Question: If one hears thunder at night while sleeping, may one recite the blessing on thunder without washing his hands? Also, must one recite a blessing on lightning every time one sees it or is once a day sufficient?
Answer: In the previous Halacha we have established that one who sees lightning recites the blessing, “Baruch Ata Hashem Elokeinu Melech Ha’Olam Oseh Ma’aseh Bereshit.” One who hears thunder recites the blessing, “Baruch Ata Hashem Elokeinu Melech Ha’Olam Shekocho Ugvurato Maleh Olam.” We have written that one must recite these blessings immediately upon seeing the lightning or hearing the thunder or at the very least, within approximately two seconds of their occurrence. Once this time has passed, one may not recite the blessing any longer; rather, one should wait for the next thunder or lightning in order to recite the blessing.
One should not recite the blessings on thunder or lightning more than once a day unless the clouds (of the storm) completely disperse, the sun shines, and the sky once again becomes cloudy, in which case one may recite the blessing again although one has recited it earlier on that day. If the night has passed, even if the clouds have not dispersed, one may recite the blessings on thunder and lightning again. (There is no connection between the two blessings and even if one has not seen lightning, one may nevertheless recite a blessing upon hearing thunder.)
We must point out though that in his lectures, Maran zt”l did not mention this Halacha of being able to recite the blessings of thunder and lightning again if the clouds disperse. When asked about this, Maran replied that this was because most people are not that meticulous and learned enough to know exactly when it is considered that the clouds have dispersed and for this reason, he did not mention it at all.
If one hears thunder while sleeping and wakes up from it and wishes to recite the blessing but if he goes to wash his hands, the time to recite the blessing will already have passed, one may recite the blessing immediately while still in bed after having rubbed one’s hands against one’s clothing or blanket (out of cleanliness).
Summary: One may only recite the blessings on thunder and lightning once a day. One who hears thunder while lying in bed at night should rub his hands in his clothing or blanket and then immediately recite the blessing on thunder.