Jerry Dennis's work can be found in other magazines including the Smithsonian and Outdoor Life. He is the author of A Place on the Water as well. Glenn Wolff's illustrations appear regularly in the New York Times and many other publications.This selection appeared in their 1993 book, It's Raining Frogs and Fishes, which offers answers to the questions countless children (and adults) ask of the world around them. The pair recently released the follow-up book, The Bird in the Waterfall: A Natural History of Oceans, Rivers and Lakes.
What child can resist tasting snowflakes? Head back, mouth open, tounge
stretched to catch that bit of coldness--for children, snowflakes on the
tongue are as essential to a northern winter as sleds and snowmen. We take
it for granted that snow will fall, that every flake, its constituent crystals
arranged in elaborate symmetry, is a wonderous and unique creation. But
is it? And how are they formed? What chemistry is at work in those dense,
dark clouds of winter?
There is more to the birth of a snowflake than Aristotle's assertation that
"when a cloud freezes there is snow." Snow is not merely frozen
rain. Rain occasionally freezes, falling to the ground as sleet or freezing
rain, but snow originates independent of atomospheric drops of water. Individual
ice crystals for high in the atmosphere when water vapor freezes around
dust or other particulates. Without particles to serve as condensation nuclei,
water vapor can be cooled to -40 degrees Farenheit before freezing occurs.
A supercooled cloud of this sort seeded with a few particles often escalates
into a snowstorm. The individual crystals collect additional molecules of
water vapor one at a time, building on one another symmetrically in a rapidly
growing, widening circle. Temperature, wind, humidity, and even barometric
pressure will determine the growth and ultimate form of the crystal. Large
and elaborate crystals for at higher temperatures and humidity while, while
the small, basic crystals such as thos common in polar regions form when
temperature and humidity are very low. As the crystals fall they bump against
each other, breaking off pieces of ice that in turn serve as nuclei for
new crystals. As they pass through warmer layers of air they adhere to one
another, congregating into snowflakes that may contain a thousand or more
crystals.
Snowflakes, then, are aggregates of snow crystals. When the temperature
is near or slightly above freezing, snowflakes become wet, adhere to other
flakes, and grow to two or three inches in diameter. On very rare occasions,
they can grow larger yet. According to a report in a 1915 issue of Monthly
Weather Review, a snowfall on January 28, 1887 dropped flakes "larger
than milk pans," measuring fifteen inches in diameter by eight inches
thick across several square miles near Fort Keogh, Montana.
Only when the temperature remains consistently below freezing will complete,
individual crystals fall to the ground. If the temperature of the cloud
they form in and the air they descend through is warmer than 27 degrees
Farenheit, the crystals tend to be flat and hexagonal. Between 27 and 23
degrees, they tend to be needle-shaped. Between 23 and 18 they are likely
to be hollow and columnar, with prismatic sides. At temperatures below 18
they can be columnar, hexagonal, or fernlike. Virtually all have six sides.
That hexagonal tendency is something of a mystery, although some scientists
suggest it is produced by electrical charges in the crystals, while others
say it is basic to the molecular structure of water molecules. The atoms
in a molecule of H20 are arranged, in physicist Hans C. von Baeyer's graphic
description, "with two little hydrogens stuck onto a big oxygen like
ears on a Mickey Mouse's head." Scientists like von Baeyer believe
that the angle at which the hydrogen molecules protrude from the oxygen
atom--about 120 degrees--causes snow crystals to grow to a six-pointed symmetry
that repeats the molecular structure of water.
The scientifically untestable notion that no two snow crystals are alike
is probably true. There are too many variables involved: Did the crystal
for around a nucleus of volcanic ash, or a bit of sea salt, or a fleck of
industrial waste? At what altitude did it form? What temperature was the
air it passed through? How much humidity did it contain? For two crystals
to be alike, they would have to form in exactly the same conditions, collect
the same number of molecules of water vapor in the same order and bump into
the same number of other crystals during their long descent to the ground.
No two snow crystals may be identical, but general categories or types have
been identified for years. Beginning in the 1880s, a Vermont farmer and
amateur photographer named Wilson Bentley began examining ice crystals and
photographing them under magnification. Armed with apparently unlimited
patience, a microscope, and a box camera, Bentley produced over 6,000 photographs,
2,000 of which appeared in his 1931 book Snow Crystals, and identified hundreds
of types of crystals.
Serious study of snow crystals was performed in 1910 by a Russian meteorologist
who identified 246 types in 176 days of observation. In the 1930s Japanese
meteorologist Ukichiro Nakaya consolidated the list to seventy-nine categories
of crystals plus anomalies and oddballs he called "mavericks."
In 1951 the International Commission on Snow and Ice simplified things immensely
by devising a classification system recognizing seven basic forms of snow
crystals: plate crystal, stellar crystal, column, needle, spatial dendrite,
capped column, and irregular crystals.
Stellar crystals (or simply "stars") are the classic, most
familiar form of snow crystals, and the basis of the "no two alike"
myth. They are not as common as aggregate flakes, irregular crystals, or
asymmetrical crystals but we are familiar with them because of countless
artists' renditions.
In addition to the basic crystals, snow can form into ice pellets when it
is buffeted by strong winds that break off the points of the crystals and
pack them into tiny balls. Graupel is formed by crystals falling
through layers of supercooled droplets of water vapor which remain liquid
as long as they are suspended in the air, but freeze the instant they come
in contact with anything solid and coat it in a dense covering of rime frost.
In places where snow is a frequent companion, it is sometimes personified
as a living thing. In Japanese folktales it is Yuki-onne, the Snow Woman,
who appears before men wandering in snowstorms and lures them to sleep and
death. In Nordic mythology snow is the Old Man, an eged king of Finland
named Snaer, whose daughters are Thick Snow, Thin Snow, and Snowstorm. To
the Inuits of the far north snow appears in so many forms and shapes it
requires an advanced vocabulary to describe it. To them, api is snow
not yet touched by wind; upsik is snow changed by wind into a firm mass;
siqoq is smoky snow blowing along the surface of the ground; annui
is falling snow; quali is snow that sticks to the branches of trees;
saluma roaq is a snow surface of very smooth and fine particles;
natatgonaq is a snow surface of rough and coarse particles; and det-thlok
is snow so deep snowshoes are required to walk in it. Dozens of variations--as
many as 200, by some accounts--make it possible for Inuits and Eskimos to
speak more precisely about snow than anyone on earth.
The winter vocabulary of the English language is growing. We have adopted
the Russian word sastrugi to describe windblown drifts, common in
the Artic and Antarctica, that look like waves of water. Cross-country and
alpine skiers have adapted a litany of descriptive slang expressions to
identify the conditions they encounter on their skis. Among them are such
colorful terms as windslab, glop, fluff, neve, breakable crust, crud,
sugar, corn, boilerplate, and cement.
Explorers in Antarctica found to their dismay that in extremely cold
temperatures (-50 is not uncommon) snow can become unskiable. At those temperatures,
the tiny ice crystals that fall almost continuously, even from clear skies,
create a dry, harsh surface more like sand than snow. Skis and sled runners,
instead of melting the points of the crystals to make them slippery, merely
roll the crystals over and over.
Snow changes continuously as it falls and after it has landed. Once on the
ground, snowflakes trap tiny air pockets and form an excellent natural insulation.
Temperatures on the surface can be more than 50 degrees colder that temperatures
beneath seven inches of loose, fresh snow. As the snow settles is metamorphoses.
"Old snow" is settled and dense, resulting from the altering of
loose, pointed crystals into small, round grains. Later it becomes firn,
with spaces between the grains shrinking, resulting in compacting and hardening
of snow. If metamorphosis continues long enough, firn can become glacial
ice.
During any ordinary snowfall in New England or North Dakota or British Columbia
or Siberia or Finland, about 1 million crystals of snow fall to cover each
two-foot square area with ten inches of snow. Snow covers about half the
land on the earth's surface, at least for part of each year, as well as
about ten percent of the oceans. About 48 million square miles of the earth
are covered year round with snow or ice.
The greatest snowfall in a twenty-four-hour period recorded in North America
occurred on April 14 and 15, 1921, when seventy-six inches of snow fell
on Silver Lake, Colorado. More recently, on April 5 and 6, 1969, Bessans,
France, was buried beneath sixty-eight inches of snow in nineteen hours.
During a snowstorm from February 13 to 19, 1959, 189 inches fell on the
Mount Shasta Ski Bowl in northern California. The snowiest place on record
in North America is Ranier Paradise Ranger Station in Washington, where
in the winter of 1971-72 a total of 1,122 inches of snow fell. The greatest
depth of snow ever measured on the ground at one time in North America was
451 inches--over 37 1/2 feet--at Tamarack, California on March 11, 1911.
Oddly enough, the interior of Antarctica recieves very little snow. Most
of the precipitation at that coldest spot on earth falls in the form of
ice crystals, with an annual precipitation equal to less than two inches
of water--only slightly more than falls on the Sahara Desert each year.
The vast ice cap at the center of the continent grows, but only slowly,
over millions of years.
"As pure as the snow," may not be the purest of metaphors. Snow,
it seems, contains much more than just frozen moisture and air. In fact
it contains enough nitrates, calcium, sulphate, and potassium picked up
from dust and atmospheric gases to make it an important source of agricultural
nutrients in many parts of the world. It also contains less savory traces
of industrial pollution. When snow crystals form in air contaminated with
sulphur dioxide the result is acid snow, which accumulates on the ground
in winter and releases highly acidic meltwater into rivers and lakes in
the spring.
The best time to catch and observe snow crystals is when the temperature
is moderately cold (about 25 degrees Farenheit is ideal), with no wind to
throw the flakes against each other and break their points. The crystals
fall individually, or more often, sticking together in loose clusters that
fall apart into separate crystals when they land. Wear a dark jacket or
carry a piece of dark-colored fabric stretched over cardboard, and after
it has been acclimated to the outside temperature, it will preserve even
the finest, most delicate crystals until you have had time to examine them.
Most crystals are an eighth of an inch or less in diameter, much smaller
than we are led to expect from the representation on Christmas card. Mixed
with those eighth inchers are occasional midgets hardly larger than the
dot on this letter i. Occasionally comes a behemoth, perfectly symmetrical
and ornate as baroque jewelry, measuring as big around as a pencil eraser.
Those giants spiral slowly downward, their flat surfaces horizontal, and
are especially satisfying to catch on sleeve or tongue. |
