Electric field measurements inside thunderstorms are essential to our understanding of thunderstorm charge structure, electrification, and lightning initiation. However, most existing measurements have been made by single instruments carried aloft by weather balloon, thus providing measurements made at a single point that moves through the storm on a timescale of tens of minutes. It is therefore difficult to interpret such data, since a change in observed field strength may be due to motion of the balloon into a region with different field or due to overall evolution of the storm's electrical structure with time. Separation of such spatial and temporal variability requires simultaneous measurements at multiple locations within the storm. This can be accomplished with a single weather balloon by carrying multiple independent electric field dropsondes aloft and releasing them one at a time, separated by short time intervals. The balloon payload design is optimized for low mass and use of off-the-shelf components whenever possible, releasing each dropsonde by a hot wire cut-down mechanism. Each dropsonde spins as it falls, measuring electric field as it rotates and sends data to a ground station in real-time. The dropsondes are designed to fall and rotate stably by use of aerodynamic simulations, with internal components robustly connected along the axis of the instrument to ensure the desired balance and alignment of the principal axes of the moment of inertia. The telemetry transmitters use simple low-cost low-power all-in-one transmitter chips. The telemetry ground station receives signals simultaneously from all dropsondes by a single software-defined radio receiver. Robust long-range communication is enabled by use of spread spectrum techniques and error correcting codes.

Thunderstorms are often described as consisting of three charge layers: upper positive, central negative, and lower positive. This simple charge structure is inferred from balloon-borne electric field measurements, however, they typically only provide information from a single moving point. Realistic charge structures in thunderclouds are expected to be more complicated than this tripole description for a variety of reasons, including non-trivial updraft geometry, turbulence, and charge deposition by lightning. In order to more fully measure such charge structures, we herein describe the development of an experiment composed of multiple low-cost electric field dropsondes. Each dropsonde consists of two pairs of electrodes where each pair has the electrodes on opposing sides of the device and connected to a differential charge amplifier. The enclosures for the dropsondes are designed so that they spin as they fall which induces charge motion and allows us to measure electric field strength. This electric field data will be transmitted to a ground station in real time along with GPS coordinates. By keeping instrument and recovery costs low, we aim to launch a single balloon payload carrying several of these instruments to drop at intervals to provide a multi-point map of electric field and infer associated charge structures inside a thunderstorm.