Image Taken From: http://www.eng.tau.ac.il/eng/Departments/Inter/edp_lab/photogal_files/
Thunder is the sound of a lightning discharge. It is produced as the lighting bolt suddenly
heats the surrounding air to temperatures around 10,000°C (18,000°F).
This causes an explosive expansion, which sends out a wave of compressed
air (a sound wave). Lightning is often
seen long before its thunder is heard.
This is because sound travels at about 343 m/s though air, much more
slowly than 3.00x108 m/s, the speed of light. While a lightning strike takes less than a
second to complete, the sound of its thunder often lasts much longer. There are many reasons for this. The shape of the lightning bolt is a major
factor, as all parts are not an equal distance from the listener. Sound produced by closer portions of the
bolt will be the first of the thunder a listener hears, and sound produced by
portions of the bolt that are farther away will be heard later. Another factor is that objects on the ground
interfere with the sound, and echoes from hills and other objects can
perpetuate thunder. Often lightning
occurs in groups, so the sounds from multiple bolts sometimes overlap to cause
longer lasting thunder.