Snow Science
From the largest bizzard to the
smallest snowflake, the
formation of snow touches on
nearly every area of science.
Some scientists study the big
picture: weather, climate, and
the water cycle. Some study the
tiniest of details: how and why
do water molecules form such
complex snowflakes? From
either persepctive, snow science
can be fascinating.
Snow and Sun
Like all precipitation, snow is
What is the science behind this winter scene?
part of a continuous cycle of
evaporation, condensation, and
precipitation known as the water cycle, or hydrologic cycle. Even
though we associate snow with cold, freezing conditions, it is
energy from the hot Sun that causes the water cycle. The Sun
warms the water on Earth’s surface. As the water warms, it
changes from liquid to a gas, and enters the atmosphere as
water vapor. High in the atmosphere, the water vapor condenses
and eventually falls back to Earth’s surface as rain, sleet, hail, or
snow—all forms of precipitation. Once it
falls to the Earth, the precipitation can
melt, flow, and evaporate, beginning the
process all over again.
Snow does not fall everywhere on Earth,
of course. It needs the right conditions
to form—conditions found in areas that
have low winter temperatures but some
moisture in the air. The level of
moisture in the air is called humidity.
Snow forms in places and at times when
humidity is high or moderate, but
temperatures are low. These conditions
are, once again, related to sunlight.
Because of Earth’s tilt, some parts of
Earth receive more consistent levels of
the Sun’s energy, because they get the
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Not all places on Earth have the right conditions for
snow. This tropical rainforest is near the equator, so it
has warm temperatures all year long.
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Snow Science
most direct light year-round. These areas are near the equator,
and it is easy to see why it does not snow
on these parts of Earth.
At higher latitudes, however, the Sun’s
light hits Earth at different angles
depending on the time of year. At these
locations, sunlight is more direct in the
summer, while in winter, less direct
sunlight is available. This causes the
familiar pattern of seasons—spring,
summer, fall, and winter. It also sets the
stage for the right combination of
humidity and cold temperatures needed
for a snow-covered winter.
Earth’s tilt causes the seasons. Many locations farther
Snowflakes
from the equator have cold winters with the right
The conditions that make it possible for
conditions for ice and snow.
snow to fall also contribute to the wide
variety of snowflake shapes. Scientists actually prefer to use the
term “snow crystal” when talking about the different shapes,
since the general term “snowflake” can refer to a single snow
crystal, or several snow crystals that have clumped together.
Snow forms high in the atmosphere when
temperatures there drop to or below 32˚C, the
freezing point of water. The crystals begin to
form as water vapor condenses to ice around
a tiny particle, such as a piece of dust. More
water vapor freezes onto the small “seed
crystal,” causing it to grow in intricate
patterns.
The ultimate shape of a snow crystal depends
on both the temperature and humidity of the
atmosphere. Snow crystals are classified as
dendrites, plates, columns, or needles. They
can be solid or hollow; they can be smooth or
This star-like snow crystal shape is
known as a dendrite.
intricately patterned. Snow crystals that form
in low humidity tend to have simpler shapes,
like smooth hexagonal plates and solid hexagonal columns. In
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Snow Science
higher humidity, the snow crystals tend to form branching
patterns, such as the star-like dendrites.
As it forms, a snow crystal moves up and
down and back and forth in the atmosphere.
As it passes through different areas, with
different temperatures and humidities,
crystal growth can change. By the time a
snow crystal reaches the ground, it has a
final shape that shows evidence of its
complex path through the atmosphere. This
is the main reason that snowflakes are
unique—each one takes a slightly different
path to the ground.
Although they are all slightly different, one
feature that all snow crystals share is their
six-sided shape. This basic shape—whether
The arrangement of water molecules forms a
it is a six-armed dendrite or a hexagonal
hexagonal pattern as ice crystals form.
plate—is caused by the arrangement of water
molecules as they form ice crystals. As the crystal grows, the
hydrogen and oxygen atoms in the water molecules align to
form a hexagonal lattice. This molecular alignment is what
determines the six-sided shape of snow crystals.
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