Here's some basic thunderstorm safety advice. Thunder is the sound that lightning makes, so you should equate the two, even if the lightning isn't visible. If you can hear the thunder, you're close enough to be struck by the lightning.

In fact, even if you can't hear thunder or see lightning, you could be at risk from a nearby thunderstorm. So-called "bolts from the blue" can strike ten or more miles away from the main body of the storm.

Light travels much faster than sound, so unless the lightning strikes very close to you, you'll see the lightning before hearing its thunder. It's easy to estimate the distance to the lightning: Count the number of seconds between seeing the lightning and hearing the thunder, and divide by 5. That gives you the distance, in miles, to the lightning. So a 5 second delay means the lightning is one mile away. By repeatedly using this rule, you can tell if the thunderstorm is moving towards you or away from you.