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It appears that in many (most) places on earth, a pressure oscillation of a 100-200 Pa takes place twice a day : that is, with maxima around 12AM and 12PM and minima around 6AM and 6PM.

I've found almost no information about it, except highly technical (and not freely available) papers. I think it's called "semidiurnal oscillation", and it seems to be a vertical oscillation of the full atmosphere. But I'm curious : what is the basic mechanism of such an oscillation, and what makes its period a half-day (instead of a full day)?

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Solar atmospheric tides? Some things are maximal at noon and midnight, minimal at sunrise and sunset, or vice versa, and could cause 12-hour signals. –  gerrit Oct 21 '13 at 15:52
    
That seems just like it! I'll try and understand that as soon as I can! –  Nicolas Oct 22 '13 at 10:22
    
I'm not sure if it's really it, and I don't count myself as an expert, that's why it's a comment and not an answer. But I've come across 12-hour signals in time series analysis of atmospheric components. –  gerrit Oct 22 '13 at 12:14
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