# 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.

• The abundance can be different because they are not the same location. So what does the abundance have to do with the speed of molecules' exchange? Commented Dec 18, 2019 at 13:59
• Here is another way to picture it. The droplet forms low in the front, where the vapour has a given abundance, let's call it A. So the droplet also gets abundance A. Now the droplet migrates upwards (Fig. 2), where the surrounding vapour has another abundance, say B. Before falling as rain, the droplet can exchange with this new vapour. But if the exchange is slow, the droplet won't have time to equilibrate with it, so it will fall with abundance A, inherited from its formation. If the exchange is fast, the droplet will be able to acquire the new composition, hence will fall with abundance B. Commented Dec 18, 2019 at 14:10
• How do you want to picture it? Commented Dec 18, 2019 at 14:16