The journey of water from the atmosphere to a permanent, massive body of ice on land involves a specialized sequence of the hydrological cycle. This transformation requires two distinct physical processes. The first converts suspended moisture into solid precipitation, and the second physically restructures that accumulated solid into the dense, flowing substance recognized as glacial ice.
Water Delivery: How Clouds Precipitate Snow
Glacier formation begins with the atmospheric process that converts cloud moisture into solid precipitation. This mechanism is most efficient in cold clouds, which contain a mixture of tiny ice crystals and supercooled water droplets existing below freezing. Supercooled water remains liquid because it lacks a nucleus, but ice crystals readily form when water vapor deposits directly onto an ice nucleus, such as a dust particle.
The Bergeron process drives the rapid growth of these initial ice crystals at the expense of the surrounding supercooled water. At the same sub-freezing temperature, the saturation vapor pressure over an ice surface is lower than that over a water surface. This difference causes water molecules to evaporate from the supercooled droplets and immediately deposit onto the growing ice crystals. The ice crystals become heavy enough to fall toward the surface, continuing to grow by collecting more supercooled droplets, a process called riming, until they land as snow.
The Transformation of Snow into Glacial Ice
Once the crystalline precipitation settles on the ground, the second, multi-stage physical process known as firnification begins, converting the light, airy snowpack into dense glacial ice. Freshly fallen snow is composed of intricate, hexagonal crystals that are approximately 90% air. Under the weight of subsequent layers of snowfall, the delicate crystal arms break and the snowpack begins to settle and compact.
The structure of the snow changes from complex flakes to small, rounded, granular particles, similar to coarse sugar. This change is driven by recrystallization, where water molecules sublimate from the sharp points of the crystals and deposit into the hollows, resulting in a more spherical, denser grain. After surviving at least one melt season, this intermediate material, which has a density ranging from 0.4 to 0.84 grams per cubic centimeter, is formally called firn.
The transformation from firn to true glacial ice continues as burial depth and pressure increase over time. As more snow accumulates above, the firn grains are forced closer together, and the air spaces between them become progressively smaller and less connected. The final transition to glacial ice occurs when the density reaches approximately 0.83 grams per cubic centimeter, sealing off the air passages. The air is then trapped as isolated bubbles within the solid ice structure, marking the point at which the mass can begin to flow under its own weight and is classified as a glacier.
The Climate Conditions for Glacier Formation
For these two physical processes to result in a lasting glacier, a specific climatic context must be maintained over many years. The requirement is that the long-term accumulation of snow must consistently exceed the loss of ice mass through ablation. Ablation includes melting, water runoff, sublimation (ice turning directly to vapor), and calving of icebergs.
Glaciers form in an accumulation zone, typically at high latitudes or elevations, where temperatures remain low enough to preserve the snowfall year-round. The input of new snow (Process 1) must create a perennial snowpack that allows the deep burial and compression (Process 2) to continue uninterrupted. If the mass balance remains positive, the growing body of ice will eventually achieve enough thickness to deform and flow, completing its transformation into a dynamic glacier.