Snow on the Ground

large snow dunes
Photo adapted from adege on Pixabay (Pixabay License)

When referring to snow on the ground, one often comes across the term “snowpack.” Just what is a snowpack? It refers to the snow that builds up on the ground over the course of a winter, eventually melting in the spring. In some places, snow only stays on the ground for a few hours or days after it snows, and a snowpack never accumulates. A lasting snowpack is common in areas where it stays cold enough that snow doesn’t melt faster than it accumulates.

High latitude places in both the Northern and Southern Hemispheres usually develop a snowpack, as do high elevation areas (often mountains) where temperatures are cooler than they are closer to sea level. We use the plural term “snowpacks,” because there is variation in the amount, timing, and physical characteristics of lasting snow on the ground from place to place and year to year.

Read on to learn more about snowpacks, how they form, their characteristics, and what they can tell us.

How much snow?

Snowfall rates vary greatly, with records being a coveted prize. The 24-hour U.S. record is currently the 1.6 meters (63 inches) that fell in one day at Georgetown, Colorado in 1913.  An interactive version of the map below and for individual states, as well as a list of records for every state and county and can be viewed on the National Oceanic and Atmospheric Administration’s Snowfall Extremes web page.

Map showing 1-day snowfall extremes for the continental U.S.
High elevation areas in the Rocky Mountains (Colorado), Sierra Nevada Mountains (California), and on Mt. Ranier (Washington) have experienced the most extreme one-day snowfall amounts in the U.S., besting the Valdez-Cordova Census Area in Alaska by eight to thirteen inches. NOAA image / National Climate Data Center (Public Domain)

The number of times that it snows per winter also varies greatly from place to place. On average in Aspen, Colorado, it snows 26 times during the winter, or about every five days, while in other places, it might only snow two or three times per winter. 

Seven distinct climate classes of snow are widely recognized, and these align well with the major ecoregions of the world, because temperature and precipitation also control what plants will grow in a given area.

A map of different snow classes of Canada and the U.S. with an illustration of typical snow depth for tundra and maritime regions, person for scale
Snow classes map and illustration from Sturm et al., 1995.
Major ecoregions map from Ricketts et al., 1999 on Wikimedia Commons (CC-BY-SA-3.0)

The distribution of snow classes (left) of North America corresponds with the locations of its major ecological regions (right). Note the difference in typical snowpack depth (human silhouette for scale) between the tundra and maritime snow classes.

Changing Snow

Sequence of illustrations of stellar dendrite snowflake metamorphosing and rounding off
A snowflake held in a glass case at below freezing temperatures for 57 days rounds off. Illustration by H. Bader (1962)

The moment snowflakes land on the ground and start to build a snowpack they begin to change.  No longer in a super-moist environment like that of the clouds in which they formed, the crystals’ arms or branches break off, and they become more rounded, even at temperatures below freezing. At the same time, the force of gravity compacts the snow, making it more dense. Yet even when compacted, the snowpack remains more air than ice by volume.

A scanning electron microscope image of a new snow crystal and an old snow crystal
Scanning Electron Microscope (SEM) images show a new snow crystal (left) and an old snow crystal (right) that is becoming rounded in shape. The crystal in the right image is approximately 4 mm in diameter. Image from Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture (Public Domain)
In this video, a snowpack with a density of 100 kg/cubic meter compacts to a density of 350 kg/cubic meter, with air content decreasing from 90% to 65%, as gravity works on the snow grains, pulling them closer together and reducing air spaces. Animation by Matthew Sturm

Snowpack Layers

Through the course of the winter, the snow pack builds up. It snows, then stops; the new layers settle and compact, the snow grains in each layer changing (metamorphism) with time. Then it snows again. Each layer preserves some characteristics that tell us about its deposition and post-deposition history. The sum of all the layers blankets the landscape and affects the ecosystem, the climate, and the hydrology (movement of water). 

Two kids sit in front of a thin wall of snow illustrating layers deposited during discrete snow events
Layers deposited during individual snow events are visible in a snowpack cross-section. The symbols represent different types of snow crystals characteristic of each layer. Photo by Matthew Sturm

Depending on the local air temperature, the thickness of the snow pack, and the temperature of soil, the grains in the snow layers can either become faceted and ornate (shown with blue arrows below), or more and more rounded (shown with red arrows below).

Snow crystals in a snowpack change through one of two different pathways, depending on air and soil temperature and depth of the snowpack. The crystals start as equilibrium grains, shown in the lower right photograph. Diagram by Matthew Sturm