How is snow like wool?
Radak: It is white; "ka'Tzemer Yihyu" (your sins will be white like wool). It mentioned whiteness, for when there is much snow, the whiteness is seen. It is compared to wool, even though it is less white than snow, for there is nothing whiter in the land to compare it to.
Veha'Ish Moshe (Soloveitchik) p.90 #1: Hashem gives snow according to the amount of wool that people have to guard themselves from the cold.
Veha'Ish Moshe p.90 #2: In freezing weather, snow is like a wool blanket that preserves the heat of vegetation in the ground, lest it freeze and get ruined. After the snow melts, the vegetation is fresh!
Why does it mention snow and [in verse 18] rain?
Radak (15): They are the cause of Brachah in fruits of the ground, and satiation.
Radak (17): Just like He brings down rain amidst mercy, for the need of His creations, and sometimes He sends torrential rain for Mishpat, to punish for Aveiros, also frost and snow is needed for vegetation and trees. However, when it is very strong (ice), it is Mishpat; it kills and dries seeds and tree flowers. The same applies to hail, even though it is not mentioned.
Malbim: Hashem's word for bad is depicted as snow, frost and ice, which freeze water. So Midas ha'Din freezes the water of Chesed, and stops the influence that flows from springs of salvation. If Hashem sent His word to the land, and this word gives snow like wool and scatters frost like ashes, and "He sends ice like crumbs", to the point that "who can stand up to His cold?!" (17), He quickly retracts it; refer to 147:18:2:2.
What is "Kefor"?
Radak: It is frost that descends in the morning on cold days. It is compared to ashes, for it is scattered and little. Unlike snow, its whiteness is not seen.
