What causes these beautiful frost patterns?

Clear nights and plunging temperatures can deposit a thick frost by daybreak. For frost to form, the temperature of the surface must be below 0°C. But what causes these pretty, leaf-like patterns? The patterns are the result of very tiny imperfections on the glass, such as scratches, specks of dust and salt, or the residue from washer fluid. These variations in the surface affect the way that the ice crystals form and branch out, forming the beautiful patterns captured in this image, taken in North Yorkshire on an smartphone by Paula Davies.