Friday, January 9, 2009

While widely believed that no 2 snowflakes are alike, a researcher caused a frosty flurry when she made the first discovery of a matching set of snow crystals over 2 decades ago. Here we will take a look at the wintery wonderland of Mother Nature all her splendor and glory as she reveals her spectacular artwork with incredible geometry of ephemeral snowflakes, frost and ice.

Snow flakes by Wilson Bentley : “Studies among the Snow Crystals, The Snowflake Man”
from Annual Summary of the “Monthly Weather Review” for 1902. Bentley was a bachelor
farmer whose hobby was photographing snow flakes.

Although it’s extremely unlikely for any 2 macroscopic objects in the universe to bear an identical molecular structure, there are nonetheless no known scientific laws that prevent it. The theory was put under the microscope when Nancy C. Knight, of the National Center for Atmospheric Research in Boulder, Colorado, reported that she discovered matching snow crystals in 1988. The crystals were however not exactly snow flakes in the usual sense, but hollow hexagonal prisms.

But how are snowflakes actually created? The process begins in a saturated cloud with subfreezing temperatures. Snow crystals grow as water vapor is deposited on microscopic particles, and the snow flakes form when the crystals collide and stick together. Crystals can be shaped like stars, columns, needles, plates or lumps.

The exact details of the sticking mechanism remains controversial — possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a ’sticky’ liquid-like layer on the crystal surface.

The shape of the snowflake is determined largely by the temperature and humidity at which it forms. Rarely, triangular snowflakes can form in 3-fold symmetry at a temperature of around 28 °F (−2 °C). The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they’re more visually appealing.

Planar crystals — thin and flat — grow in the air between freezing temperature of 32 °F (0 °C) and 27 °F (−3 °C). Between 27 °F (−3 °C) and 18 °F (−8 °C), the crystals will form needles or hollow columns or prisms — long thin pencil-like shapes. From 18 °F (−8 °C) to −8 °F (−22 °C), the habit goes back to plate-like, often with branched or fern-like features.

The maximum difference in vapor pressure between liquid and ice is at about 5 °F (−15 °C) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −8 °F (−22 °C), the crystal habit again becomes column-like, although many more complex habits also form such as side-planes, bullet-rosettes and planar types depending on the conditions and ice nuclei.

Most samples of snow crystals are observed by researchers at moderate magnifications of 30X to 500X, often using a low temperature scanning electron microscope (LT-SEM).

Snow samples are very fragile and exposure to the light necessary to photograph them using light microscopes can change structures and even melt them. Using LT-SEM, samples are frozen to temperatures below −170 °C where they can be placed in a vacuum and observed for many hours with no structural changes.

Ghost of a melting snowflake. Photo Audreyjm529

The 4 classes of snowflakes include:

• Columns — a class of snow flakes shaped like a 6-sided column.

• Dendrites — the classic snow flake shape that has 6 points, making it somewhat star shaped. A crystal dendrite is a crystal that develops with a typical multi-branching tree-like form. Dendritic crystal growth is very common and illustrated by snowflake formation and frost patterns on a window. Dendritic crystallization forms a natural fractal pattern.

• Needles — a class of snow flakes that are acicular in shape — their length is much longer than their diameter, like a needle.

• Rimed snow — snowflakes that are partially or completely coated in tiny frozen water droplets called rime. Rime forms on a snow flake when it passes through a super-cooled cloud.

Watermelon snow is a reddish-pink colored snow that smells like watermelons, and is caused by a red-colored green algae called Chlamydomonas nivalis.
Magnification of a snow crystal using a low temperature scanning electron microscope.
Photo Agricultural Research Service, USDA
Nature’s Fantastic Frost
When Jack Frost celebrates winter leaving his dazzling crystal patterns on windows on cold winter mornings, it’s a magnificently fleeting sight to behold, captured here for eternity in photographs.
If a solid surface is chilled below the dew point of the surrounding air and the surface itself is colder than freezing, frost will form on the surface. Frost consists of spicules of ice which grow out from the solid surface. The size of the crystals depends on time, temperature, and the amount of water vapor available, and is usually translucent in appearance.

Because cold air is denser than warm air, in calm weather cold air pools at ground level. This is known as surface temperature inversion, and explains why frost is more common and extensive in low-lying areas. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as “frost pockets.”
Photo Clearly Ambiguous

There are many types of frost, such as radiation — also called hoar frost or hoarfrost — and window frost.

Hoar frost refers to the white ice crystals loosely deposited on the ground or exposed objects that form on cold clear nights when radiation losses into the open skies cause objects to become colder than the surrounding air. A related effect is flood frost which occurs when air cooled by ground-level radiation losses travels downhill to form pockets of very cold air in depressions, valleys, and hollows. Hoar frost can form in these areas even when the air temperature a few feet above ground is well above freezing.
Hoar frost on a birch tree by a river. Photo Ptrktn
Located between Calcutt and Stockton, Warwickshire. Photo Dave Hamster
Hoar frost may have different names depending on where it forms:

• Air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, and wires.

• Surface hoar is formed by fernlike ice crystals directly deposited on snow, ice, or already frozen surfaces.

• Crevasse hoar consists in crystals that form in glacial crevasses where water vapor can accumulate under calm weather conditions.

• Depth hoar refers to cup shaped, faceted crystals formed within dry snow beneath the surface.

Depth hoar is a common cause of avalanches when it forms in air spaces within snow, especially below a snow crust, and subsequent layers of snow fall on top of it. The layer of depth hoar consists of angular crystals that do not bond well to each other or other layers of snow, causing upper layers to slide off under the right conditions, especially when upper layers are well bonded within themselves.
Hoar frost that grows on the snow surface due to water vapor moving up through
the snow on cold, clear nights. Photo JossDude

Window frost — also called fern frost — forms when a glass pane is exposed to very cold air on the outside and moderately moist air on the inside. If the pane is not a good insulator, such as a single pane window, water vapor condenses on the glass forming magnificent patterns. The glass surface influences the shape of crystals, so imperfections, scratches or dust can modify the way ice nucleates.

Advection frost — also called wind frost — refers to tiny ice spikes forming when a very cold wind blows over branches of trees, poles, and other surfaces, and can rim the edge of flowers and leaves. It usually forms against the direction of the wind, and can occur at any hour of day and night.

Nature’s Beauty in Ice
Nature paints her land with ice appearing in forms as varied as snowflakes and hail, icicles, glaciers, pack ice, and entire polar ice caps.
Icicles form similar to stalactites in appearance, as water drips and re-freezes. Over time continued water runoff will cause the icicle to grow. If an icicle grows long enough to touch the ground — or its corresponding ice spike growing up from the ground — it’s called an ice column.
Feather ice on a plateau near Alta, Norway. The crystals form at temperatures
below −22 °F (−30 °C). Photo Craig Thom

The subsequent photos are the effects of a storm followed by sub zero temperatures in Versoix, a town near Geneva City, Switzerland, on the Leman Lake.

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