Regarding atmospheric processes, I understand that energy is required to evaporate water by moving molecules further apart i.e. a phase change from liquid to gas. The air then ascends into the atmosphere before cooling and releasing latent heat as the water molecules condense. This is often stated to be an important heat transfer from the surface to the atmosphere. I understand how the latent heat is transported by the evaporated molecules(convection)but how exactly is it released? Gas is a poor conductor of heat so it can't be from conduction to lower temperature, slower moving air molecules. Is it radiated somehow? It can't be an adiabatic process.
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3 Answers
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As I understand your question you are interested in the releasing mechanism of latent heat during condensation.
Both answers before me somehow don't relate to this, only to the transport before condensation.
The thing to understand here is, that as your air rises adiabatically and cools at the same time, it will hit a low temperature that allows it to condense again. Now in the phase transition, just as we put in the energy before to separate the molecules, we get it back upon condensation. But the condensed molecules will always be attached to some nucleation cores which can absorb the leftover energy.
If the nucleation cores were small (of molecular size), they could thus gain kinetic energy, but as they are usually dusty particles (~$\mu$m) they can be directly heated.
I think you can visualize this process perhaps in a very simplified version as follows:
You have 2 spheres unattached, flying around as pair(our test molecule). Now upon condensation a magical rubber band appers and accelerates one particle to the other, until the rubber band reaches equilibrium (the newly reached solid state). But the leftover kinetic energy (our latent heat) will then either accelerate the solidified molecule, or bounces with the much bigger dust grain where the kinetic energy thermalizes.
answered May 24, 2014 at 14:27
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$\begingroup$ Thank you for that response. You knew exactly what I meant, which is great because I wasn't sure if I had made myself clear or not. Great analogy too. It really helped me visualise the heat release mechanism. So just to summarise, the released latent heat of condensation goes into either heating the condensation nuclei or contributes to an increase in the internal energy of the newly condensed water molecules. $\endgroup$ May 24, 2014 at 19:56
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$\begingroup$ Yes. Actually if you think a bit more detailed about it, as the internal energy of the condensed molecules must always equilibrate with the kinetic energy, this would mean, that they either re-evaporate or give it away. This is a reason why we actually will always see condensation as soon as there are condensation nuclei available. $\endgroup$ May 24, 2014 at 21:43
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In fact it is an adiabatic process.
Pressure decreases with increasing height, so as air rises it expands adiabatically, and as it expands it cools. At the same time condensation of water vapour heats the rising and cooling air and you end up with some equilibrium state.
answered May 24, 2014 at 11:54
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I think the answer you are looking for is radiation.
But there are a few questions here, this process has to be broken down a bit:
- Cold air absorbs heat near the surface,
- It is carried to the top of the troposphere by convection, adiabatically cooling in the process.
- Meanwhile the whole mass of air is a black-body radiator, but since it's not very hot all the radiation is in the IR range.
The important point is that earth only looses heat by black-body radiation, and that the earth's atmosphere is opaque at the frequencies where a 300 K object radiates. Thus radiation (and therefore heat) can only escape from the upper atmosphere. The water is important because it increases the heat capacity of the air via the latent heat you're asking about, which in turn carries more heat to the upper atmosphere where it can escape.
