I read an article about atmosphere and the article has this sentence:
At all events, however, they will cause an exchange of water molecules between the vapour and the droplet. If this exchange is extremely rapid, the O18-abundance in the droplet must correspond to that in the surrounding vapour. Whereas if the exchange is extremely slow the O18-abundance in the droplet will correspond with that of the vapour at the place where the droplet was formed, i.e., because of the oblique movement of the droplet, at a considerably lower level of the front.
Can you tell me what is the difference between rapid exchange and slow exchange? In these two circumstances, the O18-abundance in the droplet always correspond with that of the vapour.