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



Snowflakes and Snow Crystals



The Structure of Crystalline Ice



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.



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.



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.



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.



The Morphology Diagram



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.



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.

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3_A Snowflake Primer