Lighting can strike up to 16 kilometers (10 miles) away from its origination point. This means that if you're within a 16-kilometer radius of a thunderstorm, you're in danger and need to take shelter immediately in a building or hard-topped car. A quick counting exercise will tell you if you're in the danger zone.
Look in the direction of the storm until you see a flash of lightning. Hit the start button on your stopwatch or count seconds by saying, "One-Mississippi, Two-Mississippi ..."
Listen for thunder. It will be low rumbling sound that accompanies lightening. Note how many seconds occurred between seeing the lightning and hearing the thunder.
Divide the number of seconds calculated in Step 2 by five. The resulting number will be the rough distance in miles to the storm. For example, if lightning strikes and you count to 10 before you hear thunder, the distance is 2 miles. Because you are in danger of being struck by lightning, you should take cover.