What causes charge buildup in a cloud before a lightning storm? I am curious what causes the charge build up in a cloud before lightning occurs. I have seen in a few places such as this, that the process is not fully understood. Is this some form of static electricity? Here are a few of my thoughts:


*

*It seems that rain or ice particles occur are present in the clouds where this happens since thunderstorms are usually accompanied by rain.

*These clouds tend to be cumulus or cumulonimbus clouds which are much taller than other types that are commonly encountered. Is the physical size of the cloud related to this type of buildup of charge or is it a result of the moisture content? 

*If the charge buildup is caused by static electricity which two surfaces are contacting to create the charge? Is this just rain and ice particles? Can it be something else? (I think I recall something about lightning storms without rain in the desert) Why do some clouds with lightning have rain and others do not if the rain drops are the source of the static?

*Is there a minimum solid or liquid particle density needed for static electricity within a cloud? 
Any thoughts would be interesting to hear! 
 A: Charge separation occurs because of collision of particles.  The surface of an
ice pellet is not exactly pure H2O, it contains an excess of OH- ion
(for crystal-boundary reasons that I do not understand).   That's why
ice is slippery.
So, if there is a mixture of sizes of ice pellets suspended in air (not unlikely,
since we see hail from time to time), and (for instance) an updraft carries
the lightest (smallest radius) particles UP past larger slower particles, we
can confidently expect particles to collide.
The collision of a small and large particle makes a pointy spot (where the
small particle is), and the surface charge in the OH- ions flows preferentially
to that spot (because the negative charge repels).  When that small ice particle
detaches, it carries more negative charge and the large ice particle carries less.
So, a consequence of turbulent airflow and suspended water ice is net
separation of charges inside a cloud.   That charge separation generates
the electricity we see as lightning.Charge Separation...
A: I found this lecture transcript helpful. It seems to have been one of a series given by Richard Feynman. http://www.feynmanlectures.caltech.edu/II_09.html
