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The other day I saw this life-hack:

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And I was wondering how true it is. First of all, I always thought(listening to weather forecasts) that low-pressure atmosphere is what correlates with rain; although I never learned the argument behind it.

Anyway, even if low atmospheric pressure correlates with storms and rain; how much(and in what sense) does this affect the bubbles in my morning coffee(if any)? My initial guess is that there should be a threshold pressure that will differentiate whether the bubbles will stick to an edge or suspend in the middle. Since this is an everyday-life question, I should say that experimental answers are welcome as well rigorous theoretical ones.

Also, I am wondering if we can we make a barometer based on the bubbles in a mug of coffee? If so, how sensitive it would be?

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@JohnRennie I wasn't aware of that post! Perhaps the true home of that question should be Physics SE. Anyway, the question doesn't have an answer, and as a physicist I am more concerned about the physics; not just an answer. –  Ali Aug 21 '13 at 8:24
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To be honest I think the whole idea is rubbish, though I have no experimental evidence to justify my claim. A quick Google makes claims that increased pressure makes the coffee surface concave or affects bubble formation within the coffee. However the former is clearly wrong and I'd guess bubbles are due to entrained air during stirring. I'd guess the bubble behaviour is dominated by capillary forces, and it's hard to see how these would be affected by pressure. Humidity could affect the wetting propeties of the cup, so I suppose humidity could have some effect. –  John Rennie Aug 21 '13 at 8:34
    
@JohnRennie Yeah, honestly something inside me agrees with you(the possibility of the idea being rubbish ;-), but I would rather see an explicit answer before judging its truth. I think that's how science should work. –  Ali Aug 21 '13 at 9:32
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1 Answer

up vote 4 down vote accepted

This is quite humorous.

In an 1883 offical US military publication, "Weather Proverbs" by 1st Lt. Dunwoody, at page 107 it is stated "When coffee bubbles collect in the centre of the cup expect fair weather. When they adhere to the cup, forming a ring, expect rain." This is the converse of the lifehack proverb!

In 1997 Dave Thurlow, using a grant from the US National Science Foundation, insisted that the "bubbles in the middle mean sunny" version was supported by theory. http://www.weathernotebook.org/transcripts/1997/01/31.html However, he bases this on the coffee/air interface switching from concave up to convex with change in atmospheric pressure, which is extremely doubtful. Instead, the material of the cup wall and its interaction with the coffee would determine concavity.

The Young-Laplace equation describes the air/liquid interface.

The fact that the proverb and its converse are both professed should be a strong indication that there is no truth in the proverb, absent some logical reason based upon the Young-Laplace equation.

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Slight quibble: by "converse" I think you mean "negation" or "opposite" (logical complement). (A theorem and its converse can both be true: converse of $A\Rightarrow B$ is $B\Rightarrow A$). I only mention it because it did throw me for a bit, and so it might do other readers as well. BTW good answer. –  WetSavannaAnimal aka Rod Vance Feb 27 at 23:20
    
you're right, sorry –  DavePhD Feb 27 at 23:26
    
@DavePhD That's what I call a nice answer! –  Ali Feb 28 at 0:19
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