Crystals on window panes

Frost - Barely freezing cold and slow moving damp air are conducive to grow frost crystals on sheltered surfaces.
When temperatures are at the edge of freezing, crystals grow on their edges, molecules hitting the broader sides bouncing off. The orientation of the molecules and their polar charged edges being more likely to capture stray water molecules that bounce against them than the non-charged surfaces along the crystal's flatter side.