If you can hear thunder, you are close enough to be struck by lightning.
A general rule of thumb is 1 mile for every 5 seconds between the flash of lightning and the rumble of thunder.
However, when a storm rolls in people often don't have the ability to see the lightning associated with the thunder due to trees, mountains, or buildings obstructing their view.
With that in mind and knowing that water is a conductor of electricity it is always wise to remove yourself and anyone else from any body of water the moment you hear the thunder -- rain or no rain.
Allentown, PA 18102