An example of a hexagonal plate (top) and a hexagonal column (bottom), typical ice crystal shapes.
At ambient temperature and pressure,water molecules have a V shape. The twohydrogen atoms bond to theoxygen atom at a 105° angle.[3] Ice crystals have a hexagonalcrystal lattice, meaning the water molecules arrange themselves into layeredhexagons upon freezing.[1]
Slower crystal growth from colder and drier atmospheres produces more hexagonal symmetry.[2] Depending on environmentaltemperature andhumidity, ice crystals can develop from the initial hexagonal prism into many symmetric shapes.[4] Possible shapes for ice crystals are columns,needles, plates anddendrites. Mixed patterns are also possible.[1] The symmetric shapes are due todepositionalgrowth, which is when ice forms directly from water vapor in the atmosphere.[5] Small spaces in atmosphericparticles can also collect water, freeze, and form ice crystals.[6][7] This is known asnucleation.[8]Snowflakes form when additional vapor freezes onto an existing ice crystal.[9][10]
Further freezing of water on an ice crystal producessnowflakes.
Supercooled water refers to water below itsfreezing point that is still liquid.[11] Ice crystals formed from supercooled water havestacking defects in their layered hexagons. This causes ice crystals to displaytrigonal orcubic symmetry depending on the temperature. Trigonal or cubic crystals form in the upper atmosphere where supercooling occurs.[12][13]
Water can pass throughlaminated sheets ofgraphene oxide unlike smaller molecules such ashelium. When squeezed between two layers ofgraphene, water forms square ice crystals at room temperature. Researchers believe high pressure and thevan der Waals force, anattractive force present between all molecules, drives the formation. The material is a new crystalline phase of ice.[3][14]
Cirrus clouds andice fog are made of ice crystals.[1][16] Cirrus clouds are often the sign of an approachingwarm front, where warm and moist air rises and freezes into ice crystals.[17][18] Ice crystals rubbing against each other also produceslightning.[19][20] The crystals normally fall horizontally,[21] butelectric fields can cause them to clump together and fall in other directions.[22][23]
Theaerospace industry is working to design a radar that can detect ice crystal environments to discern hazardous flight conditions. Ice crystals can melt when they touch the surface of warm aircraft, and refreeze due to environmental conditions. The accumulation of ice around the engine damages the aircraft.[24][25] Weather forecasting uses differential reflectivityweather radars to identify types ofprecipitation by comparing a droplet's horizontal and vertical lengths.[26] Ice crystals are larger in the horizontal direction[15] and are thus detectable.
