We show that atmospheric electric fields as exist in thunderclouds can strongly affect the radio emission from cosmic air showers.
We also show, using data from LOFAR, that from the measured radio footprint of cosmic-ray air showers, i.e. intensity, linear and circular polarization at various distances from the shower core, one can determine the direction and strength of the electric field as function of height along the path of the cosmic ray. This method can be regarded as tomography of thundercloud electric fields using cosmic rays as probes. We will present an analysis of selected events measured during thunderstorm conditions in the period from December 2011 till August 2014. The fields we extract are consistent with the generally accepted charge structure in thunderclouds consisting out of three charge layers.