One of the important open questions in atmospheric physics concerns the physical mechanism that initiates lightning in thunderclouds [1]. Crucial to the answer is knowledge of atmospheric electric fields. Existing in situ measurements, from balloons or airplanes, are limited due to the violent nature of thunderstorms. Furthermore, they are limited to balloon trajectories or perturbed by the presence of the aircraft. Here we present a new method to probe atmospheric electric fields through their influence on... more on Probing atmospheric electric fields in thunderstorms through radio emission from cosmic-ray induced air showers.pdf