Subject: How small can pockets of dry air be in the atmosphere

I would like to know this, and what the movement patterns/speed are like for 
these dry 
pockets of air.  Also, how sharp of a contrast can there be between dry and 
moist air?  Can it be clear cut almost like a razor, or is it always a gradual 
blending of the two?  The reason I ask is because this photograph shows a gap in 
the contrail of an aircraft.  I think it was because the engines were shut off, 
but others say that it passed through a pocket of dry air, which I don't think 
is possible since it's so small.  Here's the site. http://img432.imageshack.us/
img432/4612/
image20086ss.jpg

Re: How small can pockets of dry air be in the atmosphere