When rain first falls, expose part of your body to it. Anas said rain fell while they were with the Prophet ﷺ, and he uncovered part of himself to it; asked why, he said: “Because it has just come from its Lord.” (Muslim). Also say: “Allāhumma ṣayyiban nāfiʿan.”
العربية: عن أنس رضي الله عنه قال: أصابنا ونحن مع رسول الله ﷺ مطر، فحسر رسول الله ﷺ ثوبه حتى أصابه من المطر، فقلنا: يا رسول الله لِمَ صنعتَ هذا؟ قال: «لأنه حديثُ عهدٍ بربّه». (مسلم).
At the first rain, let a few drops touch you and say ‘Allāhumma ṣayyiban nāfiʿan’ — it is fresh from your Lord.