When It Rains
اللَّهُمَّ صَيِّبًا نَافِعًا
Allahumma sayyiban nafi'an.
O Allah, make it a beneficial downpour.
What it means
Rain can water a field or wash a village away, so the dua asks specifically for the useful kind. It is a small lesson in how to receive any blessing: ask for its benefit rather than just its arrival.
When and how
Say it as the rain starts. It is a time to ask for whatever else you need, so follow it with your own duas.
Its virtue
Aishah reported that when the Prophet ﷺ saw rain he would say these words, reported in Sahih al-Bukhari 1032. He also said that dua is answered at the time rain is falling.
Source
Al-Bukhari 1032sahih