What is the ruling on saying Subhanallah when thundering or lightning?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Saying Subhana Allah when seeing the lightning or hearing the thunder is a supererogatory act. Abu Dawood reported in his book of 'Maraseel' from Ubaidullah Ibn Abi J'afar that the Prophet said: “If you hear the thunder say ‘Subhana Allah’ but do not say ‘Allahu Akbar’”. Ibn Jarir also reported from Abu Hurairah that he said: “The Prophet  used to say when hearing the thunder: “سبحان من يسبح الرعد بحمده” (Glorified is He Whom the thunder glorifies and praises)”. It is also reported that Caliph Ali Ibn Abu Talib “used to say when hearing the thunder: سبحان من سبّحت" بحمده" (Glorified is He Whom the thunder glorifies and praises)”. This was also reported from Abdullah Ibn Zbair and others. Al-Tirmithi, Ahmad and Al-Bukhari in his book ‘Adab al-Mufrad’ reported that “The Prophet used to say when hearing the thunder or the thunderbolt: “O Allah do not kill us with Your Anger and do not destroy us with Your Punishment and give us Your Peace before that all”.

Allah knows best.