The flakiest snow
How do snowflakes know how to be symmetrical? Are there any circumstances in
which they can be persuaded to be asymmetrical?
Answer: The snowflakes that you can see falling slowly from the sky are loose
collections of tiny snow crystals. These snow crystals are indeed hexagonally
symmetrical and can develop the elegant, feathery star shapes that are so
beloved of Christmas card designers. However, snowflakes are not themselves
symmetrical. Unfortunately, confusion often arises because snow crystals are
also commonly referred to as snowflakes.
Snow crystals are hexagonally symmetrical because their constituent water
molecules initially join up in a set of interlocking rings of six molecules as
the water freezes (see Diagram, bottom left).
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A snow crystal will contain more than a million trillion water molecules, but
because the pattern from which the crystal is assembled is hexagonally
symmetrical, the final structure will be too.
Within that hexagonal symmetry, snow crystals can take a huge variety of
forms. The starting point is a simple six-sided prism, like that shown
(see Diagram, bottom right).
But depending on the exact conditions it encounters in
the atmosphere, that prism can grow in different ways.FIG-mg22927501.JPG
If water molecules from the air attach themselves largely to the six prism
faces, then the snow crystal will grow into a hexagonal plate (top centre). But
if they attach more to the top and bottom basal faces, then a needle or
column-shaped crystal can result (top right). The temperature and humidity
conditions that favour different types of crystal growth are well known,
although we still don鈥檛 fully understand the physical processes involved.
At certain temperatures and when the air is supersaturated with water, the
growth of the crystal is determined by how fast water vapour can diffuse though
the air and stick to the crystal surface. The growth pattern then becomes
unstable. Parts of the crystal that stick out further into the surrounding air
will be able to mop up more water vapour and grow faster. The corners of a
hexagonal prism will begin to grow the six characteristic dendritic arms of a
Christmas card 鈥渟nowflake鈥� (top left).
Protuberances on these arms will themselves grow faster, and so on. The most
complex snow crystals are formed when a developing crystal experiences different
conditions as it passes through the atmosphere, each favouring a different type
of crystal growth.
Because of the sensitivity to growth conditions in the atmosphere, it is
generally true that no two snowflakes are exactly the same. Exceptions to this
occur when atmospheric conditions are stable and favour only a simple type of
snow crystal. In Antarctica most snow falls as simple hexagonal prisms.
Colin Bover
Derby
Answer: Snowflakes are formed by atmospheric water vapour freezing onto a
central core. Because of the chaotic nature of weather, the chances are
extremely small of the same conditions occurring for two snowflakes, so no two
are exactly the same.
However, because of the small size of the snowflakes, the variations in
temperature and air current are not large enough to vary across the diameter of
the snowflake, so the shape forms symmetrically.
To make an asymmetric snowflake, you would have to engineer an environment in
which 鈥渁tmospheric鈥� variations were on a scale considerably smaller than the
size of the snowflake.
Saajan Singh Chana
St Albans, Hertfordshire
Answer: Although snow crystals are approximately symmetrical, it has long
been known that they are never perfectly symmetrical down to the very last
detail.
The first person to photograph snow crystals was Wilson Alwyn Bentley
(1865-1931), a native of Vermont. He started studying snow crystals in his teens
and took the first ever photomicrographs in 1885. During his life he took more
than 6000, some of which have become classics. Studies of the pictures show that
absolute, perfect symmetry is hard to find.
Alan Richardson
Watford, Hertfordshire
An excellent account of the physics of snow crystal can be found at
www.its.caltech.edu/~atomic/snowcrystals鈥抬诲
This week鈥檚 question
Sticky solution: I have a friend who uses molasses to clean rust off old iron
items collected from the bush. The rusty iron article is placed in a jar of
molasses solution (nine parts water, one part molasses) and left for two weeks.
After this time, the article comes out clean and almost shiny.
What is happening here?
Steven Onley
By e-mail, no address supplied