Never seen a lightning storm before?

The part where they're in the desert and they see lightning in the distance, their first instinct is to run for their lives. How did they know how dangerous it was at the time? Hell, if I were wandering the desert for days my first instinct would be "thank god, some rain!" and stand there with my mouth open. True, you're more susceptible to be struck by lightning if you're standing out in the open, but it's still fairly unlikely and not an emergency situation. Do they live in a future where malicious lightning storms are commonplace? It would have been more believable if they saw a bunch of zombies or something get zapped and then realized this wasn't normal lightning and then decided to haul ass.