Hazrat Aisha (رَضِيَ ٱللَّٰهُ عَنْهَا) narrated that the Prophet (ﷺ) guided us to recite dua when it rains (Sunan an-Nasa'i 1523). The dua to be recited when it is raining is given below. It is essentially a request to Allah Almighty to send down a beneficial downpour. According to Hazrat Aisha (رَضِيَ ٱللَّٰهُ عَنْهَا), the Prophet would recite this dua whenever he would see rain (Sunan Ibn Majah 3890). It is also narrated by Sharik bin `Abdullah bin Abi Namir that the Prophet made dua when it rained for many days so that the rain would fall on plateaus, mountains, hills, valleys, and places with vegetation (Sahih al-Bukhari 1013).
Rain is a gift from Allah, but relentless rain can cause destruction. Even in these modern times, we see that an abundance of rains destroys infrastructure, crops, and affects the daily life of people. In worst cases, rains can cause accidents, leading to loss of life. Reciting dua when it rains is a beautiful Sunnah of the Holy Prophet (ﷺ) to beseech Allah to send rain that is beneficial for humankind. Following his practice, we should say Bismillah before reciting the dua while looking at or standing in the rain.
Reciting this dua when it rains is a way of thanking Allah (سُبْحَانَهُ وَتَعَالَى) for His generosity and blessings. This dua also invokes His mercy and asks for the rain clouds to bring beneficial and productive showers rather than calamities such as floods.