Siman 227

We learned at the beginning of the chapter "Ha-Roeh" (Berakhot 54a): "Over zikin, and over earthquakes, and over lightnings, and over thunders, and over winds, one recites a blessing with the Name and Sovereignty: 'Blessed… Whose strength and might fill the world.'" Zikin are a type of star that shoots like an arrow across the length of the heavens from place to place, and its light is drawn out like a rod; alternatively, a star that appears to have a long tail (Bartenura). Earthquakes are the trembling of the earth. Lightnings are known to be called "blitzen," and there are two types of them: one that comes with thunder, and one that on summer nights when the day is hot, many lightnings are seen at night. Thunders are also known, and they are sounds heard in the firmament from clouds that pour water one to another, as it is written: "At the sound of His giving a multitude of waters in the heavens." And winds are winds that are uncommon and blow with great fury, and they are only common occasionally. He counts five things here because there are on earth the heads of the four elements: fire, wind, water, earth, and the heavens are the fifth substance. This is zikin, the comet that comes from the heavens, and earthquakes come from the earth, lightnings are fire, thunders are the sounds of water, and winds are the element of wind (The Gra in his commentary to the Mishnayot).

Upon all these things, one blesses: "Whose strength and power fill the world," because they are seen and heard across the great expanse of the earth. And one may bless the blessing of "Who makes the work of creation" upon all these things, for they and their matters are from the creation of the six days of Genesis; so it is in the Gemara, and so the Tur and the Shulchan Aruch ruled. And according to this, at the time of thunder, which was preceded a moment before by the appearance of lightning, one blesses one blessing over both of them (Magen Avraham), whichever blessing he desires. And the world is accustomed to bless over the lightning "Who makes the work of creation" and over the thunder "Whose strength etc.," and in my humble opinion, the second blessing is a blessing in vain (the Taz also complained about this, see there, and the Eliyah Rabbah wanted to resolve it but it did not avail, see there; and this is only according to the Ra'avad, whom the Beit Yosef cited, who wrote to bless both of them, and so it appears from Rashi. But why should they enter themselves into a blessing that according to most poskim is in vain? Therefore, one must warn them not to do so, and delve into this).

When one has recited the blessing over thunder and over lightning, he should not recite the blessing over additional thunder and lightning as long as the rain continues and the clouds are gathered. But when the clouds have scattered and the rains have ceased and the sun has shone, and afterward there was lightning and thunder—he recites the blessing over them another time, and so always, for this is like a new matter. And it is stated in the Yerushalmi: One who was sitting in the beit hakise and heard the sound of thunder or saw lightning, if he is able to exit and recite the blessing within the time of a toch k'dei dibbur, he should exit from there, and if not, he should not exit. And behold, this is almost impossible, and it is found in a case where he did not perform his needs and did not touch a place of filth and there is water before him (see Magen Avraham). And in my humble opinion, the intent of the Yerushalmi is not that he entered to perform his needs, but rather that he entered for some other matter, and in this manner he is able to exit within a toch k'dei dibbur, even though the phrasing "was sitting in the beit hakise" does not seem so suited to be interpreted in this manner.