Snow Morphology for 4th Graders Fernlike Stellar Dendrite Crystal Matt Hoobler – Wyoming State Engineer’s Office

The beauty of snow is fascinating, and millions of Americans enjoy the snow-covered landscape as a playground. But beyond its aesthetic and recreational appeal, snow plays a vital role in our lives as a primary source of the water supply in the Western United States. Increasing and often conflicting demands for water in the West have heightened public awareness of the need for sound management decisions concerning water. Although the West's high mountain ranges hold a vast snowpack that provides 50 to 80 percent of the year's water supply, nature cannot be relied upon to provide an uninterrupted, dependable supply of meltwater to meet all the downstream requirements. To moderate this variability, reservoirs and canals have been built to serve the growing needs of agriculture, industry, and communities. But successful water management begins with an adequate knowledge of the primary source of water in the West: snow.

The story of a snowflake begins with water vapor in the air The story of a snowflake begins with water vapor in the air.  Evaporation from oceans, lakes, and rivers puts water vapor into the air, as does transpiration from plants.  Even you, every time you exhale, put water vapor into the air.   When you take a parcel of air and cool it down, at some point the water vapor it holds will begin to condense out.  When this happens near the ground, the water may condense as dew on the grass.  High above the ground, water vapor condenses onto dust particles in the air.  It condenses into countless minute droplets, where each droplet contains at least one dust particle.  A cloud is nothing more than a huge collection of these water droplets suspended in the air. In the winter, snow-forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing.  The water is said to besupercooled, meaning simply that it is cooled below the freezing point.  As the clouds gets colder, however, the droplets do start to freeze.  This begins happening around -10 C (14 F), but it's a gradual process and the droplets don't all freeze at once.   If a particular droplet freezes, it becomes a small particle of ice surrounded by the remaining liquid water droplets in the cloud.  The ice grows as water vapor condenses onto its surface, forming a snowflake in the process.  As the ice grows larger, the remaining water droplets slowly evaporate and put more water vapor into the air.   Note what happens to the water -- it evaporates from the water droplets and goes into the air, and it comes out of the air as it condenses on the growing snow crystals.  As the snow falls there is a net flow of water from the liquid state (cloud droplets) to the solid state (snowflakes). 

Snowflakes and snow crystals are made of ice, and pretty much nothing more.  A snow crystal, as the name implies, is a single crystal of ice.  A snowflake is a more general term; it can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals that form flakes that float down from the clouds.

The Structure of Crystalline Ice The water molecules in an ice crystal form a hexagonal lattice, as shown at in red (the two structures show different views of the same crystal).  Each red ball represents an oxygen atom, while the grey sticks represent hydrogen atoms.  There are two hydrogens for each oxygen, so the chemical formula is H2O.  The six-fold symmetry of snow crystals ultimately derives from the six-fold symmetry of the ice crystal lattice. The most basic form of a snow crystal is a hexagonal prism, shown in several examples at right. This structure occurs because certain surfaces of the crystal, the facet surfaces, accumulate material very slowly (see Crystal Faceting for more details).     A hexagonal prism includes two hexagonal "basal" faces and six rectangular "prism" faces, as shown in the figure.  Note that a hexagonal prism can be plate-like or columnar, depending on which facet surfaces grow most quickly.   When snow crystals are very small, they are mostly in the form of simple hexagonal prisms.  But as they grow, branches sprout from the corners to make more complex shapes.  Snowflake Branching describes how this happens.

Morphology Diagram By growing snow crystals in the laboratory under controlled conditions, one finds that their shapes depend on the temperature and humidity. This behavior is summarized in the "morphology diagram," shown at left, which gives the crystal shape under different conditions.  Click on the picture for a closer view. The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions.  For example, we see that thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F).  Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F).  Furthermore, we see from the diagram that snow crystals tend to form simpler shapes when the humidity (supersaturation) is low, while more complex shapes at higher humidity's.  The most extreme shapes -- long needles around -5C and large, thin plates around -15C -- form when the humidity is especially high.    Why snow crystal shapes change so much with temperature remains something of a scientific mystery.  The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. 

Crystal Faceting When water freezes into ice, the water molecules stack together to form a regular crystalline lattice, and the ice lattice has six-fold symmetry.  It is this hexagonal crystal symmetry that ultimately determines the symmetry of snow crystals. But then one must ask how molecular forces, which operate at the molecular scale to produce the crystal lattice, can control the shape of a snow crystal some ten m Facets appear on many growing crystals because some surfaces grow much more slowly than others.  If we imagine beginning with a small round ice crystal, then mostly we would find that the surface was quite rough on a molecular scale, with lots of dangling chemical bonds.  Water molecules from the air can readily attach to these rough surfaces, which thus grow relatively quickly.  The facet planes are special, however, in that they tend to be smoother on a molecular scale, with fewer dangling bonds.  Water molecules cannot so easily attach to these smooth surfaces, and hence the facet surfaces advance more slowly.  After all the rough surfaces have grown out, what remains are the slow-moving facet surfaces.  The picture at right shows the idea for a crystal with four-fold symmetry (which is easier to draw). Faceting in snow crystals produces hexagonal prisms like the ones at left, which are the simplest form of snow crystals. 

Snow Grain Shapes

Ablation: Process of snow melting or removal of snowpack

Snowpack Energy Exchanges

This is why snow melts. The ground is always forcing heat into the snow pack. Solar radiation drives snow melt, not air temperature (thermal). Ice has more optical thickness than snow. Therefore there is more chance for the short wave energy to be absorbed before it can be reflected. Spring snow has a lower albedo due to a larger grain size. If you find snow in the spring in the shade? It’s because the blocking of solar radiation is more important than the longwave radiation being emitted by the trees and ground. Trees absorb solar radiation and re-emit it as longwave energy. To have spring snow melt, the snowpack has to become isothermal. This means that the temperature through out the entire snow pack is more or less equal. In the spring it pushes the zero degree higher into the snowpack. Then solar energy is moving from the top down, with albedo governing the process. These temps meet closer to the top of the snowpack, as cold persists longer in the upper part of the snowpack than the lower part of the snowpack. You can only raise the temperature of snow to zero, then the obvious happens.

Center for Snow and Avalanche Studies Silverton, Colorado

Saddle Ridge Science Fair 1st Place Jenna H. 4th Grade

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