What is the text of the hadith that mentions snow and hail, and what is the specificity of snow and hail in the process of purification and cleansing, both Islamically and scientifically?

Regarding your question about the opening supplication in prayer: "O Allah, distance me from my sins as You have distanced the East from the West...", and concerning the Prophet's saying, "O Allah, wash away my sins with water, snow, and hail," scholars have stated that combining water, snow, and hail is an exaggeration in cleansing and purifying from sins, for whatever is washed with these three reaches the utmost purity. Scholars have interpreted this to mean purification through various forms of forgiveness. Some have indicated that snow and hail are two types of water untouched by hands, or that each of these things is a metaphor for a quality by which effacement occurs, or that it is a reference to the comprehensiveness of types of mercy and forgiveness to extinguish the heat of punishment. It is also probable that these three are specifically mentioned because they are sent down from the sky.