From John Polasek:

Re: Thunderstorms produce antimatter, scientists find (Jan. 10, 2011): This sounds like the normal pair production process which we cannot achieve in the laboratory because of the high electrical fields>10^18V/m required but apparently are attained in thunderstorms. Gamma Rays >1.02MeV can expel pairs out of the quantum vacuum, but they can only be detected as fuzzy curved tracks in magnetic field cloud chambers. Amazing that they can accomplish this in an orbiting capsule.