Subject: How does positive stroke lightning work?

I've found out that positive stroke lightning comes from the positively charged 
upper cloud to negative ground, but how?  Don't electrons carry the current, 
meaning that lightning would strike UP in this case??  If the positive charges 
(protons?) moved from the clouds down to earth, that would mean transfer of 
MASS too, but that doesn't make sense.  What are the physics involved?

Re: How does positive stroke lightning work?