3_A Snowflake Primer
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Transcript of 3_A Snowflake Primer
7/29/2019 3_A Snowflake Primer
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A Snowflake Primer The basic facts about snowflakes and
snow crystalsRef: http://tinyurl.com/3ervp
Assembled by
Ken Mitchell
Livermore TOPScience
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Snowflakes and Snow Crystals
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The Structure of Crystalline Ice
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Snowflakes grow from water vapor
Snowflakes are not frozen raindrops.
Sometimes raindrops do freeze as they
fall, but this is called sleet.
Sleet particles don't have any of theelaborate and symmetrical patterning
found in snow crystals.
Snow crystals form when water vapor condenses directly into ice, which
happens in the clouds.
The patterns emerge as the crystals grow.
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The simplest snowflakes The most basic form of a snow crystal is a hexagonal
prism, shown in several examples at right. This
structure occurs because certain surfaces of the crystal,
the facet surfaces, accumulate material very slowly.A hexagonal prism includes two hexagonal "basal"
faces and six rectangular "prism" faces, as shown in the
figure. Note that a hexagonal prism can be plate-like or
columnar, depending on which facet surfaces grow
most quickly.
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The life of a snowflake
The story of a snowflake begins with water vapor in
the air. Evaporation from oceans, lakes, and riversputs water vapor into the air, as does transpiration
from plants. Even you, every time you exhale, put
water vapor into the air.
When you take a parcel of air and cool it down, at
some point the water vapor it holds will begin tocondense out. When this happens near the ground,
the water may condense as dew on the grass. High
above the ground, water vapor condenses onto dust
particles in the air. It condenses into countless
minute droplets, where each droplet contains at leastone dust particle. A cloud is nothing more than a
huge collection of these water droplets suspended in
the air.
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In the winter, snow-forming clouds are still
mostly made of liquid water droplets, evenwhen the temperature is below freezing.
The water is said to be super cooled ,
meaning simply that it is cooled below the
freezing point.
As the clouds gets colder, however, the
droplets do start to freeze.
This begins happening around -10 C (14 F),but it's a gradual process and the droplets
don't all freeze at once.
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The Morphology Diagram
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If a particular droplet freezes, it becomes a small
particle of ice surrounded by the remaining liquid
water droplets in the cloud. The ice grows as water vapor condenses onto its surface, forming a
snowflake in the process. As the ice grows larger, the
remaining water droplets slowly evaporate and put
more water vapor into the air.
Furthermore, we see from the diagram that snow
crystals tend to form simpler shapes when the
humidity (super saturation) is low, while more
complex shapes at higher humidity. The most
extreme shapes -- long needles around -5C and large,
thin plates around -15C -- form when the humidity isespecially high.
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Why snow crystal shapes change so much withtemperature remains something of a scientific
mystery. The growth depends on exactly how
water vapor molecules are incorporated into the
growing ice crystal, and the physics behind this iscomplex and not well understood. It is the subject
of current research in my lab and elsewhere.