Referees are taught the 30-30 rule when it comes to lightning, and coaches should obey it, too. It is as simple as this: Go inside if you hear thunder within 30 seconds of a lightning flash. Wait at least 30 minutes after you hear thunder before going back outside.
Here are the top ten myths about lightning safety. The most pertinent one, given today's lapse of judgment, is this:
Myth: If it's not raining, or if clouds aren't overhead, I'm safe from lightning.
Truth: Lightning often strikes more than three miles from the thunderstorm, far outside the rain or even thunderstorm cloud.
Here is a great quiz about lightning prepared by the National Weather Service. It has only ten questions, and you are bound to learn something new.