Snow ecology question?

Question

You are studying a deep snow pack and examining the bottom and surface. Discuss how the two depths would differ with respect to temperature, type of crystals, and insulation. Account for the differences you observe. The more detailed the better! Thank you!

Answer 1

Snow forms when water vapor condenses directly into ice crystals, usually in a cloud. Floating cloud particles (ice nucleators, often of biological origin [1]) are needed in order for snowflakes to form at temperatures above -40C. 85% of these nuclei are airborne bacteria, with dust particles making up the rest.[2] The ice crystals which form around the ice nucleators typically have a diameter of several milimetres and usually have six lines of symmetry. A snowflake is an aggregate of such ice crystals and may be several centimeters large[3]. The term "snowflake" is also used below for the symmetrical ice crystals themselves. The individual ice crystals are clear but because of the amount of light the individual crystals reflect snowflakes appear white in color unless contaminated by impurities

Large, well formed snowflakes are relatively flat and have six approximately identical arms, so that the snowflake nearly has the same 6-fold dihedral symmetry as a regular hexagon or hexagram. This symmetry arises from the hexagonal crystal structure of ordinary ice. However, the exact shape of the snowflake is determined by the temperature and humidity at which it forms.[3]. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. [4] Snowflakes are not perfectly symmetrical however. The most common snowflakes are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Snowflakes can come in many different forms, including columns, needles, bricks and plates (with and without "dendrites" - the "arms" of some snowflakes). These different forms arise out of different temperatures and water saturation - among other conditions. Six petaled ice flowers grow in air between 0 °C (32 °F) and −3 °C (27 °F). The vapor droplets solidify around a dust particle. Between temperatures of −1 °C (30 °F) and −3 °C (27 °F), the snowflake will be in the form of a dendrite or a plate or the six petaled ice flower. As temperatures get colder, between −5 °C (23 °F) and −10 °C (14 °F), the crystals will form in needles or hollow columns or prisms. When the temperature becomes even colder from −10 °C (14 °F) to −22 °C (−8 °F) the ice flowers are formed again, and at temperatures below −22 °C (−8 °F), the vapors will turn into prisms again. If a crystal has started forming at around −5 °C (23 °F), and is then exposed to warmer or colder temperatures, a capped column may be formed which consists of a column-like design capped with a dendrite or plate-like design on each end of the column.[3] At even colder temperatures, the snowflake design returns to the more common dendrite and plate. At temperatures approaching −20 °C (−4 °F), sectored plates are formed which appears as a dendrite, with each dendrite appearing flattened, like the design of a snowflake plate.[3]



Snowflakes by Wilson Bentley, 1902

There are, broadly, two possible explanations for the symmetry of snowflakes. First, there could be communication or information transfer between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and other atmospheric conditions. This environment is believed to be relatively spatially homogeneous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes. The sixfold symmetry happens because of the basic hexagonal crystalline structure from which the snowflake grows. The exact reason for the threefold symmetry of triangular snowflakes is still a mystery[citation needed] although trigonal symmetry is a subsymmetry of hexagonal.

There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the National Center for Atmospheric Research.[citation needed] The crystals were not flakes in the usual sense but rather hollow hexagonal prisms