  We present measurements of radio emission from cosmic ray air showers that
took place during thunderstorms. The intensity and polarization patterns of
these air showers are radically different from those measured during
fair-weather conditions. With the use of a simple two-layer model for the
atmospheric electric field, these patterns can be well reproduced by
state-of-the-art simulation codes. This in turn provides a novel way to study
atmospheric electric fields.
