When two vampires run into each other, or when they hit the ball, the noise is really loud, as loud as thunder. So they need the sound of the thunder to disguise that sound, otherwise people would call the police and stuff.
The reason they need the thunder to play is because of the following:
- They are vamps. so they are supper strong and fast
- when they hit the ball it sounds like thunder, so they need to deisguise it
-when they run in to each other it's like boulders hitting each other
thats all the reasons i can think of at the moment
They need the thunder to play baseball because the force of one of them swinging the bat into the ball makes a thunderous sound. It is very loud and could bring attention to them, which they do not want. Also, the sound of two vampires colliding sounds like two boulders being slammed together.
Ok This is soooooooooooooo obvious!
Vampires rock and are so strong that when they hit the ball it makes a load sound just like thunder!!! So if there is already thunder, the humans won't notice and the Cullens can stay in forks without causing attention, happily!!