Snow redistribution is the movement of fallen snow from one part of a landscape to another, especially by wind. It explains why an exposed ridge can be almost bare beside a hollow containing a deep drift, even when both received the same storm. The pattern reflects transport and deposition after snowfall, not simply differences in precipitation.
Redistribution matters because where snow accumulates influences when water is released, how soil is insulated and which habitats remain snow-covered into spring. Some transported snow also sublimates, changing directly from ice to vapour. A catchment can therefore experience both internal rearrangement and a net loss of snow water. Understanding the difference requires following mass across the landscape rather than treating one depth measurement as the snowfall total.
Key takeaways
- Wind can erode exposed snow and deposit it in sheltered locations.
- Snow depth after a storm does not necessarily equal local snowfall.
- Drift water content depends on density as well as depth.
- Transported snow can sublimate before settling elsewhere.
- Redistribution changes melt timing even when total catchment snow water is unchanged.
How wind picks up snow
Moving air applies stress to the snow surface. When that stress exceeds the resistance holding particles in place, grains can begin to move. Some roll or creep near the surface, others bounce in short hops, and smaller particles can become suspended in turbulent air.
The hopping movement is called saltation. Impacts from moving grains can dislodge additional particles, helping maintain transport. Suspension carries particles higher and can move them farther, depending on wind and the time they remain airborne.
The threshold is not one universal wind speed. Fresh loose snow, older bonded snow and a hard melt–freeze crust respond differently. Temperature history, grain shape and surface roughness all affect how readily particles are mobilised.
This means a windy day can produce extensive drifting after one snowfall but little after another. The difference may lie in the snow surface rather than the wind record alone. Measurements of snowfall and wind therefore need information about antecedent snow conditions.
Why ridges and hollows look different
Terrain changes the speed and turbulence of airflow. Exposed crests and open slopes can lose snow, while sheltered lee slopes and depressions can accumulate it. The lee side is the side sheltered from the incoming wind, so its location changes when the wind direction changes.
Deposition occurs where transport capacity falls and particles settle. A hollow can collect snow from an area much larger than its own surface. Its depth therefore reflects an upwind supply zone as well as precipitation falling directly onto it.
Vegetation changes the pattern too. Shrubs, grasses and forest edges increase roughness and create sheltered pockets. As snow buries shorter vegetation, the effective roughness changes, so the same place can behave differently later in winter.
Canopy interception is a separate process: snow is caught above ground by branches. Redistribution across the ground and interception can interact, but they should not be merged into a single loss term without tracking where the water goes.
Redistribution is not always a net loss
Imagine a boundary drawn around an entire catchment. Snow moved from one slope to another remains within that boundary, so internal transport alone does not change the total snow mass. It changes the spatial distribution.
Snow blown across the catchment boundary is different: it becomes an export or import. Sublimation during transport is another loss from the local snow store, even if the vapour remains part of the wider atmosphere.
The distinction follows the conservation principles in the water cycle. A snowbank disappearing from one field does not establish that all its water evaporated; much may now occupy another field or a nearby ditch.
A hypothetical mass balance
Suppose a small study area begins with 100 tonnes of snow water. During a windy period, 15 tonnes moves from its ridge to its hollow, five tonnes leaves across the boundary, two tonnes arrives from outside and three tonnes sublimates.
The internal transfer cancels in the whole-area balance. Final snow water is 100 − 5 + 2 − 3 = 94 tonnes, assuming no snowfall or melt. Yet the hollow can become much deeper while the area’s total water store decreases.
The figures are illustrative. Their value is to show why the study boundary and the distinction between transport and phase change are essential. A map of increasing drift depth alone cannot establish increasing catchment storage.
Depth and water equivalent
Snow water equivalent, or SWE, is the depth of liquid water represented by the snow if it melted. It depends on snow depth and density. Wind-packed snow can be denser than freshly fallen powder, so equal depths need not contain equal water.
For a hypothetical one-metre drift with average density 300 kilograms per cubic metre, each square metre holds about 300 kilograms of snow water, equivalent to 300 millimetres of liquid water. A half-metre layer at 150 kilograms per cubic metre holds only 75 millimetres.
Neither density should be treated as a universal value. Drifts have layers, and density changes with settling, wind packing and melt–freeze processes. Field estimates need representative measurements rather than one assumed conversion everywhere.
Snowfall gauges add another complication. Wind can divert falling flakes around a gauge opening, producing undercatch. Differences between gauge precipitation and ground snow can therefore reflect measurement bias as well as real redistribution.
Why blowing snow can sublimate
Sublimation converts ice directly into water vapour. Airborne particles expose a large surface area to moving air, allowing exchange of heat and moisture. Under sufficiently dry conditions and with available energy, some mass is lost before the particles settle.
The rate depends on humidity, temperature, particle properties and ventilation. As sublimation adds vapour and consumes energy, it can modify the surrounding air and limit further loss. A simple assumption that every transported grain survives unchanged is therefore not always appropriate.
Conversely, it is wrong to assume that all blowing snow vanishes into vapour. Deposition remains important, particularly over shorter transport distances or under conditions less favourable for sublimation. The balance is a question for measurement or an appropriately tested model.
How drifting changes spring runoff
A uniform snow cover can release water over a broad area, while a redistributed cover contains thin patches that disappear early and deep drifts that persist. Even with the same total SWE, these patterns can produce different melt timing.
Deep drifts can maintain local water inputs after nearby slopes are bare. Their energy balance also changes as dark ground emerges around them. Heat and moisture exchange with surrounding snow-free surfaces can influence melt, making depth alone insufficient to predict disappearance.
The resulting runoff also depends on the receiving ground. Meltwater may infiltrate, refill soil storage or reach a stream rapidly through connected pathways. A late-lying drift is a potential delayed source, not a guarantee of a particular downstream discharge.
This connects snow distribution with seasonal river-flow patterns. A catchment average can hide the small areas that continue supplying meltwater late in the season.
Soil insulation and ecosystems
Snow contains air and can insulate the ground from cold atmospheric conditions. Wind-scoured sites may therefore experience different soil temperatures from nearby snow-covered hollows. The relationship depends on snow thickness, density and the timing of accumulation.
Persistent drifts can delay the start of the growing season while supplying moisture later. Exposed sites may become snow-free earlier but face different frost and water conditions. Plant communities can reflect these contrasting microenvironments.
Animals also encounter changes in shelter, access to food and movement. These ecological effects are highly site-specific, so it is better to explain the mechanisms than to label all deep snow beneficial or harmful.
Redistribution is common in exposed mountain and tundra settings and open agricultural landscapes. Its importance depends on vegetation height relative to snow depth, wind climate and terrain, not latitude alone.
How snow redistribution is measured
Repeated depth surveys reveal erosion and accumulation patterns. Density measurements convert those patterns into water storage. Measurements should span exposed, sheltered and intermediate terrain rather than focusing only on convenient access points.
Remote sensing can map snow-covered area and, with suitable methods, changes in snow depth. However, cover is not equivalent to SWE. A thin continuous layer and a deep drift can both appear simply as snow-covered pixels.
Weather observations provide the forcing: wind direction, speed, temperature and humidity. A station in a sheltered hollow may not represent the exposed ridge from which snow is transported. The placement of instruments is part of the interpretation.
Testing a distributed model
A distributed snow model represents different parts of the landscape separately. It may estimate erosion, transport, deposition and sublimation using terrain and vegetation information. Matching only total catchment snow water can conceal errors in where that water is stored.
Testing should therefore include spatial snow observations as well as downstream flow. The principles behind calibrating and validating river models apply here: agreement with one output does not prove every internal process is correct.
What snow fences illustrate
A porous barrier can alter airflow and encourage snow deposition in a selected zone. Snow fences demonstrate that small changes in roughness can reorganise accumulation over an area larger than the barrier itself.
The outcome depends on fence design, wind direction, terrain and snow supply. It is not enough to say that a solid wall will “stop” drifting snow; turbulence and deposition patterns matter. Practical installations require local design rather than a universal placement rule.
From an educational perspective, the important point is that drifting is a transport problem. Managing where snow settles means changing movement and deposition, not creating additional precipitation or removing the need to account for water later.
Interpreting photographs and snow maps
A photograph of a bare ridge beside a deep drift can support an explanation of redistribution, but it cannot quantify the mass moved without measurements. The scene may also contain differences in snowfall, sunlight or melting.
Repeated observations around a documented wind event provide stronger evidence than one image. Comparing before and after conditions helps separate transport from later melt, while density data prevent changes in packing from being mistaken for equivalent changes in water mass.
The most useful question is not merely “where is the snow deepest?” but “what combination of snowfall, transport, sublimation and melt produced this distribution?” That framing keeps competing explanations visible.
Conclusion: the landscape rearranges the snowfall
Snow does not remain exactly where it lands. Wind, terrain and vegetation redistribute it, creating contrasting stores whose melt timing and ecological effects differ across short distances.
Following both mass and location makes those patterns intelligible. It prevents a deep drift from being mistaken for greater snowfall and shows why spatial detail can matter as much as a catchment-wide average when interpreting winter water resources.
Frequently asked questions
Does a deep drift mean more snow fell there?
Not necessarily. Wind may have delivered snow from surrounding areas after it fell.
Is blowing snow always lost from the catchment?
No. Internal redistribution leaves total mass unchanged unless snow crosses the boundary or changes phase and leaves the snow store.
Why can an old crust resist drifting?
Bonding between grains increases resistance to wind erosion, so the transport threshold differs from that of loose fresh snow.
Can drifting affect river flow months later?
Yes. Concentrated drifts can persist and release water after thin snow elsewhere has disappeared, although soil and groundwater affect the downstream response.
Is snow depth enough to measure stored water?
No. Density is also required to estimate snow water equivalent.
