{{DISPLAYTITLE:Ice Ih}} Ice Ih (hexagonal ice crystal) (pronounced: ice one h, also known as ice-phase-one) is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic that is occasionally present in the upper atmosphere. Ice Ih exhibits many peculiar properties that are relevant to the existence of life and regulation of global climate. For a description of these properties, see Ice, which deals primarily with ice Ih. The crystal structure is characterized by the oxygen atoms forming hexagonal symmetry with near tetrahedral bonding angles. Ice Ih is stable down to , as evidenced by x-ray diffraction and extremely high resolution thermal expansion measurements. Ice Ih is also stable under applied pressures of up to about where it transitions into ice III or ice II. Physical properties The density of ice Ih is 0.917 g/cm3 which is less than that of liquid water. This is attributed to the presence of hydrogen bonds which causes atoms to become closer in the liquid phase. Because of this, ice Ih floats on water, which is highly unusual when compared to other materials. The solid phase of materials is usually more closely and neatly packed and has a higher density than the liquid phase. When lakes freeze, they do so only at the surface while the bottom of the lake remains near because water is densest at this temperature. No matter how cold the surface becomes, there is always a layer at the bottom of the lake that is . This anomalous behavior of water and ice is what allows fish to survive harsh winters. The density of ice Ih increases when cooled, down to about ; below that temperature, the ice expands again (negative thermal expansion). The latent heat of melting is , and its latent heat of sublimation is . The high latent heat of sublimation is principally indicative of the strength of the hydrogen bonds in the crystal lattice. The latent heat of melting is much smaller, partly because liquid water near 0 °C also contains a significant number of hydrogen bonds. The refractive index of ice Ih is 1.31. Crystal structure The accepted crystal structure of ordinary ice was first proposed by Linus Pauling in 1935. The structure of ice Ih is roughly one of crinkled planes composed of tessellating hexagonal