I asked Cecil this question, but it doesn’t have quite the entertainment value as the questions he selects for his column. I’ll start with just a little background info to set the mood. Many years ago in physics class, the teacher was giving a lecture about how lightning is created. There’s a net positive charge in the clouds and a net negative charge on the earths surface, and as we all know charge flows from negative to positive, and BLAM, lightning. Then I asked, ‘Are you trying to say that lightning comes from the ground?’ The fact he said yes was not nearly as surprising as half the class turning around and looking at me like, ‘what, you didn’t know that?’. I have never been comfortable accepting this fact. Storm chasers have a term for lightning that strikes the ground (or emanates as it were) called simply enough, ground strikes. Why would they need a special term for something that happens all the time. I’ve also seen lighting in the sky that doesn’t appear to touch the ground. Therefore, if someone could shed some ‘light’ on the topic - 
This site (among others) disagrees. It says that the bottom of the cloud has the negative charge and the ground has the positive.
The Wikipedia article below has a really good and thorough explanation. The short answer is that a path of ionized air comes down from the cloud to the grounded object in a jumpy pattern called a “stepped ladder” and the main burst of electricity travels up the path to the cloud.
There are more complexities and different types of lightning.
Lightning doesn’t come from the ground, exactly. The electrons that flow through the air in a ground strike do–the Earth is just a convenient source of electrons here, but it has no net charge, itself. What is actually happening is that the cloud bottoms attain a net positive charge, while the tops attain a net negative charge, due to a charge separation mechanism that is not fully understood, but most likely has to do with the strong updrafts found in thunderstorm cells. There is thus a strong potential difference between the Earth and clouds, which causes a stepped leader to work its way downwards, seeking a point of lower potentential, i.e., the Earth, which is at zero potential. Once this stepped leader gets close to ground level–a few hundred feet up–the charge differential pulls streamers of electrons up from the ground and everything attached to it. When the stepped leader touches one of the streamers, BAM!! Electrons flow upwards in several spurts until the charge is dissipated and the potential difference is reduced to near zero. Cloud-to-cloud lightning works the same way, only it’s striking between points of potential difference from cloud to cloud. Lightning really comes from the clouds. The direction of clow is irrelevant.
I remember watching a Nova episode a couple a weeks ago, and saw segment on lightning. In the episode they said that a professor Joseph Dwyer from the Florida Institute of Technology had discovered that the charge comes from space. On the link provided there is a list of his publications (for those who like to read). Here is the link to that episode page.
Hmm. I had it in my head that the cloud bottoms generally acquired a positive charge, but it looks like this is usually NOT the case, in fact. About 90% of the time,. the cloud bottoms are negatively charged, but are positively charged about 10% of cases.
Actually, Zeus is the cause of lightning. 
Dang, thought that was Thor. My mistake.