Re: How exactly does a cloud charge?

Many atmospheric scientists are scratching their heads over this question, and nobody knows precisely what the answer is. Somehow, thunderclouds are acting as electrical generators. To do this, they must carry charge against the ambient electric field in the cloud.

It is generally agreed that some form of electrostatic charging followed by gravity does this. For example, suppose you have two raindrops or ice crystals, one large enough to fall downward, one so small it's carried upward in the updraft that forms the center of all thunderstorms. Suppose that when they collide the smaller one transfers a few electrons to the heavier one. If this happens a zillion times, the falling heavy drops will cause negative charge to build up at the base of the cloud. This will happen until the electrostatic field gets so strong that it prevents the light drops from rising and the heavy ones from falling or until the charge gets so large that a lightning flash occurs.

How do electrons tend to be transferred to the heavy particles? That's the question nobody can agree on. Some research suggests that if you break a piece off an ice crystal, the larger chunk often gets an electron or two. My favorite theory, though, notes that even before the thundercloud develops, there is negative charge near the ground and positive charge higher up. Since like charges repel and opposites attract, positive charge in a single drop will be pushed to the bottom surface and negatives will be pushed to the bottom. If a rising small drop hits the bottom of a falling large drop, charge will be transferred until they are both neutral at the point of contact. To do this, the small drop gives electrons to the large one. Voila! Here's an ASCII figure.

+++++++++++         ++++++++++++           ++++++++++++
                                           
  ____                                           (O)   ||
 / - -\                                          +  +  \/
|      |  ||          ____
 \____/   \/         / - -\  ||
  +  +              |      | \/              _____
                     \____/                 /  - -\
      - -  /\             (O)  /\          |       |  ||
      (O)  ||             + +  ||           \_____/   \/
      +  +            

-----------        ---------------       --------------
Top and bottom     Drops collide;         Drops continue
of drops charge    charge transferred     falling/rising