Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: changing upper atmosphere

Learn more physics!

Most recent answer: 11/01/2017
Q:
I've been reading about changes in the upper atmosphere and it is said that when the upper atmosphere cools it contracts, bringing lower-density gas to lower heights. Because of this, there appears to be a long-term decline in the density of the thermosphere. I see that I don't understand this, because it seems to be the opposite of what would happen if a space contracted. If things contract they must get denser - hence my problem. If you have the same number of neutrals and ions in the air, then if the space they exist in gets less (contracts) then they must get closer together, hence the air would get denser, not less dense.I would be grateful if you could explain what it is that I don't understand about this scenario.The 2 articles that I have seen on this topic are: https://www.nrl.navy.mil/media/news-releases/2004/increasing-greenhouse-gases-lead-to-dramatic-thinning-of-the-upper-atmosphere and file:///C:/Users/sandraprobook/Downloads/ShunrongZhangA-AIM%20(7).pdf Thankyou very much, Sandra Jacobs. P.S. I have sent this a second time because my email address was omitted from the first send.
- Sandra Jacobs
New Zealand
A:

Yes, you're right that the wording in that article was confusing. As I understand it, what they're saying is that increasing density in the lower part of that sector of the atmosphere goes with decreasing density higher up. That does make complete sense. Keeping the total amount constant requires that a density increase in one part be balanced by a density decrease elsewhere. It's just what you expect if that part of the atmosphere gets colder, based on simple physics. The Boltzmann factor says that density falls off faster with height if the temperature is reduced.
The whole effect of cooling part of the upper atmosphere has long been predicted by the same models that show that increasing greenhouse gas concentrations will raise the surface temperature. It's one of the many signatures to show that the current surface warming is indeed caused by our CO2 and CH4 emissions, not some miscellaneous mystery effect.

Mike W.


(published on 11/01/2017)

Follow-up on this answer.