Snow forms when water in the atmosphere freezes or deposits into ice crystals and those crystals grow into sufficiently large particles that fall to the ground without completely melting. Most snow develops inside clouds at temperatures below or near the freezing point, although the exact temperature range depends on humidity, crystal type and atmospheric conditions. Water vapour can deposit directly onto ice nuclei, forming solid ice without first becoming liquid.
Snow crystals then grow as additional water vapour deposits onto their surfaces, and individual crystals can collide and stick together to form snowflakes or aggregates. For snow to reach the ground as snow, the atmospheric temperature profile below the cloud must generally remain cold enough to prevent substantial melting. Surface air temperatures slightly above zero degrees Celsius do not automatically prevent snowfall because snowflakes can sometimes survive a shallow above freezing layer, particularly when the air is dry and evaporation produces cooling.
The shapes of snow crystals arise from the molecular structure of ice and the environmental conditions under which the crystals grow. Ordinary atmospheric ice forms a crystalline structure in which water molecules arrange into a hexagonal lattice, giving many snow crystals sixfold symmetry. Temperature and supersaturation strongly influence whether a growing crystal develops plates, columns, needles, dendrites or other forms.
The famous snow crystal diagrams associated with Wilson Bentley and later classifications demonstrate that crystal morphology changes systematically with temperature and humidity, although real atmospheric crystals often become irregular through collisions, riming and variable growth histories. Dendritic snowflakes develop intricate branching structures under suitable combinations of temperature and water vapour supersaturation. The diversity of snow crystal shapes therefore reflects the microscopic physics of ice growth rather than random decoration.
Snow can form through several processes within clouds. Deposition occurs when water vapour changes directly into ice on an ice surface. Aggregation occurs when separate ice crystals or snow particles collide and stick together, producing larger snowflakes. Riming occurs when supercooled liquid droplets collide with and freeze onto ice particles, producing graupel or heavily rimed crystals depending on the degree of accumulation.
In mixed phase clouds, the Bergeron process can promote ice growth because water vapour pressure over ice is lower than over supercooled liquid water, encouraging vapour transfer towards ice crystals. Vertical air motions are important because they repeatedly move particles through regions with different temperature and moisture conditions. Snow formation is therefore controlled by interactions between thermodynamics, cloud dynamics and ice microphysics rather than simply by low temperature alone.
Whether precipitation reaches the ground as snow depends strongly on the vertical temperature profile. A snowflake descending from a cloud may encounter layers that are above freezing and begin to melt. If the warm layer is shallow, the particle may remain partly frozen and refreeze or remain snow after entering colder air below. If the warm layer is sufficiently deep, the snowflake can melt completely and become rain.
If it then enters a sufficiently cold layer near the surface, it may freeze into an ice pellet rather than returning to its original snowflake structure. This is why two locations with the same surface temperature can experience different precipitation types if their atmospheric profiles differ. Meteorologists use radiosonde observations, numerical weather prediction models and remote sensing to analyse temperature, moisture and wind with height when forecasting snowfall.
Snowfall intensity and accumulation depend on more than the presence of subfreezing air. Moisture supply, vertical motion, cloud depth, crystal growth efficiency, wind and ground conditions all influence accumulation. Heavy snowfall can occur when moist air is lifted strongly over mountains, during intense frontal systems or within convective snow showers. Orographic snowfall occurs when moist air is forced upward over elevated terrain, producing cooling and cloud development.
Downwind areas may receive substantially less snow because descending air warms and dries, producing a precipitation shadow. Lake effect snow can develop when cold air passes over relatively warm unfrozen water, gaining heat and moisture before rising over nearby land. These processes explain why snowfall can vary dramatically over relatively short distances, especially near mountains, coastlines and large lakes.
Snow is an important part of the climate and hydrological systems because it acts as temporary or long term water storage. Snowpack accumulates when snowfall exceeds melting and sublimation and can persist for months or, under appropriate climatic conditions, much longer. When snow melts, the resulting water contributes to streams, rivers, soil moisture and groundwater recharge. Mountain snowpack is especially important for seasonal water supplies because melting can occur during spring and summer after the main precipitation season.
Snow also has a high surface reflectivity, or albedo, meaning it reflects a substantial fraction of incoming solar radiation. A fresh snow surface can therefore remain relatively cold and can influence local and regional energy balances. As snow ages, melts, becomes contaminated or undergoes grain metamorphism, its reflectivity can decline, increasing solar absorption and accelerating warming and melt.
Snow has distinctive physical properties that affect ecosystems, transport and human activities. Snow contains a mixture of ice and air, so its density can vary greatly depending on crystal structure, wind compaction, temperature history and metamorphism. Fresh powder can be relatively low density, whereas old compacted snow can become much denser. Snowpack undergoes continuous metamorphism as temperature gradients drive changes in crystal structure, and melting and refreezing can create layers of hard ice or crust.
These layers influence avalanche behaviour, water movement and ecological insulation. A snowpack can also insulate the ground from extreme cold, providing a relatively stable environment for plants and small animals. However, heavy snow can damage vegetation and infrastructure, while rapid melting can contribute to flooding. Snow therefore functions simultaneously as precipitation, water storage, a thermal insulator and a reflective surface.
A common misconception is that it must be freezing at ground level for snow to fall. Snow can reach the surface when the near surface temperature is slightly above zero degrees Celsius if the above freezing layer is shallow or if evaporative cooling helps maintain sufficiently cold conditions. Conversely, temperatures below zero at the surface do not guarantee snow because a warm layer higher in the atmosphere may melt falling snow and produce rain or another precipitation type.
Another misconception is that all snowflakes are large six sided star shapes. Individual ice crystals can have many forms, and observed snowflakes frequently consist of aggregates, rimed crystals or partially melted structures. Snow formation is fundamentally a problem of ice nucleation and growth within atmospheric clouds combined with the thermal and moisture structure encountered during descent. Its final form depends on the entire atmospheric history of the particle rather than on surface temperature alone.
