Thunder is not always preceded by lightning but lightning is always followed by thunder. Why?

Thunder is always generated only by a lightning bolt. The bolt, 1 to 5 cm in diameter, could cause the air temperature to rise up to 30,000 degrees C (Compare it with Sun's surface (Photosphere) temperature of 5,400 degrees C). Such a sudden rise of air temperature causes explosive expansion of the bolt area to produce compression (or shock) waves that are heard as thunder or thunder rumblings.

The time lapse between the two in seconds multiplied by 330 gives roughly the distance of the lightning in metres.When there is a series of thunderclouds and the most rear one some 20 to 30 km away produces a lightning, the thunder may not be heard; especially so when the prevailing surface wind is towards the thundercloud.

Also, during a bright day, an isolated thundercloud may produce a lightning that can hardly be seen; but the thunder could be heard. Also, in a maturing thunder cloud with updrafts of 100 kmph or more, a lightning in the middle area of the cloud could be seen as a bright flash illuminating the upper portions of the cloud; the thunder may not be heard as the updrafts will disperse the sound waves. (A MIG jet aircraft approaching overhead can be seen but the sound can be heard only after it passes over your head).

When thunderclouds merge as in the `eye wall' of a very severe cyclone wherein the sustained surface wind speed exceeds 150 kmph, rapid collision of positively charged areas with negatively charged areas could produce intermittent flashes of lightning without producing thunder. This, I have personally observed for hours from very close quarters to the `eye wall'. This was confirmed under similar situations by some highly experienced meteorologists by independent judgement.