Don't worry about you ridge vent. Why? 1/ snow is porous & air moves through it (That's why they can find people buried under snow alive days later). 2/ If it gets iced over & starts holding in the heat, he heat will rise to the highest point (The ridge vent) & melt it.
Now, back to your original question & comments. As has been stated, the moisture is caused by air leakage into your attic. stop the leakage & you stop the problem. You mentioned frost on your metal chimney. That may be the source of your problem. Most chimney installers do not correctly install the firestop where the pipe goes through the ceiling In the last 10 years I can count on 1 hand the number done right on projects where I've worked, not including the ones that I just did it myself). IT MUST BE AN AIRTIGHT SEAL! check it out. If you find holes plugged with fiberglass (Not air-tight), seal them up. Yhis is one of the most common problems I run into.