Once enough charge has been separated in a growing storm, a lightning flash can occur. These normally travel within or between clouds (abbreviated CC) or from cloud to ground (CG). Most storms produce more CC than CG flashes--about six times as many in tropical storms and two times as many in midlatitudes. Sometimes a flash will travel from cloud to air or simply occur within "clear" air.
Exactly what triggers flashes is still uncertain and an area of continued research. It seems that very concentrated electric fields (perhaps at the ends of pointed surfaces or single particles) are needed to accelerate charged particles, or ions. Once moving with sufficient energy, the ions appear to blaze a path toward opposite charge in cascading fashion.