Moon Phase Affects Rainfall

In voodoo, new age, astrological, aura, garbage news, the Moon’s phase affects mental health and is a general consideration for werewolves.  In real and useful science, the Moon is that occasional sight in the sky that gives us ocean tides and casually reminds us that the Earth actually has a big ball of rock falling around it.  But in a strange twist, new data suggests that the Moon actually does affect one facet of human experience: Rainfall.

Satellite data over the tropics, between 10 degrees S and 10 degrees N, shows a slight dip in rainfall when the moon is directly overhead or underfoot. Credit: Tsubasa Kohyama/University of Washington

A new paper from the university of Washington suggests that there is a slight dip in rainfall when the Moon is overhead or underfoot, due to the tidal force exerted by the Moon on the Earth.  For a good rundown of the tidal force and how that affects your weight, see this post.

Of course, if this was significant, we would have known about it long ago, so how much of a difference does this make? Well as expected, since the Moon’s gravity only changes a person’s weight by about a millionth of a kilogram, it only changes rainfall by 0.78 micrometers, an imperceptible amount.  It took an analysis of 15 years of data to prove this, so nobody will be walking outside during an overhead Moon and noticing a slight lack of rainfall anytime soon.

Okay so we know it’s happening, but why would this happen?

We know from the link above that the Moon has a tiny pull for matter on Earth.   That tiny pull tugs on the atmosphere and causes it to bulge on the Moon-facing side.  The extra atmosphere creates extra pressure, in the same way that deeper water has higher pressure because of the weight of the water above it.  Extra pressure on the atmosphere in turn creates a slightly higher temperature.  So the air is warmer, albeit imperceptibly.  Here’s where it connects to rain – warmer air can hold more moisture, meaning slightly less moisture will fall as rain because the air can hold slightly more of it.

This is where it all comes from, that tiny little pull of gravity is making it rain less.  Isn’t science fascinating?


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