There is a "30-30 Rule", which gives you an easy to follow lightning safety guidance. When you see lightning, start counting the seconds, until you hear thunder. Now, if this time is 30 seconds or less than that, understand that the thunderstorm is close enough to you to be dangerous. In this case, you must seek shelter as soon as you can. Now, even if you are unable to see the lightning, you will definitely be able to hear the thunder, so wait for half an hour after the last flash, before you leave the shelter.