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Readings from the Tropical Rainfall Measuring Mission, shown in this artist’s conception, provided evidence to support a link between lunar tides and rainfall patterns. (Credit: NASA)

When the moon is high in the sky, its gravitational pull warps the atmosphere enough to reduce rainfall ever so slightly. At least that’s the conclusion that researchers from the University of Washington reached after reviewing 15 years of detailed rainfall data.

The evidence is laid out in a paper to be published in Geophysical Research Letters.

Rainfall and pressure
Satellite readings show a slight dip in rainfall when the moon is directly overhead or underfoot. The top panel shows air pressure, the middle panel shows the rate of change in air pressure, and the bottom panel shows the rainfall difference from the average. (Credit: UW)

Readings from the U.S.-Japanese Tropical Rainfall Measuring Mission, collected between 1998 and 2012, suggest that rainfall is reduced by about 1 percent if the precipitation falls when the moon is directly overhead or underfoot.

Those findings are in sync with a 2010 study that laid out a similar link between phases of the moon and precipitation. Both papers show that lunar tides have an effect on the atmosphere.

The more recent study, conducted by UW doctoral student Tsubasa Kohyama and atmospheric scientist John Michael Wallace, delves deeply into the mechanism behind the moon’s effect.

Here’s how it works: The moon’s gravitational pull causes Earth’s atmosphere to bulge toward it, just like the ocean’s tides do. As a result, the pressure or weight of the atmosphere in the bulge rises. This atmospheric tide effect on air pressure has been documented in studies going back to 1847.

Higher pressure tends to increase the temperature of the air parcels below. The warmer air is capable of retaining more moisture. “It’s like the container [for moisture] becomes larger at higher pressure,” Kohyama said in a UW news release. That would reduce the amount of moisture falling out of the atmosphere as rain.

The relative reduction is too small to be noticed when you’re walking through the rain. It only shows up in a statistical analysis of rainfall, like the one conducted by Kohyama and Wallace. But the researchers say it’s another factor that should be taken into account when scientists develop and test climate models.

Now Wallace wants to find out whether the phases of the moon affect the frequency of rainstorms, or whether heavy downpours are more susceptible to lunar tidal influence than Seattle-style drizzles.

In the meantime, don’t forget your umbrella … even if the moon is high.

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