What did Tolkien mean by a thunder-battle?

Tolkien was talking about that awesome occasion when two large thunderstorms collide in the atmosphere.

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The "thunder-battle" described in The Hobbit is a "game" played by the Stone Mountain Giants of the Misty Mountains. They throw large boulders back and forth, when a boulder is not caught by a giant it crashes against the mountainside making the sound of "thunder".

This is described in first edition copies of the Hobbit, but Tolkien removed it from later editions when he edited the Hobbit to make it compatible with the LOTR story. Most likely it was removed as he felt it might be considered "too childish" as he was elevating the target reading age of the Hobbit to match the LOTR as well as matching the stories. I have not checked the current edition of the Hobbit, Christopher might have added it back (especially as it is depicted in Jackson's movie version of the Hobbit).