Properties of Snow

Why is it that skis slide, septic systems in Minnesota don’t freeze in winter, and the Earth has a climate that can support life? Part of the answer to these questions lies in the properties of snowpacks: their mechanical, thermal, optical, and other characteristics. 

Thermal properties of snow

Snow is a good thermal insulator, because just like a down parka, it contains a lot of small air spaces. Heat moves through snow in four ways, with most of the heat conducted from one ice grain to another where they touch. If these grains barely touch (like in depth hoar snow) the insulating ability of the snow will be 10 times greater than that of denser snow in which the grains are in close contact with each other and have thicker connections between them.

Depth hoar, snow that forms at the bottom of the snowpack under some conditions, is a good insulator because of the air spaces it contains. The denser the ice grains are, the more heat is transferred from grain to grain, reducing the insulative ability of the snow. Illustration by Matthew Sturm.
A down jacket on display in a store
Down jackets keep you warm because they contain a lot of small air spaces to hold heat. Photo by Matti Blume on Wikimedia Commons (CC BY-SA-4.0)

Mechanical properties of snow

graph showing relationship between pressure (force per unit area) and sinkage (depth) in snow
Wearing snowshoes reduces the amount of pressure that a person exerts by more than half, resulting in less “sinkage” into the snow. Figure by Matthew Sturm using data from Alger and Osborne, 1989

Some mechanical properties of snow are familiar, others less so. Anyone who has post-holed through deep snow knows that the fluffier the snow (more air and less ice), the farther they will sink. It is not the weight (force) that the snow has to support, but rather the pressure (force/unit area) that matters most. Wearing skis or snowshoes greatly reduces the pressure and therefore the sinkage. The less we sink the easier it is to move.  Cars can exert four to five times the pressure of a human in boots, which is why snow on roads becomes packed so hard.

Sintered snow hanging from a tree trunk in snowy forest
Curved, curled, or bent snow features result from the process of sintering, in which molecules in the snow near its melting point form strong bonds. Photo by Natalia Kollegova from Pixabay (Pixabay License)

Snow can also resist a tensile (pulling) force because it sinters, meaning that the molecules bond and cause the snow to harden.  This is why snow can “hang” off a roof or powerline.  Sintering is an atomic process that bonds two ice molecules when they touch each other; it is also used in metallurgy to create solid parts from metal powders.  Sintering happens the fastest when a material (like ice) is close to its melting point (0°C / 32°F). The longer the particles are in contact, the thicker the bonds and the stronger the snow. In the North, it is well known that if snow slides off a roof, it will be powdery and easy to shovel for a few hours, but the next day it will be rock-hard.

Optical properties of snow

Every surface reflects light, and some surfaces do so more than others. The term albedo refers to the reflectivity of a surface. Albedo is a measurement of the proportion of light that reaches a surface that is reflected by that surface.

Albedo = Reflected light / Incoming light

Snowy mountain view with arrows showing albedo of dark and light surfaces
Light colored surfaces, like snow, reflect much of the incoming energy they receive from the sun and absorb little of it (high albedo), whereas dark colored surfaces absorb most of the incoming energy from the sun and reflect little of it (low albedo). Photo by Liam Toney, 2020

Substances that reflect a large proportion of incoming radiation have a high albedo value, and substances that reflect little incoming radiation have a low albedo value. New snow has an albedo of about 0.9, meaning that it reflects 90% of the incoming light it receives, whereas the albedo value of the open ocean is about 0.06, meaning that it reflects only 6% of the incoming light it receives. A pure black surface has an albedo value of zero, because it does not reflect any light (which is why it appears black to our eyes).

Any light that is not reflected by a surface is absorbed by it. Light is a form of energy, electromagnetic (EM) radiation. Therefore, the more light that a surface absorbs, the more energy it holds, and the warmer it can get.

A black surface that absorbs all incoming radiation and reflects none of it can get very hot. If you have ever stood barefoot on a freshly paved, asphalt surface on a sunny summer day, you will probably recall that it was uncomfortably hot! In contrast, surfaces that reflect a lot of light absorb less of it, holding less energy, and staying cooler, which is why standing barefoot on light-colored sand on the same day can feel pleasantly warm, but not too hot.

aerial view of Nabesna Glacier
Although fresh snow reflects as much as 90% of incoming radiation from the sun, old, dirty snow, and debris-coved glaciers can reflect as little as 20%. NPS Photo / Bev Goad (Public Domain)

At a value of about 0.9, fresh snow has the highest albedo of any naturally-occurring substance on Earth. The albedo value of a black surface is 0. Between these extremes lie a variety of surfaces with varying albedo values, such as sea ice (0.5-0.7), sand (0.4), grass (0.25), old/dirty snow (0.2-0.4) and conifer forest (0.09). The overall albedo of the Earth is determined by the relative area covered by these and other surfaces. Therefore, the Earth’s albedo varies throughout the seasons, from year to year, and over longer time scales.

Life is possible on our planet in part because the Earth’s albedo maintains a thermal equilibrium, meaning a relatively stable temperature. Snow cover plays an important role in maintaining this thermal equilibrium by acting like a giant reflector, sending energy back to the atmosphere rather than absorbing it.

With increasing global temperatures due to climate change, sea ice, which is typically covered in snow, is melting rapidly. When sea ice and the snow on top of it melt into liquid water, the local albedo changes from about 0.5-0.9 (moderately and highly reflective sea ice and fresh snow, respectively) to 0.06 (dark sea water, one of the lowest albedo values of all naturally occurring surfaces on Earth). The darker water absorbs more sunlight, which in turn leads to the water to warm up. Warmer water melts more sea ice, which in turn reduces the overall albedo of the Earth leading to an increase in solar radiation absorption. This polar amplification cycle (“polar” because it is happening in the planet’s polar regions and “amplification” because it accelerates the rate of change in the system), is an example of what climate scientists call a positive feedback loop. In this case, the word “positive” doesn’t mean “good;” rather, it means a change to one part of a system (e.g. warming, causing melting snow and ice) causes more of the same type of change to occur (e.g. melting snow and ice, causing reduced albedo, causing more warming).

Hydrological properties of snow

Water from snow melt is used by nearly two billion people world-wide, and in the U.S., this runoff (water that “runs off” the land surface) from snow is valued at nearly a trillion dollars per year. To predict how much runoff will be available in a given year, snow surveyors sample the snowpack in thousands of locations each spring. Snow samples are taken using some variation of a tool called the Federal core tube.  A core is taken from the snow, the core volume computed, the core weighed, the density of the snow determined, and then from that the snow-water-equilavent (SWE)….which is essentially the water depth if all the snow were to melt instantaneously.

The Snow Water Equivalent (SWE) in the Tuolumne River Basin of the Sierra Nevada mountain range, which supplies water to the San Francisco Bay Area and California’s Central Valley, was 21 times larger on April 1, 2017 than it was on April 1, 2015. NASA Image (Public Domain)

Person in snow with long metal tube for taking snow core samples
Scientists take core samples from the snowpack and calculate the amount of water it contains, called the “Snow Water Equivalent (SWE).” Photo by Matthew Sturm