I'm not trying to be a smart ass and I don't have to worry about this snow crap you speak of but, if the defrost melts the snow how is ice going to form on the same glass?
The defrost doesn't melt the snow directly to water. It melts it enough to get it "slushy."
The wipers then remove the slush, temporarily. Then the wiper blades start accumulating the slush.
After awhile, they can no longer sweep the slush off the windshield, they just start spreading it around. The slush then cools the windshield and the slush turns to ice on the windshield and on the wipers.
The defroster cannot keep up. Before long, your windshield is covered and you cannot see.
If you keep the heat blowimg on the floor, not defrost, the snow doesn't melt and the wipers can keep the windshield somewhat clear.
In heavy snow, this doesn't work all the time. The windshield eventually ices up.
I have got pretty good at moving my seat all the way forward, raising it all the way up and reaching out the window every now and then as the wiper comes all the way to the left side and flicking it away from the windshield. When it slaps back against the windshield, the ice knocks off the blades and then they can clear some of the windshield.
There are some days that I pull off the highway and clear the windshield because the passenger side is completely covered and I only have a 2 foot area on the drivers side that I can see out of.
During the 3 day before Christmas blizzard of 2004, I was stopping every 10 miles to clear the windshield. We were only going 30 mph or so on the highway, so it wasn't that dangerous, until you could no longer see out of the windshield. Stop on a get off ramp and clean again.
I made it 120 miles in 10 hours. They finally told me to stop and grab a motel.
The next day was just as slow heading back home.. People had just abandoned their cars on the highway the night before and bunches of cars being pulled out of ditches.
Took me 9 hours to go 120 miles to get back home.
Fun, fun, fun but was not really that dangerous. Never got going above 30 mph or so.