Upward electrical discharges from thunderstorms

Thunderstorms occasionally produce upward discharges, called blue jets and gigantic jets, that propagate out of the storm top towards or up to the ionosphere(1-4). Whereas the various types of intracloud and cloud-to-ground lightning are reasonably well understood, the cause and nature of upward discharges remains a mystery. Here, we present a combination of observational and modelling results that indicate two principal ways in which upward discharges can be produced. The modelling indicates that blue jets occur as a result of electrical breakdown between the upper storm charge and the screening charge attracted to the cloud top; they are predicted to occur 5-10 s or less after a cloud-to-ground or intracloud discharge produces a sudden charge imbalance in the storm. An observation is presented of an upward discharge that supports this basic mechanism. In contrast, we find that gigantic jets begin as a normal intracloud discharge between dominant mid-level charge and a screening-depleted upper-level charge, that continues to propagate out of the top of the storm. Observational support for this mechanism comes from similarity with 'bolt-from-the-blue' discharges(5) and from data on the polarity of gigantic jets(6). We conclude that upward discharges are analogous to cloud-to-ground lightning. Our explanation provides a unifying view of how lightning escapes from a thundercloud.