# A question about Earth's atmosphere/fluids

1. Mar 27, 2009

### fluidistic

Hi,
I'm currently learning about simple fluid mechanics at university and I fall into a problem I couldn't solve but this isn't the point here. This problem made me realize that if for example you drive a car and inside your car you have a hydrogen filled toy balloon, if you accelerate forward then maybe a bit counter-intuitively the balloon will go forward and might bother you when driving.
I thought a bit more about it and now I realize that more the fluid has inertia (density I'd say), the less it will move compared to a fluid that has a lower density (if the fluids are under the action of a force/acceleration). I don't know how to explain this well in English so I understand you if you don't understand what I'm trying to say.
Nevertheless, here is my question : as Earth has an acceleration (centripetal one), its atmosphere should suffer changes during the day. There should be a larger quantity of atmosphere during the night since during the night, Earth's surface is opposed to the centripetal acceleration caused by the sun. But Earth's also hasn't a "simple" motion since it rotates on itself inclined with respect to its orbit. So I guess that it could also be possible to have a large amount of atmosphere during the day, but I'm unsure.
Is this particularity really happening?
If you didn't understand anything, please let me know and I'll explain it in a better way.

2. Mar 27, 2009

### D H

Staff Emeritus

A couple of key references:

Jacchia, L.G. & Slowey, J., "The Shape and Location of the Diurnal Bulge in the Upper Atmosphere", SAO Special Report #207 (1966).
Note the first author. While we now have better models of the Earth's upper atmosphere, the http://modelweb.gsfc.nasa.gov/atmos/jacchia.html" [Broken] was the standard model for the upper atmosphere for a long time.

Vallado, D.A., McClain, W.D., "Fundamentals of Astrodynamics and Applications, Second Edition", Springer (2001).
The summary at Google books says this text "is rapidly becoming the standard astrodynamics reference for those involved in the business of spaceflight." That's not quite right. This book (later revised) is the standard astrodynamics reference for those involved in the business of spaceflight.

Last edited by a moderator: May 4, 2017
3. Mar 27, 2009

### fluidistic

Thank you very much D_H. It seems interesting to me. However it seems that the effect is more due to sunlight warming the atmosphere than the centripetal acceleration as I was expecting. In the document http://books.google.com/books?id=PJ...uNOIfhtgeatIjNCQ&sa=X&oi=book_result&resnum=5, it says "These variations occurs everyday as the Earth rotates". Does they mean rotates around itself or around the Sun? If it's around itself then I don't understand the effect at all.

Last edited by a moderator: May 4, 2017
4. Mar 27, 2009

### D H

Staff Emeritus
The centripetal acceleration you are speaking of is caused by gravity. The atmosphere is not immune to gravitational acceleration. The gravitational acceleration of the Earth's atmosphere toward the Sun and Moon are nearly the same as that experienced at the center of the Earth. There is a slight difference in acceleration because the atmosphere is separated from the center of the Earth by one Earth radius. The atmosphere is subject to tidal forces. Tidal forces generated by the Moon and Sun are nearly symmetric toward and away from the Moon and Sun.

If the tidal forces were the dominant forces in the shape of our atmosphere, the lunar atmospheric tides would be about twice as large as the solar atmospheric tides. However, these atmospheric tidal forces are tiny compared to the heating of the atmosphere by the Sun. The diurnal bulge overwhelms the tidal forces.

5. Mar 27, 2009

### fluidistic

Ah thanks, this makes sense. I totally forgot about tidal forces! I realize now that the effect I was trying to describe was in fact atmospheric tides!