GIANT CHARGE SEPARATION. “Combining the collisions with the freezing, we can begin to understand how a cloud may acquire the extreme charge separation that is required for a lightning strike. When there is a charge separation in a cloud, there is also an electric field that is associated with the separation. Like the cloud, this field is negative in the lower region and positive in the upper region. The strength or intensity of the electric field is directly related to the amount of charge buildup in the cloud.”
http://science.howstuffworks.com/nature/natural-disasters/lightning1.htm