Some thunderstorm clouds stand alone while others can be seen in groups. These groups are known as a squall line. Lightning always accompanies a thunderstorm. Electricity produced in a thunderstorm can be as strong as 100 million volts. Lightning strikes before it thunders.
Thunder is the sound of air suddenly expanding due to the heat of the lightning.
When lightning is near, thunder sounds like a sharp crack. Thunder from distant lightning sounds like groaning and rumbling by the time it reaches you. Because the speed of light is about a million times that of sound, we see a lightning bolt before the sound of thunder reaches us. Hail storms often accompany thunderstorms. They're usually round but can be cone-shaped or irregular in shape with pointed projections.