Snowpack strata

What it is

Snow that stays on the ground through a winter is not one thing. Each storm lays down its own layer, and each spell of weather between storms marks the top of the pack: a crust after sun or rain, a sparkle of hoar after a clear night, a hard slab where the wind packed it. By March a mountain snowpack is a stack of these layers, each with its own grains, density and strength. Fig. A draws one, cut open in a snow pit.

The layers do not stay as they fell. They settle, their grains change shape, and the whole pack gains water as it gains weight. Fig. B follows one winter at a mountain station, and Fig. C draws the grain types a pit reveals.

What it is not

It is not the snow depth. Depth says how deep the snow is, not how much water it holds or how its layers are built. Two packs of the same depth can hold twice the water of each other, and one can be sound while the other is ready to slide.

It is not a glacier. A seasonal snowpack melts every summer; a glacier is snow that has lasted from year to year and turned to ice.

Lookalikes

Snow depth: How deep the snow is. It says nothing of the water in it or of its layers; two packs of the same depth can hold very different water and very different danger.
New snow: What fell in the last day. It is the top layer only, the one a storm adds to the stack.
A glacier: Snow that has lasted from year to year and turned to ice under its own weight. A seasonal snowpack melts every summer.

The machine

Settling and rounding

New snow is mostly air, commonly about 100 kg m⁻³ (6.2 lb ft⁻³), a tenth the density of water. Under the weight of the snow above it and with time, it settles. The delicate arms of the crystals break and evaporate, the grains round, and they bond to each other where they touch. A settled layer of rounded grains is two or three times as dense as it fell, and stronger for it.

The water the pack holds is its depth times its density, divided by water's; the first equation below gives it. Forty inches of settled snow at 300 kg m⁻³ hold 12 in (30 cm) of water, the same as 120 in of new snow. That is why the water in the pack rises smoothly through a winter while the depth jumps with each storm and sinks between them.

Faceting: the gradient that weakens

The ground keeps the bottom of the pack near 32 °F (0 °C), while the surface follows the air and can be far colder. The difference drives water vapour up through the pack, from the warmer grains below to the colder ones above. Where the gradient is strong, about 5.5 °F a foot (10 °C m⁻¹) or more, the vapour builds flat faces and steps on the grains instead of rounding them. Faceted crystals bond poorly. At the base of a shallow early season pack they grow into depth hoar, large cup shaped grains that can stay weak all winter. The second equation gives the gradient: a thin pack under a cold sky facets, and the same sky over a deep pack does not.

Hoar, crusts and slabs

On a clear, calm night the snow surface cools below the air, and frost grows on it in feathers: surface hoar. Buried by the next storm intact, it is one of the most persistent weak layers there is. Sun or rain on the surface melts it, and the melt freezes into a crust. Wind picks snow up from exposed slopes and packs it onto sheltered ones as a slab, dense and cohesive.

A slab over a weak layer is the machine of a slab avalanche. The weak layer collapses or shears, the crack runs across the slope, and the whole slab slides at once.

Melt

In spring the pack warms through until the whole of it is at 32 °F (0 °C). Meltwater runs down through it, rounds the grains into clusters and refreezes at night. When the pack is saturated it releases its water to the streams, Melt and runoff FB-WAT-032.

A painted chart of one modelled winter at a high mountain station, November to June, before a pale mountain skyline: the snow depth as a pale blue area whose edge jumps with every storm and sinks as the snow settles, reaching its deepest, 72 inches (183 centimetres), in March; the water in it as a deep blue area that climbs in steps to its most, 24 inches (61 centimetres), in early April; and both falling together through the melt of May to nothing in June. — **Fig. B** One winter at a mountain station. The snow depth jumps with each storm; the water in it climbs steadily.Maximize The drawing, to scale

Eight painted round lens views, white snow grains against a dark field, each named with its class code and symbol: precipitation particles, a six armed star; decomposing fragments, broken arms; rounded grains, a small bonded cluster; faceted crystals, flat sided blocks; depth hoar, a large ribbed cup; surface hoar, an upright feather; melt forms, a wet cluster of rounded grains; and ice formations, a flat lens of ice; with a note that machine made snow is the ninth class, named and not drawn. — **Fig. C** The grain classes. The snow grains a pit reveals, as a hand lens shows them.Maximize The drawing, to scale

The drawings

Each figure drawn as an engineering sheet, its parts numbered, to print at 11 by 17.

Fig. A, the drawing A snow pit wall. Section through a winter snowpack 60 in (152 cm) deep, layer by layer, with its temperature and hardness HEIGHT TO SCALEMaximize The sheet, SVG, 11 by 17 The painting

New snow
Decomposing fragments
The slab, rounded wind packed grains
Buried surface hoar, the weak layer
Faceted crystals
A melt freeze crust
Depth hoar
The temperature through the pack
The hardness of each layer

Fig. B, the drawing One winter at a mountain station. Snow depth and snow water equivalent through one winter, November to June, at a typical high mountain station in the West TIME TO SCALE, DEPTH TO SCALEMaximize The sheet, SVG, 11 by 17 The painting

Snow depth
Snow water equivalent
A storm, and the settlement after it
The deepest snow, March
The most water, early April
The melt

Fig. C, the drawing The grain classes. The main grain classes of the international classification, each drawn enlarged with its symbol NOT TO SCALE, GRAINS ENLARGEDMaximize The sheet, SVG, 11 by 17 The painting

Precipitation particles
Decomposing fragments
Rounded grains
Faceted crystals
Depth hoar
Surface hoar
Melt forms
Ice formations

Ingredients

Storms, each laying down its own layer of new snow
The spells between storms: clear nights that grow surface hoar, sun and rain that make crusts, wind that builds slabs
A temperature gradient through the pack, warm at the ground and cold at the surface, that drives water vapour upward
Settlement under the weight of the snow above, which rounds and bonds the grains

Scales

time: a storm for a layer to fall; days to weeks for it to change; a winter for the whole stack
horizontal: a slope, for one layer's character; a mountain range, for the weak layers a season leaves behind
vertical: an inch to several feet a layer; the whole pack, a few feet to more than 10 ft (3 m)
orlanski: micro

Equations

Snow water equivalent

\mathrm{SWE} = h\,\frac{\rho_s}{\rho_w}

$\mathrm{SWE}$: the depth of water the snow would make if it melted, in
$h$: the depth of the snow, in
$\rho_s$: the density of the snow, kg m⁻³
$\rho_w$: the density of water, 1,000 kg m⁻³

Assumes One density for the whole depth; a real pack is denser at the bottom than the top, and the equation is applied layer by layer and summed.

Working form 40 in (102 cm) of snow at 300 kg m⁻³ (18.7 lb ft⁻³) holds 12 in (30 cm) of water: the same water as 120 in (305 cm) of new snow at 100 kg m⁻³ (6.2 lb ft⁻³).

The temperature gradient

G = \frac{T_{\mathrm{top}} - T_{\mathrm{bottom}}}{\Delta z}

$G$: the temperature gradient across a layer, °C m⁻¹
$T_{\mathrm{top}},\ T_{\mathrm{bottom}}$: the temperature at the top and the bottom of the layer, °C
$\Delta z$: the thickness of the layer, m

Assumes A steady gradient across the layer; the rule of thumb below is for the dry snow of a cold winter pack.

Working form The ground holds the base of the pack near 32 °F (0 °C). Under a surface at 14 °F (−10 °C), a pack 20 in (51 cm) deep has a gradient of about 11 °F a foot (20 °C m⁻¹), twice the 5.5 °F a foot (10 °C m⁻¹) at which grains grow facets; a pack 60 in (152 cm) deep, under the same surface, about 3.6 °F a foot (6.6 °C m⁻¹), under it.

Signatures

surface: surface hoar sparkling on the snow after a clear; calm night; a crust after sun or rain; wind drifting snow off ridges onto lee slopes
sounding: a warm layer aloft that brings rain onto the snow; a clear; calm night that grows hoar

The numbers

Quantity	Value, and the kind of number it is
New snow density	50 to 150 kg m⁻³ (3 to 9 lb ft⁻³), commonly about 100 (6.2)Typical, McClung 2006
Settled, rounded snow	200 to 350 kg m⁻³ (12.5 to 21.8 lb ft⁻³)Typical, McClung 2006
Spring snow, melted and refrozen	350 to 500 kg m⁻³ (21.8 to 31.2 lb ft⁻³)Typical, McClung 2006
Faceting gradient	About 5.5 °F a foot (10 °C m⁻¹) or more across a layer of dry snowTextbook, McClung 2006
The grain classes	Nine: precipitation particles, machine made, decomposing fragments, rounded grains, faceted crystals, depth hoar, surface hoar, melt forms, ice formationsStandard, The International Classification for Seasonal Snow on the Ground
Hand hardness	Fist, four fingers, one finger, pencil, knife, ice: what can be pushed into the layer with moderate forceStandard, The International Classification for Seasonal Snow on the Ground
The base of the pack	Near 32 °F (0 °C), held there by the groundTypical, McClung 2006

How the station sees it

A station sees the pack from above and from beneath. Cooperative and volunteer observers measure snowfall and depth by hand each morning, and the water in a core of new snow. Snow pillows weigh the pack from below and report its water, SNOTEL and the snow pillow FB-INS-007. The snow tracker on snowpack.fyi reads both for the ski mountains. What no station records is the layers themselves: those come from people digging pits, and from knowing the weather each layer was made in.

Cooperative observers: snowfall and snow depth by hand, once a day
Volunteer rain and snow gauges: new snow, its water, and the depth on the ground, every morning
The snow tracker on snowpack.fyi: new snow, base depth and water at sixty three mountains, from the snow pillows and the stations near them

How it is warned

Avalanche danger is forecast by avalanche centers for the mountains they cover, and the Weather Service carries their Avalanche Watch, Avalanche Warning and Avalanche Advisory to the public. The danger is rated on a five level scale, from low to extreme, by elevation and aspect. The layers drawn here are what those forecasts are about.

Sources

Fierz, C. and others, UNESCO International Hydrological Programme. The International Classification for Seasonal Snow on the Ground, IHP-VII Technical Documents in Hydrology No. 83 (2009).
McClung, D. and P. Schaerer. The Avalanche Handbook, 3rd ed. (2006).
National Weather Service. Watch, Warning, Advisory definitions.
American Meteorological Society. Glossary of Meteorology.

Definition after the The International Classification for Seasonal Snow on the Ground, IHP-VII Technical Documents in Hydrology No. 83 (2009). Plate FB-WAT-031, revision 1, 2026-09-25. The number is permanent; cite it.