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I have just read in the Telegraph an article entitled Moon overhead makes rainfall lighter, scientists conclude. In that article there is the following statement:

When the moon is overhead, its gravity causes Earth’s atmosphere to bulge toward it, so the pressure or weight of the atmosphere on that side of the planet goes up.

I am interested in understanding the Physics behind that increase in pressure.

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Like tides on the ocean, it is a consequence of the tidal forces (if the claim is correct at all which I am verifying now). Tidal forces result from inhomogeneities of the gravitational acceleration.

If we approximated the external gravitational acceleration $\vec g$ caused by the Moon or the Sun to be constant all over the Earth and its vicinity, the Earth and all its parts would just "freely fall" and one couldn't feel the effect of the Sun or the Moon at all.

However, the field $\vec g$ isn't quite uniform – it points to the direction of the center of the Sun or the Moon (instead of having parallel lines); and it decreases as $1/r^2$ with the distance from the Sun or the Moon.

When the Moon is overhead, the points in your head (closer to the Moon) have a greater $\vec g$ than those in your feet. This difference tries to make your body stretch in the vertical direction, relatively speaking. This goes the sea level goes up at this point. Because of the even symmetry, the sea level also goes up if the Moon is directly underfoot. However, on the maximum circle in between these two extreme points on the Earth defined by the Moon, the effect is reverted and the sea level goes down.

These people argue that the same "stretching" acts on the atmosphere as well. It's somewhat plausible but it seems to me that the difference of the pressure will only be equivalent to the 1-meter-or-so change of the altitude that you get from the increased sea level i.e. from the deformation of the equipotential surfaces. Due to omnipresent oscillations, this change pressure by 0.01% should be almost unobservable.

Addition: the paper in Geophysical Research Letters is here:

http://onlinelibrary.wiley.com/doi/10.1002/2015GL067342/full

They claim that the main effect of the tidal forces is on the relative humidity and the effect may actually be seen in the precipitation data.

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  • $\begingroup$ Thank you for such a detailed response. The reason I asked the question was that it seemed to me that the local value of g on the Earth would be reduced by the presence of the Moon and thus just because the atmosphere had a greater height it did not necessarily mean that the pressure had increased. $\endgroup$ – Farcher Feb 4 '16 at 10:39
  • $\begingroup$ I am sure we have similar confusions. I don't know the full theory myself. Their paper is too phenomenological for me. A discussion could also start here: motls.blogspot.com/2016/02/does-moon-cause-more-rain.html?m=1 $\endgroup$ – Luboš Motl Feb 4 '16 at 11:11
  • $\begingroup$ I like the word "phenomenological" as it encapsulates my understanding of the paper without knowing what the word means! $\endgroup$ – Farcher Feb 4 '16 at 11:47
  • $\begingroup$ Thanks ;-) In particle physics, it's being used in this way the two of us understood since early 1990s when Paul Ginsparg established two archives at arxiv.org, particle physics "theory" and "phenomenology". The latter - new term in particle physics - was meant to be an application of science to actual phenomena, with the focus on what is observed (phenomena), not what is behind it. It contrasts with "theory" that tries to explain the reasons as carefully as you can. That's very different from psychology and philosophy where "phenomenology" studies (phenomenon of) consciousness only. $\endgroup$ – Luboš Motl Feb 4 '16 at 13:47
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Their claim that atmospheric pressure would increase if moon is over head is wrong.

Atmospheric pressure at same height is same. This follows directly from Pascals law.

So, even if moon is directly upwards or whatever is the shape of bulge out. It does not matter.

If there was an increase in atmospheric Pressure , it will create a pressure gradient. This difference will cause air to move from high pressure to lower . Hence , equalizing it.

So, at all places on Earth at same height, atmospheric pressure remain same, regardless of height of atmosphere above it.

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