Lightning is a giant electric discharge between the bottom of a charged cloud and the surface of the Earth. Lightning is an electric spark, but on a huge scale. It is caused by accumulation of charges in the clouds. This vigorous movements cause separation of charges. This leads to the positive charges collect near the upper edges of the clouds and the negative charge to accumulate near lower edges of the clouds. Check the figure below and try to answer the following questions

Explain why lightning is always accompanied by thunder?

Explanation

Lightning occurs when a massive electrical discharge takes place between two oppositely charged clouds. This rapidly heats up the air along its path which expands very fast producing shock waves which we hear as thunder.