TPO18-3 Lighting 闪电
1. The electrical discharge takes place when the attractive tension between a region of negatively charged particles and a region of positively charged particles becomes so great that the charged particles suddenly rush together.
2. At such temperatures, the collision between the ice pellets and the ice crystals causes electrical charges to shift so that the ice pellets acquire a negative charge and the ice crystals become positively charged. Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud. The heavier negatively charged ice pellets are left to concentrate inthe center. This process explains why the top of the cloud becomes positively charged, while the center becomes negatively charged.(解释该自然现象的主要段落，信息量大，两道事实信息题一道推断题的出处。)
3. Belowthis large, cold, negatively charged region, the cloud is warmer than -15℃, and at these temperatures, collisions between icecrystals and falling ice pellets produce positively charged ice pellets that then populate a small region at the base of the cloud.(考察因果关系的事实信息题出题点。)
4. Most lightning takes place within a cloud when the chargese paration within the cloud collapses. However, as the storm cloud develops,the ground beneath the cloud becomes positively charged and lightning can take place in the form of an electrical discharge between the negative charge of the cloud and the positively charged ground.
5. Lightning that strikes the ground is the most likely to be destructive, soeven though it represents only 20 percent of all lightning, it has received a lot of scientific attention.(前半句为修辞目的题出处，和后半句有因果关系，向后在本句找答案。)
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