Fresh, dry snow has an average density of about 100 kg m-3, but this ranges from 20 to 200 kg m-3 or more, depending on the temperature, wind, and liquid water content during deposition. ![]() Snowflakes are composed of an arrangement of ice crystals, but snow that accumulates on the ground is a porous medium dominated by air pockets. The density of snow is less than that of crystalline ice. The volume compressibility is about 1.2 X 10-10 Pa-1, giving a density of 921 kg m-3 under the load of 4 km of ice, which is typical of the East Antarctic plateau. Ice density also increases slightly with pressure. Once frozen, ice behaves like most solids, with increasing density as temperature drops pi = 920 kg m-3 at -23☌ (figure 2.2b) and values reach 922 kg m-3 for the coldest ice to be found, in the Antarctic ice sheet. ![]() Seawater with a salinity of 35 ppt freezes at -1.9☌. This is the case in most of the world's oceans. Where salinity exceeds 24.7 ppt, density increases continuously as temperature drops to the freezing point. Water molecules, causing it to behave more like a regular liquid. Density of (a) pure water and (b) ice as a function of temperature at mean sea level pressure. Dissolved ions in salt water also interfere with the molecular packing ofįigure 2.2. Salt content in seawater makes it more dense: 1028 kg m-3 for surface water with a temperature of 0☌ and a salinity of 35 ppt. This density inversion is specific to freshwater. Water density continues to decrease below 0☌ in supercooled water droplets (figure 2.2a). To quote James Trefil, "water never quite forgets that it was once ice." The reason for this is not fully understood, but it is related to the angle of the hydrogen bonds in the rigid, low-density crystal lattice that characterizes water in its solid phase. Pure water has its maximum density at 4☌, and it becomes less dense as it cools below this. This plot also illustrates the unusual density inversion for freshwater. Imagine the sight of sunlight sparkling off of a diamond-berg in a sea of liquid diamond! But this is not to be found at Earth's surface temperatures and pressures.įigure 2.2 plots the density of pure ice and water as a function of temperature. ![]() Diamonds, germanium, gallium, and bismuth, all structurally similar to ice, also float in their own liquid. Ice floats in its own melt, one of few substances to do so. At 0☌, water has a density of 1000 kg m-3, whereas pure ice (Jh) has a density of 917 kg m-3. The geometric arrangement of this lattice structure is spacious, giving water the most unusual property of having a solid phase that is less dense than its liquid phase.
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