Hoar Frost patterns
If solid and smooth surfaces like window panes and car roofs in contact with humid air are chilled, the water vapor resublimes to form small spicules of ice - actually a thin layer of hoar frost. The frost patterns, sometimes called fern frost because of their floral appearance, either need small nuclei of dirt to form patterns or follow small scratches on the otherwise smooth surfaces. They do not occur after rain, in this case the water would first condense in small droplets and then freeze into clear ice.
Hoar frost patterns on a car top
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