What is the physics behind this kind of frost, seen on the sunroof on my car? Why does it form so linearly, even though every line has feathery offhsoots? (Why don't they all form radially, more like snowflakes?) Why are lines able to cross each other with apparently no interaction? I could see if these lines were being formed from pre-existing scratches in the windshield - that would explain both the above questions, since it would mean the structure of the lines has much more to do with the surfacfe and much less to do with the formation of crystals. But wouldn't the scratches have a more regular pattern, such as in line with the car's motion?