eight times a day. The researchers separated typical weather variations from those that came and went periodically alongside the moon’s monthly loop.
The data revealed that during atmospheric high tide, rising air pressure slightly increases air temperature. That temperature boost allows the air to hold more water vapor, lowering the relative humidity and making rain less likely. During low tide, pressures drop slightly, cooling the air, raising the relative humidity and making rain more likely. This effect amounts to about a hundredth that of the typical background weather variability, Kohyama says.
Understanding the lunar influence on rainfall won’t change how we predict the weather, says Eduardo Agosta Scarel, a climate scientist at the Pontifical Catholic University of Argentina in Buenos Aires. The effect is so small that it quickly disappears into the background noise with time, he notes.