As the molecules seek more room, they collide violently with layers of cool air, and set up a great air wave that has the sound of thunder. Thunder has many different sounds. The deep rumbling roar of thunder is caused by the air wave from the lightning trunk that is farthest away. The sharp crackle of thunder is set up when the large trunk of lighting forks out into many branches. The loud crash of thunder is cause by a main trunk that is near the observer.
Lightning occurs in a variety of forms.
Also, a single flash of lightning often varies in appearance, depending on the position of an observer in relation to it. The major forms of lightning include forked lightning, streak lightning, ribbon lightning and bead, or chain, lightning. Forked lightning refers to a flash in which multiple branches of a stroke are visible. Streak lightning is a flash that seems to illuminate a single jagged line. Ribbon lightning appears as parallel streaks of light.