  Cosmic ray air showers produce radio emission, consisting in large part of
geosynchrotron emission. Since the radiation mechanism is based on particle
acceleration, the atmospheric electric field can play an important role.
Especially inside thunderclouds large electric fields can be present. We
examine the contribution of an electric field to the emission mechanism
theoretically and experimentally. Two mechanisms of amplification of radio
emission are considered: the acceleration radiation of the shower particles and
the radiation from the current that is produced by ionization electrons moving
in the electric field. We selected and evaluated LOPES data recorded during
thunderstorms, periods of heavy cloudiness and periods of cloudless weather. We
find that during thunderstorms the radio emission can be strongly enhanced. No
amplified pulses were found during periods of cloudless sky or heavy
cloudiness, suggesting that the electric field effect for radio air shower
measurements can be safely ignored during non-thunderstorm conditions.
