Let It Snow!
The coolest form of precipitation is
snow. Snow falls when water in the atmosphere freezes
and forms crystals that
fall to the ground. Snow accumulation varies from a light dusting to a dumping
of a meter or more. Pretty, flaky, and fun, snow is important to climate.
Many animals and
people depend on it to live. But snow has a dark side. Slippery, snowy roads
make driving dangerous. People die in avalanches and
from cold exposure. Just shoveling heavy snow off a sidewalk can be dangerous
for someone with health problems.
Mountains accumulate a lot of snow. An area of naturally-formed, packed
snow and ice that hasn’t melted yet is called snowpack, and mountains
get the deepest snowpacks.
©G.Ellis/GLOBIO.org Forming a Flake
The lacy arms of this complex snowflake are called dendrites. The snowflake
is almost the same on all sides. It is called a symmetrical flake. Different
environmental conditions such as temperature help form symmetrical or uneven
snowflakes.
©Jasper Nance Water in the atmosphere is in the form of
a gas called
water vapor.
When water vapor is cooled high in the sky, it condenses around
tiny particles of dust into water droplets. These droplets clump together into
a cloud. If it’s cold enough, some of the droplets freeze into ice crystals.
More molecules of water vapor condense and freeze on the surface of the crystal.
Eventually the crystals grow into snowflakes heavy enough to fall from the
cloud.
Different Shapes from Different Conditions
Temperature and the amount of water vapor in the air affect snow crystal formation.
The colder and drier it is, the simpler the shape. With more water vapor and
warmer temperatures, crystals can grow into lacy, frilly flakes. Wind affects
the shape of snowflakes too. If winds are light, the flakes can grow larger
than when they are knocked around by high winds.
Simple to Complex
The snowflake on the right is a simple star crystal. The one on the left
is more complex. Which snowflake do you think formed in warmer temperatures?
©Jasper Nance The simplest crystal shape is a hexagon.
The crystals may be flat like plates or shaped like columns with six surfaces,
like a pencil. From this shape, additional water molecules can
grab on and freeze, branching out to form lacy arms. These crystals are called
dendrites. Dendrites may be shaped like stars or have branches that resemble
ferns. Other crystal shapes include needles, triangles, and bullets. Scientists
can grow snow crystals in a lab. These “designer” snowflakes help
scientists better understand how crystals form in nature.
Made by Machine
If you’ve ever been skiing or snowboarding, you know that snow can also
be made by machines. Snow makers turn air and water into snow so that people
can ski even when the weather doesn’t cooperate. Snow from a snow maker
is not made of crystals at all, but tiny frozen ice pellets. The machine breaks
up a stream of water into droplets and cools them off as it blows them out
onto the slope. Tractors then move and shape the snow to cover the slopes.
Ski and snowboarding resorts around the world have machines that make snow
so people can enjoy playing in the snow whenever they want. Another machine
that makes “snow” is a snow cone or shave ice maker.
©Anders Ljungberg Where on Earth Does It Snow?
Look at a map of the earth and
imagine a belt circling the equator.
In this area, at sea level, snow doesn’t usually fall. However, even
here snow can fall at high elevations where
cold temperatures keep it from melting before reaching the ground.
The water and land at Earth’s North and South Poles are always covered
in snow and ice, but the air is so dry that the snowfall is really quite low.
High winds blow the existing snow around, making it seem like it’s snowing
when it isn’t. The Arctic Circle (North Pole) is mostly ice-covered ocean
but also includes parts of Canada, Iceland, Russia, Alaska, and all of Greenland.
Ice covers land on the continent of Antarctica (South Pole).
Where Snow Never Melts
Glaciers are made of ice and snow and store the largest amount of fresh
water on Earth. The largest glaciers are continental ice sheets and Antarctica
is one of the only places where they exist.
©G.Ellis/GLOBIO.org In spring and summer, snow melts at lower elevations and outside the polar
regions. But snow never melts in some places. It gets packed into thick layers
of ice called glaciers.
You can think of a glacier as a very slow-moving river of ice. Antarctica has
the most glaciers, but they are also found in the Arctic Circle, and at high
elevations such as the South American Andes, the European Alps, and Tanzania’s
Mt. Kilimanjaro.
A Flurry of Snow Terms
Small hail collecting on the ground can sometimes be confused with snow.
Hail can be much larger and cause damage to property. When it’s large,
it’s best to run for cover!
©Michael Henderson Some types of precipitation seem like snow,
but really aren’t. Sleet is the term for raindrops that freeze before
they hit the ground. So what’s freezing rain? That’s rain that
reaches the ground in drops, THEN freezes on contact. Some scientists also
define sleet as snow crystals that have melted and refrozen on their way to
the ground. In either case, sleet is made of ice pellets, not crystals, and
is not a form of snow. Hail is also not a form of snow. Hail is made up of
balls of ice that have several layers. A frozen raindrop can swirl around in
a storm cloud and gather more water that clings to it and freezes. When the
raindrop or hail hits the ground, it can be as small as a pea or as large as
a plum. Hail falls during severe thunderstorms.
Blizzards and Avalanches
You’ve probably heard of a blizzard—a
windy snow storm that lasts a long time and dumps a lot of snow. But have you
heard of a ground blizzard? That’s when high winds lift already fallen
snow from the ground and swirl it around. Both types of blizzards can create
dangerous whiteout conditions, where the snow is so heavy you can’t see
very far.
There are several kinds of snow avalanche and different things can trigger
the slide. The photo above shows the largest kind, a powder avalanche. A
falling piece of rock or ice may have begun the loose snow’s destructive
release.
©C.Monteath/Hedgehog House/Minden Pictures When huge amounts of snow slide off the slope of a mountain, it’s called
an avalanche. Avalanches occur when the top layer of snow gets too heavy for
the snow at the bottom to “hang on” any longer. The snow breaks
free and falls, covering anything in its path. Vibrations from earthquakes
or weapons fire can also trigger an avalanche. A huge avalanche caused by an
earthquake in China destroyed
a whole town in 1970, killing 18,000 people. Avalanches kill 150 people every
year, and injure many more.
Snow Birds
Many animals are well adapted to live in
snow. One adaptation is
that they change color to better blend in to their snowy surroundings. Foxes,
hares, and birds such
as grouse and ptarmigans all grow white fur or feathers for
winter—even on their feet. The wider furry or feathery feet act like
snowshoes, keeping animals on top of the snow instead of sinking.
In the winter, the Arctic fox’s fur changes from gray to white. The
coat’s color change helps the fox stay hidden when hunting. The prey
won’t see the fox until it’s too late.
©K.Campbell/GLOBIO.org Snug in the Snow
Many animals rely on snow as shelter. Mice, hares, grouse, and ptarmigans
make snow burrows in which they can rest and be protected from wind. Snow is
a good insulator.
It traps warmth from the ground which keeps it and anything else under the
snow blanket several degrees warmer than it is on top of the snow. The layer
of air and snow pack just above the ground in which animals tunnel and burrow
is called pukak.
Plants benefit
from the insulating ability of snow too. During winter plants go dormant.
But even if a plant looks dead above the ground, its roots are protected under
the soil and snow. When the snow melts and the plants can get enough sunshine,
they “wake up” and begin growing. Farmers may use manmade snow
to protect crops from freezing.
Winter Fun and Water Source
Despite snow’s dangers, it has many
benefits. For one thing, snow can be a lot of fun. You know this if you’ve
ever thrown a snowball, built a snowman or snow fort, or made snow angels.
Sports such as skiing, snowboarding, sledding, and snowmobiling wouldn’t
exist without snow, and resorts with snowy slopes and trails are big business.
In some parts of the world, kids like to play in the snow when the temperature
isn’t too cold. Some animals enjoy playing in the snow too.
©Bruce Dennis Snow is also very important as a source of water for many people. When snow
melts each spring, the water that runs off fills streams and rivers that provide
drinking water. In some places, snowmelt accounts for three-quarters of the
water in streams. This is also why shrinking glaciers are a concern. Glaciers
store most of the world’s fresh water in the form of ice. Global
warming is causing many glaciers to melt, which may lead to rises in sea
level worldwide. At the same time, rivers the glaciers feed may recede or dry
up.
They Know Their Snow
Scientists study how seasonal snowpack affects plants and animals. Getting
samples during this time of year can be a lot of cold work!
©Courtney Meier We already mentioned scientists who study
snowflakes and their delicate structure. Weather scientists or meteorologists study
snow and weather patterns to try to predict snowfalls. Other scientists study
snow to learn about climate, especially climate
change. Because snow and ice are very sensitive to temperature, they are
indicators of change. Scientists measure snow depth and use images from satellites
to track the size of glaciers. Aside from the water snow provides, it is also
important in keeping the earth cool. It does this by reflecting sunlight. Snow
and ice reflect up to 90 percent of the Sun’s energy. Without that reflection,
the energy would get absorbed by the ground, making it even warmer. Studying
snow is serious business, but snow scientists probably still like a good snowball
fight!