If there is a ‘part 1’, there was probably going to be a ‘part 2’, right?  In the second paper up on my Publications page about the tropical precipitation pickup, I break apart the standard total column humidity metric.  I show that most clouds don’t actually care what the column humidity is.  In some senses that is not at all surprising, but in other senses that is.  I separate the column moisture into two parts.  Armed with these two new measurements of moisture, we can describe why the pickup value occurs where it does.  The other interesting suggestion in this paper is that of a discrete set of rules cloudy atmospheres seem to follow.  The moisture state of the convective tropical moisture is transient.  At long enough time scales, the atmosphere is always moving toward the pickup value (either from below or above it).  Local layers are also always trying to limit their local humidity.  If you combine these two rules, you can compactly describe the evolution of the tropical convective atmosphere.  Pretty cool.