Snow
A fresh snowfall in Colorado's (USA) high forests.
Snow is precipitation in the form of crystalline water ice,
consisting of a multitude of snowflakes. Since it is composed of small rough
particles it is a granular material. It has an open and therefore soft
structure, unless packed by external pressure.
Snow is commonly formed when water vapor undergoes deposition high in the
atmosphere at a temperature of less that 0°C (32°F), and then falls to
the ground. Snow can be also manufactured using snow cannons, which actually
create tiny granules more like sleet. This is sometimes called "grits" by
those in the southern U.S. for its likeness to the texture of the food.)
Types
Very light snow falling is called flurries or just a flurry.
Tiny icy granules of snow are called sleet.
A snow squall is a brief, very intense snowstorm while a blizzard
is long-lasting snow storm with intense snowfall and usually high winds.
Either storm can create whiteout conditions where visibility is
reduced to zero while blizzards can also create large snowdrifts. A
ground blizzard is a wind storm which drives already fallen snow to
create drifts and whiteouts.
Occurrence
Snowfall varies by time and location, including geographic latitude, elevation
and other factors which affect weather in general. In latitudes closer to the
equator, there is less chance of snow fall, 35° is often quoted as a rough
delimiter. The western coasts of the major continents remain snowless to much
higher latitudes.
Some mountains, even at or near the equator, have permanent snow cover on
their top, including Mount Kilimanjaro in Tanzania. Conversely, many regions
of the Arctic and Antarctic receive very little precipitation and therefore
little snow despite the bitter cold (below a certain temperature, air
essentially loses its ability to carry water vapor).
It is generally accepted that an inch of rain is equivalent to ten inches of
snow. However, this is only a rough guide, since the density of fresh snow
varies widely.
Substantial snowfall sometimes disrupts infrastructure and services even in
regions that are accustomed to them. Traffic may be snarled or even completely
stop. Basic infrastructure such as [[electricity]], phones and gas supply can
be shut down. A snow day is a day on which school or other
services are cancelled owing to unusually heavy snowfall. In areas that
normally have very little snow, this may occur even with light accumulation
— something often made fun of by those people used to colder climates,
where streets would remain passable given the same amount of snow.
The highest seasonally cumulative precipitation of snow ever measured in the
world was on Mount Baker, Washington, U.S.A during 1998-1999 season when they
received 28.96 meters or 1,140 inches; this surpassed the previous record
holder, Mount Rainier, Washington, U.S.A which during 1971-1972 season
received 1,122 inches (28.5 m) of snow; and the world record daily
precipitation was recorded in Silver Lake, Colorado, U.S.A in 1921 (1.93
metres, 76 inches).
Recreation
Forms of recreation dependent on snow:
-
many winter sports, such as
skiing, snowmobiling, snowshoeing and
snowboarding
- playing with a sled or riding in a sleigh
- building a snowman or snow fort
-
throwing snowballs mutually in a snowball fight or at others to tease them.
(Humans seems to be the only animal that throw their snowballs. Pygmy
chimpanzees have been seen carrying snowballs around, but never to throw
them.)
Where snow is scarce but the temperature is low enough, snow cannons may be
used to produce an adequate amount for such sports.
Tightly packed snow may be used as a construction material in, for example,
Inuit snow houses.
Geometry
Snow flakes by Wilson Bentley, 1902.
An interesting question is why the arms of snowflakes are symmetrical, and
why no two snowflakes appear to be identical. The answer is believed to be due
to the fact that the distances between snowflakes are much greater than the
distances across snowflakes.
The symmetry of snowflake arms is always six-fold, which arises from the
hexagonal crystal structure of ordinary ice (known as ice Ih) along
its 'basal' plane.
There are broadly, two possible explanations for the symmetry of snowflakes.
Firstly, there could be communication (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 so on. This environment is
believed to be relatively spatially homogenous 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.
However, the concept that no two snowflakes are alike is incorrect: it is
entirely possible, but unlikely, that a pair of snowflakes may be visually
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 by Nancy
Knight of the National Center for Atmospheric Research. The crystals were not
flakes in the usual sense but rather hollow hexagonal prisms.
The "Japanese Tsuzumi" is an unusual variation of snow crystal, getting its
name from the hourglass-shaped tsuzumi drum.
Related topics
External links
This article is licensed under the GNU Free Documentation
License. It uses material from the Wikipedia article "Snow".
