Backgrounder on Snow, Measuring Snow, and Forecasting Water Supply

“The importance of snowpack to life in and around the Rocky Mountains cannot be overemphasized.”

Rocky Mountain/Great Basin Regional Climate-Change Assessment,
U.S. Global Change Research Project (2003, Utah State University)

This section contains a collection of background information intended to assist you when preparing lessons and activities for your particular setting and specific needs.  We’ve divided this overview into three sections, organized around key subject areas.  This trio of concepts—snow’s physical properties and characteristics; techniques for accurately measuring snowpack; and the steep challenge of using the resulting data to build meaningful forecasts of water supply—build upon each other.  Feel free to extract particulars from within this section as needed.

Snow

Frozen and crystallized precipitation falling all the way to the ground is called “snow.”  As a form of precipitation, snow is distinct from rain (visible droplets of water), sleet (freezing rain), hail (ice balls formed by updrafts), and drizzle (droplets of water to small to be visible one by one).

Except under very cold conditions (chillier than -40 ˚ F), snow formation requires tiny particles aloft in the atmosphere, which give water vapor, in air at 32 ˚ F or cooler, something on which to start freezing.  An individual particle begins to attract water molecules far above the ground, while swirling in the clouds.  A nucleus can be a fleck of volcanic ash, salt from ocean spray, windblown wood smoke, or even soot and other pollutants.  Collectively, the particles serving as snow seeds are called “cloud condensation nuclei.”

As a particle begins to attract water molecules, they freeze first to the particle, then to themselves.  Water forms crystals when frozen because of the substance’s molecular structure.  This structure is hinted at by the chemical notation of water: H₂O.  Two hydrogen atoms are each smaller than a single oxygen atom and attach toward one side of the oxygen.  This tendency gives each molecule polarity.  The positive end of one molecule shares a mild attraction with the negative end of up to four other water molecules.  Known as hydrogen bonding, this ever-so-slight stickiness between molecules of water is responsible for many of the remarkable properties of this inorganic substance without which life cannot exist.

One of water’s properties is the formation of crystals when frozen.  As droplets solidify, they branch out in six evenly-angled directions, forming a hexagonal flake.  From that classic snowflake shape, with its six sides, there are endless possibilities in exact size and details, so that no two snowflakes are exactly the same.  The old saying really is true!

Key determinants to the size and shape of each unique flake include temperature, humidity, and amount of time spent circulating in clouds as the crystals form. As amalgamations of crystals, most snowflakes are ⅛” to ½” in diameter.  Blown by winds, flakes often crash together, becoming broken and conjoined.  The biggest flakes—up to almost two inches across—require near-freezing-point temperatures, light winds, and convective air conditions.  Bigger flakes tend to be heavy, and fall from the sky more readily.  Studies in the Rocky Mountains have shown that the fluffiest snow forms when winds are light and temperatures are around 15° F.  This kind of snow is referred to as “powder.”  At even colder temperatures, flakes tend to be smaller and denser.

As a rough rule of thumb, an inch of snow makes about 0.10 inches of melt water.  It is worth keeping in mind that this approximation is quite coarse.  Snow varies widely in its density, that is, its water content per volume.  In reality, ratios as low as 100-to-one and as high as three-to-one have been found.  So, whereas 10 inches of Rocky Mountain powder might contain 0.10 inches of water, 10 inches of heavy, wet slush might be equivalent to more than three inches of rain.

Compared to other forms of precipitation, snow poses difficulties to meteorologists as they work to predict the weather.  They have a tough time detecting snow falling from a distance and even greater trouble in predicting timing and locations of snow showers.  Ironically, snow is the most visible form of precipitation once on the ground as a cool white covering across a landscape.

Why is snow white anyway?

Because of snow’s crystalline structure, it reflects most light falling on it, not absorbing any portion of the spectrum much more than any other.  So, we see the reflected light as white, an even mixture of all portions of the visible spectrum. Most other natural materials absorb some segment of sunlight to produce their non-white color.

Once snowflakes hit the ground, they accumulate.  They lose their individual nature and become part of the pack—the snowpack.  Snowpack has a set of material characteristics different from falling snowflakes.  NRCS hydrologists and other scientists have worked long and hard to understand the energetic and structural properties of snowpack.  Snowpacks are composed of crystallized water, ice, and air.  These elements come together and can be described in terms of depth, density, volume, area, porosity, and—of keen significance to water users in the West—snow water content.

For sources of more in-depth sources of information on snow’s physical properties, see Section F, “Related Topics with Suggested Activities and Resources,” especially the first subsection (p. 121).  Terms that might be unfamiliar can be looked up in Section H, “Glossary for Adopt-a-SNOTEL Site”.