Question

Why usually there is a time delay between observing the flash and hearing the thunder

Answer 1

Lightning can be seen before thunder is heard as light travels faster than sound. ... The noise of thunder is caused by the rapid expansion of heating the air. You can normally hear thunder up to 6 miles (10km) away from the lightning flash. The sound can last quite a few seconds!

Answer 2

There is usually a time delay in between seeing lightning and hearing the thunder, because light travels at a speed that is faster than sound

Speed of Light in Air = 3 * 10^8 meters/second

Speed of Sound in Air = 343 meters/second

As the speed of light is almost 10^6 times faster than sound, we see the lightning before hearing the thunder