When Water Vapor Becomes Snow

By Ron Kurtus (revised 30 December 2006)

Snow is created when water vapor—the gaseous state water—is cooled so much that it turns into solid ice crystals or snow.

Going directly from a gas to a solid is call deposition. The molecular characteristics of water causes its solid state to be in regular crystals.

The size and shape of these ice crystals is determined by the amount of water and the temperature at which snow is formed.

Questions you may have include:

• What is the process of freezing?
• Why are snowflakes regular crystals?
• How is the shape of a snowflake determined?

Deposition process

Deposition is when a gas turns directly into a solid. This happens under certain circumstances of temperature and pressure. The opposite of deposition is sublimation, when a solid turns directly into a gas. Dry ice is an example of sublimation, where a solid turns into carbon dioxide (CO₂) gas. 

Vapor becomes crystals

Clouds consist of water vapor or the gaseous state of water (H₂O). In cold weather, the conditions are right for the vapor to turn directly into solid ice crystals.

At higher temperatures, the water vapor molecules have sufficient kinetic energy to overcome any bonding forces that would hold them together as a liquid or in some solid pattern. When the temperature gets low enough, the molecules gather in a solid crystalline structure.

Snow compared to rain

Snow crystals or snow flakes are usually fluffy and not very dense. The amount of snow is about 8 times the amount of rain water. In order words, a snowstorm with 16 inches of snow would only amount to 2 inches of rain. Or 40 cm of snow would be only 5 cm of rain.

Sleet

Snow is not formed from rain drops in the clouds freezing. If the temperature in the clouds is not cold enough to create snow but the and it starts to rain, that rain can still freeze when the temperature near the ground is below freezing. In such a case, it is called freezing rain or sleet.

Snow crystals regular

As water changes its state and becomes ice, the molecular forces start to arrange the water molecules into a regular pattern or crystalline shape. This also happens when water vapor turns directly into ice crystals.

See patterns in ice

You can watch water freeze and see the patterns of ice form on its surface as the liquid turns to solid. This gives you an idea of what happens when water vapor crystallizes.

Crystals are 6-sided

Clouds consist of water vapor. When the temperature is low enough, the water vapor molecules gather together and form small crystals of ice that we call snow. Because of the shape of the water molecules the bonding is hexagonal, such that the crystals are 6-sided.

6-sided snowflake crystals

Temperature affects shape

The temperature of the air in the clouds affects the shape and type of the snow flake. This has to do with how fast the water freezes and crystallizes.

Celsius (°C)	Fahrenheit (°F)	Shape
0° to -4°	32° to 25°	Thin plates
-4° to -6°	25° to 21°	Ice needles
-6° to -10°	21° to 14°	Hollow columns
-10° to -12°	14° to 10°	Six-sided plates
-12° to -16°	10° to 3°	Intricate dendrites
-16° to -22°	3° to - 8°	Six-sided plates
-22° and below	- 8° and below	Hollow columns

Summary

Water changes from the liquid to the solid state when it freezes, creating ice crystals. When this happens in clouds, the ice crystals are in the form of snow. The temperature at which snow is formed, affects the shape of the flakes.




source: www.school-for-champions.com