1. A type of snow crust; a layer of ice, thicker than a film crust, upon a snow surface. It is formed by the freezing of meltwater or rainwater that has flowed into the snow surface. 2. Same as ice ri
nd.
Any one of a number of macroscopic, crystalline forms in which ice appears, including hexagonal columns, hexagonal platelets, dendritic crystals, ice needles, and combinations of these forms. The crys
tal lattice of ice is hexagonal in its symmetry under most atmospheric conditions. Varying conditions of temperature and vapor pressure can lead to growth of crystalline forms in which the simple hexagonal pattern is present in widely different habits (a thin hexagonal plate or a long thin hexagonal column). In many ice crystals, trigonal symmetry can be observed, suggesting an influence of a cubic symmetry. The principal axis (c axis) of a single crystal of ice is perpendicular to the axis of hexagonal symmetry. Planes perpendicular to this axis are called basal planes (a axes related to the prism facets) and present a hexagonal cross section. Ice is anisotropic in both its optical and electrical properties and has a high dielectric constant (even higher than water) resulting from its water dipole structure. The electrical relaxation time for water is much shorter than for ice (10^9 Hz compared with 10^4 Hz), resulting from a chain reaction requirement for molecules to relax through defects in the ice lattice. In the free air, ice crystals compose cirrus-type clouds, and near the ground they form the hydrometeor called, remarkably enough, "ice crystals" (or ice prisms). They are one constituent of ice fog, the other constituent being droxtals. On terrestrial objects the ice crystal is the elemental unit of hoarfrost in all of its various forms. Ice crystals that form in slightly supercooled water are termed frazil. Ice originating as frozen water (e.g., hail, graupel, and lake ice) still has hexagonal symmetry but lacks any external hexagonal form. Analysis of their sections (0.5 mm) in polarized light reveals different crystal shapes and orientations, depending on the freezing and any annealing and subsequent recrystallization process.
A cloud consisting entirely of ice crystals (such as cirrus); to be distinguished in this sense from water clouds and mixed clouds. Ice-crystal clouds have a diffuse and fibrous appearance, quite diff
erent from that typical of water droplet clouds, resulting from growth in much weaker updrafts and different fall speeds of a wider size particle spectrum.
A type of very light ice fog composed only of ice crystals (no droxtals), and at times observable to altitudes as great as 7000 m. It is usually associated with precipitation of ice crystals. Observed
from the ground, ice-crystal haze may be dense enough to hinder observation of celestial bodies, sometimes even the sun. Looking down from the air, however, the ground is usually visible and the horizon only blurred. For very sparse ice-crystal haze during daytime, sunlight reflecting from crystal faces produces sparkling in the air; hence the name diamond dust for these crystals.
Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as hal
os, coronas, and sun pillars. May be called 'diamond dust.' Precipitation of ice crystals in the form of needles, columns or plates sometimes so tiny, they seem suspended in air. They are mainly visible when they glitter in sunshine and occur only at very low temperatures and stable air masses.
Any one of a number of macroscopic crystalline forms of ice including hexagonal columns and platelets, dendritic crystals, ice needles and their combinations.
A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures
in a stable atmosphere.
A lake that exists because its water is restricted from flowing by an ice dam. Sometimes these lakes form because an advancing glacier had blocked a valley.
In climatology, a day on which the maximum air temperature in a thermometer shelter does not rise above 0C (32F), and ice on the surface of water does not thaw. This term is not used in the United Sta
tes, but is used in the United Kingdom, throughout most of Europe, and probably in many other parts of the world.
Mass flux or volumetric flux of ice through a glacier cross section or 'gate'. The gate can be anywhere on the glacier, but is often at or close to the terminus. If the terminus is a calving front, ic
e discharge is usually in discrete pieces that, when discharged into a body of water, become icebergs, and the ice discharge is equivalent to the calving flux plus the flux due to advance (positive) or retreat (negative) of the calving front. Avalanching from the glacier margin, for example from the margin of a hanging glacier, may constitute ice discharge; see also dry calving.