What causes a thunderstorm?

Thunder is caused by lightning. Thunder is the sound caused by rapidly expanding gases a channel of lightning discharge. Lightning has enough energy to heat the air up to 18,000 degrees Fahrenheit. This causes a rapid spreading of the air, creating a sound wave heard as thunder. The sharp click or crack heard very near, right before the main crash of thunder.
Thunder is the sound produced after a lightening strike. Lightening is a huge electrical discharge. The charge pushes apart the air molecules. The molecules are heated rapidly and expand quickly. This creates vibration that reaches our ears as sound. You can tell how far apart the lightening strike is by counting the number of seconds between the flash and the sound. Approximately 1 miles is equal to 1 second, more or less. This works because of the speed at which sound travels.
Thunder is the rumbling or crack of sound that can usually be heard from the sky during a storm.

Thunder is caused because lightning heats up the air, to about 30 000ºC, causing it to expand quickly. The rumbling occurs as the sound passes through atmospheric layers at different temperatures.