눈의 결정체 4.

A Guide to Snowflakes    ... A look at the different types of falling snow ...    If you look closely at falling snow, you can see a great many different crystal shapes.  There's a lot more to see than you might think!      The table at right shows the more common and/or distinctive types of snowflakes.  Click on the table for a more detailed look, then scroll down this page for examples of the different types.       This page is an abbreviated version of my Field Guide to Snowflakes.

Types of Snowflakes

Simple Prisms    A hexagonal prism is the most basic snow crystal geometry (see the Snowflake Primer).  Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns (shaped a lot like wooden pencils), or anything in between.  Simple prisms are usually so small they can barely be seen with the naked eye.

The examples at right show two stubby prisms and one thin plate.  Snow crystal facets are rarely perfectly flat, being more typically decorated with various indents, ridges, or other features.

Stellar Plates    These common snowflakes are thin, plate-like crystals with six broad arms that form a star-like shape.  Their faces are often decorated with amazingly elaborate and symmetrical markings.

Plate-like snowflakes form when the temperature is near -2 C (28 F) or near -15 C (5 F), as dictated by the snow crystal morphology diagram.

Sectored Plates    Stellar plates often show distinctive ridges that point to the corners between adjacent prism facets.  When these ridges are especially prominent, the crystals are called sectored plates.

The simplest sectored plates are hexagonal crystals that are divided into six equal pieces, like the slices of a hexagonal pie.  More complex specimens show prominent ridges on broad, flat branches.

Stellar Dendrites    Dendritic means "tree-like", so stellar dendrites are plate-like snow crystals that have branches and sidebranches.  These are fairly large crystals, typically 2-4 mm in diameter, that are easily seen with the naked eye.

Stellar dendrites are clearly the most popular snow crystal type, seen in holiday decorations everywhere.  You can see these crystals for yourself quite well with just a simple magnifier.  (See Snowflake Watching for more about observing snowflakes.)

Fernlike Stellar Dendrites    Sometimes the branches of stellar crystals have so many sidebranches they look a bit like ferns, so we call them fernlike stellar dendrites.  These are the largest snow crystals, often falling to earth with diameters of 5 mm or more.  In spite of their large size, these are single crystals of ice -- the water molecules are lined up from one end to the other.

Some snowfalls contain almost nothing but stellar dendrites and fernlike stellar dendrites.  It can make quite a sight when they collect in vast numbers, covering everything in sight. The best powder snow, where you sink to your knees while skiing, is made of stellar dendrites.  These crystals can be extremely thin and light, so they make a low density snowpack.

Hollow Columns    Hexagonal columns often form with conical hollow regions in their ends, and such forms are called hollow columns.  These crystals are small, so you need a good magnifier to see the hollow regions.

Note how the two hollow regions are symmetrical in each column.  Sometimes the ends grow over and enclose a pair of bubbles in the ice, as seen in the last picture on the right.

Needles    Needles are slender, columnar ice crystals that grow when the temperature is around -5 C (23 F).  on your sleeve these snowflakes look like small bits of white hair.

One of the amazing things about snow crystals is that their growth changes from thin, flat plates to long, slender needles when the temperature changes by just a few degrees.  Why this happens remains something of a scientific mystery.

Capped Columns    These crystals first grow into stubby columns, and then they blow into a region of the clouds where the growth becomes plate-like.  The result is two thin, plate-like crystals growing on the ends of an ice column.  Capped columns don't appear in every snowfall, but you can find them if you look for them.

The first example at right shows three views of a capped column.  The first view is from the side, showing the central column and the two plates edge-on.  The other two views show the same crystal from one end, with the microscope focused separately on the two plates.

Double Plates    A double plate is basically a capped column with an especially short central column.  The plates are so close together that inevitably one grows out faster and shields the other from its source of water vapor.  The result is one large plate connected to a much smaller one.  These crystals are common -- many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.

The first picture at right shows a double plate from the side.  The second picture shows a double plate with the microscope focused on the smaller plate.  In the third picture, note the slightly out-of-focus hexagon that is about one-sixth as large as the main crystal.  This hexagon is the second side of a double plate, connected to the main plate by a small axle.

Split Plates and Stars    These are forms of double plates, except that part of one plate grows large along with part of the other plate.  The picture at right shows all eight ways to make a split star.  Split plates and stars, like double plates, are common but often unnoticed.

You may have to stare at these pictures a bit to see how the two distinct pieces fit together.  Note how in each case the crystals are connected in the center with short axles.

Triangular Crystals    Plates sometimes grow as truncated triangles when the temperature is near -2 C (28 F).  If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal.  These crystals are relatively rare.

Surprisingly, no one knows why snow crystals grow into these three-fold symmetrical shapes.  (Note however that the molecular structure of triangular crystals is no different from ordinary six-sided crystals.  The facet angles are all the same.)

12-Sided Snowflakes    Sometimes capped columns form with a twist, a 30-degree twist to be specific.  The two end-plates are both six-branched crystals, but one is rotated 30 degrees relative to the other.  This is a form of crystal twinning, in which two crystals grow joined in a specific orientation.

These crystals are quite rare, but sometimes a snowfall will bring quite a few.  The picture at the far right shows a 12-sider where the two halves are widely separated.

Bullet Rosettes    The nucleation of an ice grain sometimes yields multiple crystals all growing together at random orientations.  When the different pieces grow into columns, the result is called a bullet rosette.  These polycrystals often break up to leave isolated bullet-shaped crystals.

Sometimes a bullet rosette can become a capped rosette, as shown in the example at the far right.

Radiating Dendrites    When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite (also called a spatial dendrite).

The first example on the right shows radiating plates.  The second example shows a fernlike stellar dendrite with two errant branches growing up out of the main plane of the crystal.

Rimed Crystals    Clouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals.  The frozen droplets are called rime.  All the different types of snow crystals can be found decorated with rime.  When the coverage is especially heavy, so that the assembly looks like a tiny snowball, the result is called graupel.

The first two pictures at right have relatively light rime coverage.  The final example is completely covered with rime, but you can still see the six-fold symmetry of the underlying stellar crystal.

Irregular Crystals    The most common snow crystals by far are the irregular crystals.  These are small, usually clumped together, and show little of the symmetry seen in stellar or columnar crystals.

Artificial Snow    Snow machines shoot a mixture of water and compressed air out of nozzles.  The water comes out as fine droplets, and the air cools as it decompresses, causing the droplets to freeze.  A fan blows the ice particles onto the slopes.    You can see from the picture at right that artificial snow is made of frozen water droplets, with none of the elaborate structure found in real snow crystals.

If you want to go outside and look at snow crystals for yourself, I recommend my new book -- Ken Libbrecht's Field Guide to Snowflakes.  This book contains a much more complete list of the different snow crystal types, along with how to find them.  once you know what to look for, snowflake watching is a fascinating recreation!

Classifying Snowflakes

How does one classify snowflakes?  It's not so easy, because how you divide the different types is somewhat a matter of taste.  There is a good analogy with breeds of dogs.  The definition of different breeds is decided upon by a committee of people, and really one can make up as many breeds as one wants.  And no matter how many different breeds you define, some dogs will be mixed, not belonging to any one breed.     Snowflakes do come in different types, and you need to give them names if you want to talk about them.  But there will never be a precise way to define the different types.  I prefer the 35 types shown at the top of this page, but others have come up with alternate classifications schemes.  Some of these are shown below.

International Classification System    In 1951 the International Commission on Snow and Ice produced a fairly simple and widely used classification system for solid precipitation [1].   This system defines the seven principal snow crystal types as plates, stellar crystals, columns, needles, spatial dendrites, capped columns, and irregular forms.   To these are added three additional types of frozen precipitation: graupel, ice pellets, and hail.  This is a fine classification system, but in my opinion it's too simplistic to be very interesting.

Nakaya's Classification    Physicist Ukichiro Nakaya (see Historic Snowflakes) created the first systematic classification scheme for snowflakes, in which he subdivided falling snow into 41 individual morphological types [2].

Magono and Lee    The most complex classification scheme is an extension of Nakaya's table, published by meteorologists C. Magono and C. W. Lee in 1966 [3].  This table includes 80 different snow crystal types.

[1] B. Mason, in The Physics of Clouds   (Oxford University Press, 1971) [2] U. Nakaya, Snow Crystals: Natural and Artificial (Harvard University Press, 1954). [3] C. Magono and C. W. Lee, Meteorological Classification of Natural Snow Crystals, Journal of the Faculty of Science, Hokkaido University, 1966.

출처 : http://www.its.caltech.edu/~atomic/snowcrystals