Why does it say "the winter passed"?
Rashi: Now it is not difficult to travel.
Rashi (13, from Shir ha'Shirim Rabah 2:24): This refers to the 400 years [decreed at Bris Bein ha'Besarim. I skipped (diminished) them, to count them from the birth of Yitzchak.
Seforno: Egypt's decree [to drown baby] boys (it passed).
Malbim (Melitzah): Chachamim depicted the Nefesh as a field, whose seeds sprout. Just like it has potential and is prepared to take out (make sprout) seeds planted in it, so the Nefesh, when it works with investigation, takes out Divine seeds hidden in it and makes Kadosh fruits. We explained this in Yeshayah (refer to Yeshayah 28:24:2:3, 28:24:1:3 and the note there). Moreh ha'Nevuchim (32:1) says that intellectual understandings, because they depend on physicality, they resemble sensory understandings. I.e. since the Nefesh is tied to physicality, physical states affect it, e.g. fatigue and weariness 1 . After it produced flowers (seeing Divine visions) and grape clusters (Nevu'ah and Ru'ach ha'Kodesh), it faints and sleeps.
Malbim: This is why Hashem put sleep in living beings, to rest from their actions and renew strength like initially. Also vegetation has a fixed sleep to renew its strength, like botany teaches. Hashem, in His Chachmah, put seasons of summer and winter, and heat and cold. This is necessary for seeds and what grows from them. After the harvest, after the land and garden sprouted their crops, their strength wearied. If summer would persist, and the land yielded fruits constantly, the fruits would be thin and dry, for the field became ruined. Therefore, Hashem made that after the harvest will begin the days of winter and snow. Then all natural powers sleep under the cover of snow and ice, until they waken in spring with new strength.
What is the significance of the rain stopping?
Rashi (13, from Shir ha'Shirim Rabah 2:24): This is the difficulty of [traveling in] winter. I.e. 86 years of difficult slavery was decreed on you, and it passed. From Miryam's birth the Egyptians made the slavery harsh. She is called Miryam because Marerum (they embittered them).
Seforno: Enslavement of our ancestors in Egypt ceased on Rosh Hashanah, before the Ge'ulah.
Malbim (Melitzah): In nature, after the ice melts, vapors rise and the days of rain begin. Then the air is not clean; it blocks the sun. So the Nefesh, after its powers rested and start to awaken, imagination begins, and thoughts that darken in front of the clear supreme light. This is until the clouds purify (depart) and the thoughts become clear; also the rain passes. Moreh ha'Nevuchim says that one can have Barakim (awesome insights) one after the other, will little separation; he is in constant light. This is the level of the great Nevi'im. Most Nevi'im had a large gap between one Barak and the next. This happened to Shlomo four times in his lifetime (the four times that Hashem spoke to him); between each were days of winter and snow.
