ORIGINALLY POSTED 7 DECEMBER 2000
Sno'
kiddin'?
Snow and ice are both precipitation -- processes that remove water from
clouds. Clouds, regions of the atmosphere with high relative humidity,
are made of droplets of water and/or bits of ice. Even though water is
much denser than air, these droplets and ice crystals are small enough
to be suspended by random upward air motion.
When these droplets or crystals join together, gravity overcomes the suspending force and we have precipitation.
Precipitation occurs in four overlapping categories:
Rain is liquid water (in middle and cold latitudes, almost all rain originates as ice in clouds).
*The
snowflake images on this page (unless otherwise noted) were made from
photos taken by Wilson "Snowflake" Bentley, a Vermont farmer-scientist
who initiated the study of snowflakes by shooting fresh flakes. Scientists
now capture snow flakes on a liquid polymer that creeps into a 3-D mold
around the flake.
Snow job
Water freezes when it drops below the freezing point and loses
an amount of energy called the heat of crystallization. When a large amount
of water freezes, we get hunks of ice.
Things
grow more interesting when water vapor freezes in the atmosphere.
Small water droplets are highly reluctant to form into larger droplets (maybe they feel perfect just as they are...). Even when the temperature is below freezing, the laws of thermodynamics, which govern the movement of energy in matter, govern the crystallization process.
In air, pure water will not freeze until far below 0 degrees C. Crystallization occurs much more easily around a particle of dust, or even a bacterium. Similarly, water vapor joins together much more easily around a tiny nucleus. Cloud seeders try to cause rain by inserting these nuclei into clouds, but their effectiveness is debatable.
At any rate, once the congregation process starts, it cascades. Just as water coalesces much faster on existing droplets, ice crystals in clouds readily gather onto existing ice crystals.
To capture this exquisite flake up-close, snow
samples are frozen to temperatures 170 degrees below zero, then placed
in a vacuum so they can be observed for many hours with no structural
changes. These photographs show the extraordinary symmetry of snow crystals
even at high magnification. Photo: USDA
Three processes cause the enlargement of ice crystals into snowflakes:
Snow joke
From our description of the four precipitation categories, you may notice
that the formation of snow crystals is just the start — the crystals
may be changed or destroyed before they blanket a ski slope — or
your driveway.
These photos were taken in 1902 by Wilson Bently,
known as "The Snowflake Man." Bentley, a bachelor farmer made a hobby
of photographing snowflakes, captured more than 5000 flakes on camera.
Photo: NOAA
Snow more
Crystals are molecular structures with a specific bond geometry. Because
water molecules only bond at 120 degrees, ice crystals and snow flakes
are all based on hexagons, which have 120-degree intersections.
Both temperature and humidity in clouds govern the growth of snowflakes:
Environmental Temperature °C °F Crystal Form 0 to -4 32 to 25 Thin plates -4 to -10 25 to 14 Columns -10 to -12 14 to 10 Plates -12 to -16 10 to 3 Dendrites -16 to -22 3 to -8 Plates -22 to -50 -8 to -58 Hollow columns
Because temperature and humidity vary from place to place, snowflakes take innumerable crystal shapes. And since both conditions change as a flake bounces around in a cloud, the basic shapes can blend, as seen in the plate-dendritic flake shown here.
Snow myth
We're getting to a pressing question: Is there any truth to the myth every
one of the countless trillions of snowflakes has a unique shape? We asked
Pao-Kuan
Wang, an atmospheric physicist who studies flakes and ice in clouds at
the University of Wisconsin-Madison, and he told us it depends on how
hard you look. "With a microscope, going down to the molecular level,
of course they're all different," he says. "But without a microscope,
at the superficial level, they may look alike."
Pao-Kuan Wang prepares to put a flake under the scope.
He suspects that the myth about snowflakes reflects "a kind of appreciation for the complicated design — and because they're so beautiful."
Ice in the atmosphere can change the climate.
NOTE: This page adapted from our avalanche feature.
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive