Blue electric streamer discharges in the upper reaches of thunderclouds are observed as flashes of 337.0 nm (blue) with faint or no emissions of 777.4 nm (red). Analyzing 3 years of measurements by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS), we find that their distribution in rise time falls into two categories. One with fast rise times of 30 μs or less that are relatively unaffected by cloud scattering and emanate from within ∼2 km of the cloud tops, and another with longer rise times from deeper within the clouds. 46% of cells generating shallow events are associated with overshooting tops compared to 31% of cells generating deeper events. The median Convective Available Potential Energy (CAPE) of the cells is ∼50% higher for the shallow events and ∼30% higher for the deeper events than for lightning cells, suggesting the discharges are favoured by strongly convective environments.