  We observe a large fraction of circular polarization in radio emission from
extensive air showers recorded during thunderstorms, much higher than in the
emission from air showers measured during fair-weather circumstances. We show
that the circular polarization of the air showers measured during thunderstorms
can be explained by the change in the direction of the transverse current as a
function of altitude induced by atmospheric electric fields. Thus by using the
full set of Stokes parameters for these events, we obtain a good
characterization of the electric fields in thunderclouds. We also measure a
large horizontal component of the electric fields in the two events that we
have analysed.
