On January 27th, 2016 we learned about

The Moon’s pull on the atmosphere creates a tiny bit more precipitation

As the Moon orbits the Earth, it exerts gravitational forces strong enough to move the less-solid bits around, leading to movements of material that rise and fall on a daily basis. While this concept most famously applies to the daily tides in our oceans, water isn’t the only substance to be pulled and collected in response to the Moon. The atmosphere, being a gas, is pulled by the Moon as well, but for the most part, we don’t notice it the way we do something more visible, like a waterline at the beach. Analysis of 15 years of data, however, has found that the tidal influence on the atmosphere appear to be strong enough to change the weather. At least a little bit.

It seems obvious that the gases in our atmosphere would moved significantly by the Moon’s gravity, since water, as a colder, heavier substance moves an average of 21 inches per tidal cycle, depending on the local terrain. But the movement of the planet’s air turns out to be almost imperceptible. The one hypothesized effect was that movement in the atmosphere would have to be changing air pressure as well, which can lead to shifts in the weather. This was first purposed in 1969, but the effect is so small that were no good data sources to test with. It wasn’t until the launching of weather satellites decades later that scientists could really look for measurable, Moon-induced shifts in the weather. After looking at 15 years of data, with eight measurements a day, the hypothesis has been supported, although even the researchers working the project stress that a rising Moon isn’t reason to bring an umbrella to the beach.

Rising moon, falling temperatures

The logic behind the Moon’s influence is based on how air pressure and temperature relate to humidity. As the Moon’s gravity gathers more air beneath it in an atmospheric ‘high tide,’ the more densely collected air has a higher air pressure and temperature. This air can then hold more water vapor in the sky, lowering the chances of rain. At low-tide, the opposite takes place. The thinner layer of air has lower air pressure and temperatures, and is therefore unable to support as much water vapor. That vapor is more likely to condense and fall as rain, leading to the one micrometer change in rainfall that was able to be attributed to the Moon.

A meteorological drop in the ocean

The essentially imperceptible effect on the weather isn’t because tidal forces are weak. Tidal forces obviously move the oceans a great deal, greatly impacting the plants and animals that live there, particularly along the shore. The issue is that the change in weather is an indirect effect of the tides, and is subject to a variety of influences, like other temperature and weather conditions, not to mention other tidal forces, like those coming from the Sun. With this many variables pushing, pulling and moving the air, the Moon’s influence is basically washed out.

Source: Atmospheric tides alter rainfall rate by Thomas Sumner, Science News

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