At ambient temperature and pressure,
water molecules have a V shape. The two
hydrogen atoms bond to the
oxygen atom at a 105° angle.[3] Ice crystals have a hexagonal
crystal lattice, meaning the water molecules arrange themselves into layered
hexagons upon freezing.[1]
Slower crystal growth from colder and drier atmospheres produces more hexagonal symmetry.[2] Depending on environmental
temperature and
humidity, ice crystals can develop from the initial hexagonal prism into many symmetric shapes.[4] Possible shapes for ice crystals are columns,
needles, plates and
dendrites. Mixed patterns are also possible.[1] The symmetric shapes are due to
depositionalgrowth, which is when ice forms directly from water vapor in the atmosphere.[5] Small spaces in atmospheric
particles can also collect water, freeze, and form ice crystals.[6][7] This is known as
nucleation.[8]Snowflakes form when additional vapor freezes onto an existing ice crystal.[9][10]
Trigonal and cubic crystals
Supercooled water refers to water below its
freezing point that is still liquid.[11] Ice crystals formed from supercooled water have
stacking defects in their layered hexagons. This causes ice crystals to display
trigonal or
cubic symmetry depending on the temperature. Trigonal or cubic crystals form in the upper atmosphere where supercooling occurs.[12][13]
Square crystals
Water can pass through
laminated sheets of
graphene oxide unlike smaller molecules such as
helium. When squeezed between two layers of
graphene, water forms square ice crystals at room temperature. Researchers believe high pressure and the
van der Waals force, an
attractive force present between all molecules, drives the formation. The material is a new crystalline phase of ice.[3][14]
Cirrus clouds and
ice fog are made of ice crystals.[1][16] Cirrus clouds are often the sign of an approaching
warm front, where warm and moist air rises and freezes into ice crystals.[17][18] Ice crystals rubbing against each other also produces
lightning.[19][20] The crystals normally fall horizontally,[21] but
electric fields can cause them to clump together and fall in other directions.[22][23]
Detection
The
aerospace 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 reflectivity
weather radars to identify types of
precipitation by comparing a droplet's horizontal and vertical lengths.[26] Ice crystals are larger in the horizontal direction[15] and are thus detectable.