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By definition, rainfalls are transitions from vapor state to liquid state of water. I can say that "by definition" rainfalls must viewed as critical phenomenon?

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useing the definition as a guide is a great tool if this "rainfall" meets all the requirements of the definition than yes but all requirements must be met to label it as critical phenomenon. Make sure you form this "Comment" as a question i would suggest asking "as critical phenomenon is defined as """" then I am lead to believe """" has meet these requirements. Is there another definition that describes the requirements of a critical phenomenon? –  Argus May 28 '12 at 20:03

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My immediate reaction is that the condensation of water vapour to form water droplets is unlikely to be a critical phenomenon, but a quick Google suggests that various authors have claimed critical behaviour in clouds. However from my cursory glance at the Google results it isn't clear if they are talking about the condensation process or some other behaviour of clouds.

The reason I don't think droplet formation is likely to be a critical process is because you generally get critical behaviour as the system approaches the spinodal curve in the phase diagram. The spinodal is the point at which the potential barrier to phase change becomes zero. However water droplet formation normally requires nuclei such as dust particles to seed the water droplet. That suggests there is still a potential barrier and therefore that the system is some way from the spinodal.

I think it's an oversimplification to say "rainfalls are transitions from vapor state to liquid state of water". This isn't my area, but I understand that raindrop formation is a complex business that can occur in different ways e.g. it sometimes procedes by ice crystal formation directly from the vapour, and subsequent melting of the ice crystals forms the raindrops.

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