How to understand the sentence below about rapid or fast exchange between vapour and droplet? 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.
The wording is not clear, but here is what I understand:

In case of extremely rapid exchange of molecules between the droplet and the surrounding vapour, the droplet gets the same O18 abundance as the vapour because they reach equilibrium.

In case of extremely slow exchange of molecules between the droplet and the surrounding vapour, the droplet keeps the O18 abundance that the vapour had where the droplet was formed. This is a different location (lower in the front, as shown by figure 2), so the abundance can also be different.