Feathery, crystalline ice deposits that form directly on exposed surfaces, such as grass, tree branches and wires, when water vapor in cold, humid air deposits straight into ice without first condensing into liquid dew. Hoar frost grows in delicate, often spectacular branching or needle-like structures under calm, clear, sub-freezing conditions, and can coat an entire landscape in white on a still winter morning.
Facts
Identification
Distinguishing AppearanceCrystals from dendritic (tree-like) and needle-like to knifelike blades of ice, white when scattering light 1 PeriodicityForms on cold, clear nights 1 Sources
1. Frost - Wikipedia
Hoar frost section
They form on cold, clear nights when conditions are such that heat radiates into outer space faster than it can be replaced from nearby warm objects or brought in by the wind.
Crystal forms
Crystals may vary in size and shape, from an even layer of numerous microscopic-seeds to fewer but much larger crystals, ranging from long dendritic crystals (tree-like) growing across a surface, acicular crystals (needle-like) growing outward from the surface, snowflake-shaped crystals, or even large, knifelike blades of ice covering an object.
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