What causes the thunder, then?
According to me, When lightning travels from clouds to the ground, it opens a little hole, or channel, in the air. Once the lightening disappears, the hole collapses and reverts back inwards. The resulting sound is thunder. Light travels at a rate of 186,282 miles per second, allowing the human eye to see a flash of lightning immediately as it happens. Sounds travels at the much slower rate of 1,087 feet, or one-fifth of a mile per second.
The rate at which the sound waves travel varies, depending on the temperature of the air and the wind's speed. Thunder appears to rumble or roll at times due to zigzag lines, or forks, that occur when lightning that strikes is not visible. These forks can branch out over several miles. These branches of lightning are further from the ground; therefore, they produce sounds of thunder, which take longer to be heard than the large bolt of lightning that originally produced the branched-out forks.