It's impossible to say what the "second source of water vapor" is without more context.

Here's why:

* "Source" can be interpreted in different ways: It could mean the source of water vapor in the atmosphere, in a specific location, or in a particular process.

* There isn't a clear hierarchy: Different sources of water vapor contribute in various ways depending on the context.

To answer your question accurately, please tell me:

* What context are you interested in? (e.g., the global water cycle, a specific geographical area, the process of evaporation)

* What is the first source of water vapor you are referring to?

Once you provide more details, I can give you a more specific and accurate answer!