Blessing for Thunder and Lightning

Shalom u'vracha

The Shulchan Aruch HaRav, in the section on blessings on benefits, chapter 12, section 24, states: Regarding lightning and thunder, one should bless, 

"Blessed are You, Hashem, our God, King of the Universe, who makes the works of Creation." 

ברוך אתה יי אלהינו מלך העולם עושה מעשה בראשית

If one wishes, they may say, 

"Blessed are You, Hashem, our God, King of the Universe, whose strength and might fill the world." 

ברוך אתה יי אלהינו מלך העולם שכחו וגבורתו מלא עולם

If one hears thunder and sees lightning at the same time, they bless both with one blessing, but if they occur separately, one blesses over this one and then returns to bless over that one. The custom in these countries is to bless over the lightning, "who makes the works of Creation," and over the thunder, "whose strength and might fill the world," because most often they come one after the other immediately, and they do not wish to bless with one blessing twice immediately, for it is better to praise the Blessed One with many praises than to praise Him with one blessing, and they chose thunder—"whose strength and might fill the world"—because thunder demonstrates greater strength and might than lightning.

Wishing you success - Binyamin Shmueli