Cloud microphysical processes are still poorly understood, despite their importance in the regulation of Earth’s climate. A little-explored aspect is charging, which influences aerosol and droplet behaviour. Theory suggests cloud droplet charging occurs in layer clouds through the action of the vertical air-Earth current Jz, which flows from the ionosphere to the Earth’s surface in undisturbed regions of the atmosphere (Zhou and Tinsley, 2007). Flow of current through a horizontal layer cloud causes the upper and lower edges to become electrified, due to the change in electrical conductivity between clear and cloudy air. The charge is continuously transferred to droplets and aerosol particles. The possible consequences for cloud microphysical processes include effects on particle-droplet collisions (Tripathi and Harrison, 2002), droplet-droplet collisions (Khain et al, 2004) and droplet activation (Harrison and Ambaum, 2008). Very few in-situ charge measurements in layer clouds have hitherto been made.