Lightning and thunder are related events. Lightning is a sudden, violent flash of electricity that occurs between a cloud and the ground or between two clouds in the sky. A lightning bolt can be several miles long and can be straight or forked. It is very hot, with an average temperature of 34 000 degrees Celsius. This causes the air around the electric bolt to expand, producing lots of sound energy. It is this sound energy that we hear as thunder. So, lightning and thunder are caused by the same event, but lightning is light energy, or electromagnetic energy, whereas thunder is sound energy. Thus, we see lightning and hear thunder.