<p>The wording is not clear, but here is what I understand:</p>

<p>In case of extremely rapid exchange of molecules between the droplet and the surrounding vapour, the droplet gets the same O<sup>18</sup> abundance as the vapour because they reach equilibrium.</p>

<p>In case of extremely slow exchange of molecules between the droplet and the surrounding vapour, the droplet keeps the O<sup>18</sup> abundance that the vapour had <em>where the droplet was formed</em>. This is a different location (lower in the front, as shown by figure 2), so the abundance can also be different.</p>

CC BY-SA 4.0
2019-12-18T13:59:01.630Z
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?
Y. zeng
CC BY-SA 4.0
2019-12-18T14:10:27.300Z
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.
Jean-Marie Prival
CC BY-SA 4.0
2019-12-18T14:16:19.470Z
How do you want to picture it?
Y. zeng
CC BY-SA 4.0
2019-12-18T09:05:28.357Z
2019-12-18T09:05:28.357Z
Jean-Marie Prival
<p>I read <a href="https://www.tandfonline.com/doi/abs/10.3402/tellusa.v5i4.8697" rel="nofollow noreferrer">an article</a> about atmosphere and the article has this sentence:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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.</p>

<atmosphere>
How to understand the sentence below about rapid or fast exchange between vapour and droplet?
Y. zeng
# How to understand the sentence below about rapid or fast exchange between vapour and droplet?

(Asked by: username_0 on 2019-12-18)

I read [an article](https://www.tandfonline.com/doi/abs/10.3402/tellusa.v5i4.8697) 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.

**Tags:** atmosphere


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## 1 answers

### Answer by username_1 on 2019-12-18. Score: 1 | Accepted answer

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.

> **Comments:**
> **username_0**: 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?  
> **username_1**: 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.  
> **username_0**: How do you want to picture it?  

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