It is fairly easy and fun to find out how close the clouds are in a thunder storm. We will use the lightning and its thunder to calculate this distance.
Thunder is just the compressed air wave we hear which is caused by the hot lightning.
If the clouds are close enough (closer than 300,000 Km) which is always the case, then the moment we see the lightning, we can assume (which is a fair assumption) that the thunder has just occurred.
Since sound travels much slower than light, we hear the thunder (of that very lightning) a couple of seconds later.
So, if we know the time difference between when we "see" the light and when we "hear" the roar along with the speed of sound, we can derive the clouds distance.
To get the time difference, as soon as you "see" the lightning start counting the seconds until you hear the thunder. Then multiply this number by 343.2 (the speed of sound in meters per second).
The result is the distance of the clouds in meters, like if you hear the thunder a second after the lightning, then the clouds are just 343.2 meters away! Which is pretty close and you better run!