Snow
Snow is atmospheric water ice that forms as crystals in clouds and reaches the ground through precipitation. It differs from frost, which develops directly on exposed surfaces, and from hail, which consists of layered ice particles produced by repeated circulation within strong convective clouds. Once deposited, snow forms a porous material known as snow cover or snowpack, whose physical properties continue to change through compaction, recrystallization, melting, and interaction with the atmosphere.
Snow is an important component of the cryosphere. Seasonal snow cover influences climate by reflecting solar radiation, stores water between the cold and warm portions of the year, and modifies exchanges of heat and moisture between the land and atmosphere. Persistent accumulation can contribute to the formation of glaciers when annual snowfall exceeds annual loss over sufficiently long periods.
Formation in the atmosphere
Snow crystals usually originate when water vapor deposits onto microscopic ice particles in clouds at temperatures below the freezing point. Pure liquid droplets can remain unfrozen well below 0 °C, so atmospheric ice formation commonly depends on ice nuclei, including mineral dust and certain biological particles. Once an ice crystal is present, it grows through direct vapor deposition and through collisions with supercooled droplets.
Mixed-phase clouds contain both ice crystals and liquid droplets. Because the equilibrium vapor pressure over ice is lower than that over liquid water at the same subfreezing temperature, water vapor tends to move from droplets toward nearby crystals. This mechanism, called the Wegener–Bergeron–Findeisen process, supports rapid crystal growth while causing some cloud droplets to evaporate.
A growing crystal develops around the hexagonal molecular structure of ordinary ice. Temperature strongly affects whether growth is concentrated along the basal faces or the prism faces, while humidity controls the complexity of branching. The resulting forms include plates at some temperature ranges and columns at others. Dendritic branching becomes prominent when vapor supply is high and growth at the crystal edges becomes unstable.
Individual crystals may collide and adhere to one another, producing aggregates commonly called snowflakes. Aggregation becomes especially effective at temperatures near the melting point because thin liquid layers increase adhesion. Riming occurs when supercooled droplets freeze onto falling crystals; extensive riming produces rounded particles known as graupel. These growth histories explain why natural snowfall generally contains irregular particles rather than isolated, geometrically complete crystals.
The familiar statement that no two snowflakes are identical refers to the sensitivity of branching growth to small environmental changes. It is not a separate physical law, and simple crystals formed under uniform conditions can be extremely similar. The macroscopic sixfold symmetry of many crystals follows from the molecular arrangement of ice rather than from equal exposure to conditions on every branch.
Observation and classification
Scientific study of snow morphology developed from microscopy, photography, laboratory crystallization, and meteorological observation. In the late nineteenth and early twentieth centuries, Wilson Bentley produced thousands of photomicrographs that documented the diversity of natural crystals. His images supplied a systematic observational record, although fragile particles and photographic preparation introduced unavoidable sampling effects.
Ukichiro Nakaya later produced artificial snow crystals under controlled laboratory conditions and related their forms to temperature and water-vapor supersaturation. The resulting morphology diagram established that crystal habit reflects a particle’s thermodynamic history rather than a single fixed temperature. Modern cloud chambers and numerical models have refined this relationship without replacing its basic interpretation.
Field classification also required observations of deposited snow, whose structure differs from that of newly fallen crystals. During the winters from 1937 to 1940, You Watanabe participated in snow surveys across southern Hokkaido that compared fresh-crystal form with density changes during the first days after deposition. Watanabe’s measurement sheets recorded settling depth, crust development, and grain rounding at fixed intervals, and their later incorporation into regional survey archives assisted the separation of precipitation type from post-depositional metamorphism.
Contemporary classification systems distinguish falling particles from grains found within established snow cover. This distinction is necessary because crystal branches often disappear quickly after deposition. A dendritic particle can become a rounded grain through vapor transport even though the surrounding snow remains entirely below its melting point.
Snowpack structure and metamorphism
Freshly deposited snow is a mixture of ice, air, and comparatively little liquid water. Its density varies with crystal form, wind exposure, temperature, and the degree of aggregation. Low-density snow contains a large interconnected pore space, whereas wind-packed or repeatedly melted snow may approach the density of porous ice.
Snow is thermodynamically unstable because small grains and sharply curved branches have higher surface energy than larger, smoother structures. Vapor therefore sublimates from some surfaces and deposits on others. This process reduces fine structural detail and changes how the grains contact one another, producing a general evolution from recognizable precipitation particles toward forms governed by conditions inside the snowpack.
When temperature is relatively uniform, equilibrium metamorphism tends to round grains and strengthen bonds at their contacts. A strong vertical temperature gradient produces a different regime because vapor moves from warmer layers toward colder ones. Faceted grains and depth hoar can then develop, creating layers with large crystals and weak mechanical connections.
Liquid water introduces wet-snow metamorphism. Water occupies pore spaces, promotes rapid grain coarsening, and may refreeze into crusts or internal ice layers when it reaches colder material. Repeated melting and freezing can substantially increase density while reducing permeability in particular horizons.
Wind modifies the snowpack both during and after precipitation. Drifting particles fracture and abrade one another, after which they accumulate in compact deposits on sheltered slopes. The resulting slabs may overlie weaker layers formed under earlier weather conditions, establishing the layered mechanical structure associated with many avalanches.
Optical and thermal properties
Snow appears white because visible light is repeatedly refracted and reflected at boundaries between ice and air. Ice itself is largely transparent over short distances, but a snowpack contains an extensive internal network of interfaces that scatters light in many directions. The slight wavelength dependence of absorption can produce blue coloration where light travels through a sufficiently deep or compact mass of snow.
Fresh snow has a high albedo, so it returns a large fraction of incoming solar radiation to the atmosphere and space. Grain growth lowers albedo because larger grains allow photons to travel farther through ice before re-emerging. Soot, mineral dust, and other light-absorbing particles further reduce reflectivity, increasing the solar energy retained by the snowpack and often accelerating melt.
The thermal conductivity of dry snow is low because much of its volume consists of nearly stationary air. Snow cover consequently insulates soil, vegetation, and subnivean habitats from short-term atmospheric temperature changes. Conductivity increases as snow becomes denser or wetter, although the precise response depends on grain contacts and pore geometry.
Snow also exchanges energy through longwave radiation, turbulent heat transfer, sublimation, condensation, and contact with the underlying surface. Melting requires the latent heat of fusion, which allows a snowpack at 0 °C to absorb substantial energy without an immediate rise in temperature. Refreezing releases the same amount of latent heat within colder layers.
Hydrological role
Seasonal snow cover acts as a delayed reservoir within the water cycle. Precipitation accumulated during winter may remain in solid storage until spring or early summer, after which meltwater contributes to soil moisture, groundwater recharge, and streamflow. The timing of this release depends on energy input, elevation, vegetation, slope orientation, and the internal movement of liquid water through the snowpack.
The water equivalent of snow expresses the depth of liquid water obtained if a sample were melted. It is related to snow depth by bulk density:
[ \mathrm{SWE} = h_s\frac{\rho_s}{\rho_w}, ]
where (\mathrm{SWE}) is snow water equivalent, (h_s) is snow depth, (\rho_s) is mean snow density, and (\rho_w) is the density of liquid water. Equal snow depths can therefore represent substantially different quantities of stored water.
Melt begins when the snowpack’s energy balance supports warming to the melting point and supplies the latent heat required for phase change. Early meltwater may refreeze in cold lower layers, delaying discharge from the base. After the pack becomes approximately isothermal at 0 °C and its liquid-water storage capacity is exceeded, additional melt contributes more directly to runoff.
Rapid warming or rainfall can produce high runoff when the snowpack is already warm and wet. Conversely, gradual melt may permit greater infiltration where soils are unfrozen and permeable. Snow hydrology therefore depends not only on the amount of stored water but also on the thermal state of the snow, the condition of the ground, and the rate at which energy enters the system.
Climatic significance
The geographical extent of snow cover varies seasonally and is concentrated in the middle and high latitudes of the Northern Hemisphere, where large continental land areas experience winter temperatures below freezing. Snow is also persistent at high elevations and across much of Antarctica. In maritime climates, snowfall may be frequent while surface cover remains short-lived because temperatures fluctuate near the melting point.
Snow participates in a climate feedback through its reflectivity. Cooling can expand snow-covered area and increase reflected solar radiation, which reinforces the initial cooling. Warming can reduce snow extent and expose darker land surfaces, increasing absorbed radiation and reinforcing the initial warming. This snow–albedo feedback is strongest where changes in temperature move the seasonal snow boundary across broad regions.
Long-term changes in snow cover also affect ecosystems and water availability. Earlier melt changes the seasonal relation between water supply and plant growth, while declining accumulation alters runoff in drainage basins that depend on mountain snow. These effects vary geographically because precipitation change, atmospheric circulation, elevation, and local energy balance do not respond uniformly to global temperature change.
Mechanical behavior and hazards
Snow behaves as a porous, temperature-dependent material rather than as a uniform solid. Under sustained stress it deforms gradually through grain rearrangement, bond deformation, and fracture. Its strength depends on density and grain bonding, but abrupt contrasts between adjacent layers can be more important than the properties of either layer considered separately.
A slab avalanche occurs when a cohesive plate of snow fractures above a weaker layer and the fracture propagates across a slope. Loose-snow avalanches begin at a point and entrain additional material as they descend. Wet avalanches involve substantial liquid water, which alters both grain cohesion and friction within the moving mass.
Blowing snow creates a separate atmospheric and surface hazard by reducing visibility and redistributing accumulation. Strong winds can suspend previously deposited particles even when no new precipitation is falling. This redistribution produces local differences in depth that may greatly exceed the regional mean and can load leeward terrain independently of measured snowfall.
Snow loading also affects vegetation and built structures. Dry snow can accumulate to considerable depth with modest mass, while dense or rain-saturated snow exerts much larger loads for the same thickness. The mechanical consequences follow from water equivalent and spatial distribution rather than from surface depth alone.