Thunder is formed as air heats up and expands when lightning occurs. The rate of expansion is so fast that the air vibrates, causing sound waves. Since light travels faster than sound, lightning is seen before thunder is heard.
Sounds of thunder are categorized into claps, peals, rolls, rumbles and close-ins. Sudden, loud thunder lasting 0.2 to 2 seconds is called a clap. When thunder has changing frequency or amplitude, it is called a peal. Irregular sound variances are called rolls. Rumbles have low frequency but are of a longer duration. When lightning clicks nd is followed by a loud crackle and continuous rumbling, it is called a close-in.
Though thunder is composed of sound waves, at very close distances it has been known to cause property damage. Pressure of sound waves may be large enough to pop nails out of drywall or break glass. The proximity of lightning can be predicted by timing the difference between when the lightning is seen and when the thunder is heard. Hearing thunder in 30 seconds or less after spotting lightning means the lightning is close by, and people need to take cover. It is best to postpone going outside until at least 30 minutes after the last lightning is seen or the last thunder is heard.