The Psalmist says in verse 18 that God's "word" melts them. Verse 19 states in a colorful way that God has revealed His word to the nation of Israel. Verse 20 reminds us that no other nation has had the privilege of God giving His word to them the same way that He has revealed it to Israel.
Verse 16 and 17 were images of bad weather covering the land. Verse 18 has references to the fact that God stirs up breezes ("He causeth the wind to blow") and the waters melt and flow. God controls the entire process, even the snow melting and becoming drinking water.
I believe the word picture simply involves the truth that God's word "melts our heart" in that it gives us understanding of how to live our lives. Snow melting is similar to our hearts "melting" when we begin to understand what God desires for our lives and how He wants us to live for His will. God allowed His word to come into this world and come into our lives so that it could be melted (understood) for us to learn.
This Psalm begins and ends with the call, "Praise ye the LORD" like all the Psalms do in this group. We must choose to always be grateful for the world He has created for us.