TOEFL Listening: TPO-TOEFL阅读TPO - HRT91XN6F31UJF82N$

Lightning takes place when a separation of positive and negative electrical particles that develop in a storm cloud suddenly collapses. A. A storm cloud first develops a positively charged layer at the top then a negatively charged middle layer and finally, a positively charged layer at the bottom B. Lightning from cloud to ground follows a channel that forms when a stream of electrons moving down meets a stream of positive particles coming up from the ground C. Lightning from a cloud to the ground is more likely to be destructive than is lightning that takes place within a cloud D. A separation of oppositely charged particles in clouds develops from collisions of falling ice pellets with ice crystals, from updrafts and from temperature variations E. Field studies laboratory simulations and high-speed photography have all been used to investigate the way charge separations develop in clouds F. Once a channel has been formed it is usually used by several successive electrical discharges that illuminate the channel as flashes of lightning