O Allah, make it a blessing and mercy upon us. — Rain
اللَّهُمَّ اجْعَلْهَا بَرَكَةً وَرَحْمَةً عَلَيْنَا
Transliteration: Allaahummaj-'alhaa barakatan wa rahmatan 'alaynaa
Translation: O Allah, make it a blessing and mercy upon us.
Reference: General supplication