שולחן ערוך אורח חיים · סימן רכ״ז

English

Upon a comet, which is like a type of star that is seen like an arrow across the sky from place to place, whose light stretches like a staff, and upon shaking of the earth, and upon lightning, and upon thunder, and upon winds that blow angrily: on each of these one says, "Blessed are you, God our Lord, king of the world, creator of the original (first) creations." And if you'd like, say, "Blessed are you, God our Lord, king of the world, whose strength and might fill the world."

English

As long as the clouds have not dispersed, one blessing suffices [for subsequent lightning/thunder]. [However,] if the clouds dispersed between one lightning flash and the next, or between one thunderclap and the next, one must recite the blessing again.

English

If one was sitting in a restroom and heard the sound of thunder or saw lighting: if it is possible to leave within toch kedei dibbur [the amount of time it takes to greet one's teacher, about two seconds], they should leave, otherwise not.