CBSE Class 10 Answered

There is an electrostatic force of attraction between the positive and negatively charged objects.

So as the clouds get positively charged, they induce a negative charge on the earth surface.

The two charges are attracting each other, and has to neutralize. But there are air molecules that separate the clouds and earth surface. So the charges could build up only, and the potential increases so much that the air ionizes, into positive ion and negative electrons. The charges rush past to the electrons on the ground to neutralize.  This is what we see is a lightening. 

A sudden rush of the lightening, increases the temperature of the air by thousands of degree celcius. The air expands very rapidly and a wave forms called shock wave. That's what we hear as thundering.

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