The Messenger of Allah (salAllahu alayhi wasalam) said

The Messenger of Allah (salAllahu alayhi wasalam) said •►‘Two will not be rejected, Supplication when the Adhan (call of prayer) is being called, and *at the time of the rain*.' [Al-Hakim 2: 114, and Abu Dawud #2540, ibn Majah]

•►‘Seek the response to your du’as when the armies meet, and the prayer is called, and *when rain falls*.’ [reported by Imam al-Shafi' in al-Umm, al-Sahihah #1469]

The time of the rain is a time of mercy from Allah (Subhanahu wa Ta’ala) so, o­ne should take advantage of this time when Allah (Subhanahu wa Ta’ala) is having mercy on His slaves.