How does snow form?
OK, I admit, this is a little geeky.  I wrote this as a correction to a technical manual (on skiing) which gave incorrect information.
The actual correction was a summery of the principles, with this text was to be placed in the references section at the back.

But I thought that a few (very few) people may be interested, so I have reproduced it in full here.

Wayne


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*  An understanding of snow conditions is best approached through knowledge of how snow crystals form and change.
*  What follows is only an outline only.
*  As a start-point for further study you may research (the items shown in brackets).
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Snow is fundamental to “Snow”sports. During the winter months mountains will be covered in a deep layer of compacted ice crystals. It is the molecular structure of these crystals which enables skis to slide over the ice/snow. For most of the 20th century it was assumed (without any empirical evidence) that ice was slippery due to pressure (e.g. an ice skate or ski pressing down on it), or friction melting of the surface. It is now known that ice is slippery as it consists of H2O molecules formed into a hexagonal shape, which bond on all sides with other hexagons. The molecules near the surface can only bond with others to the side and below them so, due to molecular vibration, the bonds are shaken loose; this is similar to liquid water. So in effect your skis are sliding over a layer of liquid. (Quasi Liquid Layer)

Ice is simply frozen water which consists of molecules in constant random motion formed by the bonding of an oxygen atom and two hydrogen atoms. (Brownian Motion / Molecular Dynamics) Due to their motion the molecules have energy (temperature is a measure of energy). In water some molecules may evaporate from the surface. When they leave the surface the molecules take energy/heat with them. One of the results of this is that, as only the most energetic molecules can escape, once they leave the average energy/heat of the body of water is reduced. (Latent Heat of Vaporization) After molecules evaporate they enter a very thin layer of air just above the surface; which is not affected by wind. Here, again due to their random motion, some molecules may head back down into the water and others will not. Any molecules which move away from the water will join the rest of the gasses in the air; in this state it is called Water Vapour. Air consists of mainly oxygen and nitrogen along with other trace gasses, one of which is Water Vapour. The amount of Water Vapour in the air varies from almost none up to approx. 4%. (Laminar Boundary Layer)

Note: “Gas” is commonly used to describe anything which would be in a gaseous state under typical conditions of temperature and pressure, “Vapour” is the gaseous state of anything which would usually be liquid or solid.

The formation of ice crystals.

As stated, each gas in the air mixture contains molecules in constant random motion so, due to their motion, they have Kinetic Energy; Temperature is a measure of the average kinetic energy. When two molecules collide they rebound and, as there are more molecules in the centre of the air parcel, those heading outwards will travel further before colliding again. The result of this is that the gas parcel will expand until it hits the edge; in the atmosphere this may be a mountain, a parcel of denser air, the ground, etc. (The Le Chatlelier Principle. Ideal Gas Law)

As each gas expands it pushes outwards with a force / pressure; this is known as that gas’s Partial Pressure. When added together the sum of all the Partial Pressures is known as the Air Pressure. (Dalton’s Law). The air parcel cannot expand downwards into the ground so it expands upwards and outward. As it does so denser cooler air moves into the low pressure space below it pushing the air parcel upwards. The parcel of air is performing work as it expands and pushes against the surrounding atmosphere, reducing its energy/heat, so its temperature reduces. As the air parcel continues to rise and expand the temperature continues to drop. (Adiabatic Cooling) As stated above the air consists of various gasses, one of which is Water Vapour. Relative Humidity is a measure of the actual water which can exist as a gas in the air compared to the amount that can exist at the air’s current temperature; it is displayed as a percentage. Warm air can have a larger percentage of water vapour than cooler air and, as the temperature changes, so does the percentage of vapour which can exist in the air. For example if there is a Relative Humidity of 100% the air is said to be Saturated as is contains the maximum percentage of Water Vapour which it can do, at that temperature. If the temperature drops then, even though no extra moisture has been added, there is now more vapour present than the air can contain e.g. 102% and the air is said to be Supersaturated. If the temperature were to rise there would less moisture present than the maximum the air can contain e.g. 98% and the air would then be Unsaturated. (Relative Humidity)

Note. The term Saturated stems from the old, incorrect, belief that water dissolved in air and, if more Water Vapour was added to already wet air, it would become saturated. This is not true and is the source of the common misconception that warm air can hold more water than cold air, or that air will somehow absorb water. Water Vapour is simply one of the gasses which make up the air mass; it is not held by it. Water Vapour can exist even if none of the other gasses in the air are present.

The parcel of air will continue to rise and cool, increasing the Relative Humidity. Assuming that the air pressure remains constant, at a certain temperature the Relative Humidity will rise to 100% and the air will be Saturated. This temperature is called the Dewpoint. If the temperature continues to drop (increasing the Relative Humidity) the vapour will start to condense out of the air and form liquid water droplets. The energy/heat the molecule took when it evaporated is released as the vapour changes from gas to liquid. This warms the atmosphere and contributes to the cloud rising further. (Dewpoint)

Even if the temperature is below 0c the droplets will remain liquid unless they come into contact with a surface e.g. aerosols, ice, dust, etc. (condensation nuclei) when they will freeze into ice; at this point the cloud may contain water as liquid super-cooled water, solid ice or vapour. Within the cloud there is a continuous dynamic process taking place with vapour either evaporating from droplets or sublimating from ice whilst other molecules are condensing back into the liquid and accreting onto ice. As it requires less energy to do so, more molecules will escape from water than from ice, so the vapour pressure above the water’s surface is higher than over ice. This means the air above the droplet is Unsaturated whereas over the ice it is Supersaturated and above the Dewpoint, so vapour will continue to evaporate from the liquid droplets but be absorbed by ice crystals. The result is that in cold clouds ice crystals will rapidly grow whilst water droplets reduce. (Bergeron Findeisen process)

The shape of ice crystals.

Due to the sub atomic structure of Oxygen and Hydrogen ice crystals will initially form into a 6 sided (hexagonal) shape. The reason for this is that each atom’s nucleus is surrounded by an area containing electrons which have a slight negative charge; Hydrogen has one and Oxygen has eight, arranged into four pairs. As like charges repel, these four pairs are forced as far apart as possible and are arranged opposite each other around the nucleus. The atoms will bond by sharing electrons. The two hydrogen atoms will share their single electron with the oxygen atom. One pair of the Oxygen’s electrons is shared by each of the two Hydrogen atoms; this reduces the negativity of these two pairs. As their repulsive force has been reduced they are forced slightly towards each other by the other, unshared, two electron pairs. This gives the H2O molecule its familiar Y shape as the oxygen atom now has two hydrogen atoms attached to it. (Covalent Bonding)

However, the bond between the atoms is not equal. The Oxygen atom has a stronger pull and so drags the shared negatively charged hydrogen electron towards itself. The result is slightly negative oxygen and slightly positive hydrogen. The oxygen atom also has two lone pairs of negatively charged electrons which have not formed part of the arrangement which, being negative, are attracted to the hydrogen in nearby H2O molecules and may form electrostatic bonds with them. In liquid water the inter-molecular bonds are continually being broken and reformed. However as the temperature is reduced the molecules move around much less, so the bonds become semi permanent. When water freezes, due to the to the Y shape of each molecule, the inter-molecular bond will form a six pointed star shape. (Hydrogen Bonding. Van der Vaals attraction)

Fig 1 Fig 2 Fig 3 Fig 4
The atoms bond by sharing electrons. This results in polarity zones within the molecule. Each H2O molecule may form Hydrogen bonds with up to four other molecules. In liquid water the molecules are in constant motion. Any bonds that form are quickly broken. As the temperature drops the polarity of the atoms forces the molecules into a hexagonal shape.


Once an ice crystal forms in a cloud it may dissolve or continue to grow through accretion (Water Vapour settling onto the surface). The final shape of the crystal depends on the Temperature and the Supersaturation in which it formed and, most importantly, the conditions it moved through after the initial formation. Ice crystals enlarge by the accretion of water vapour or, in the case of Riming, by the deposition of droplets which freeze onto their surface. (Ice Morphology. The Berg Effect)

The temperature mainly determines the crystal’s shape; higher temperature tends to produce plates, whilst lower temperature results in columns. Higher Supersaturation produces more complex crystals. (Ukichirō Nakaya)

As a general guideline it can be shown that:
• At around -2C Plates will grow. As the Supersaturation increases these will form into thicker plates and then Dendrites.
• At around -5C Columns will grow.  At higher Supersaturation they become thicker and the column end faces reduce so the columns become hollow.   At the highest Supersaturation thin needle-like crystals form.
• At round -15C Plates will develop becoming increasingly thick at the Supersaturation increases.
•• Below -25C a mixture of thick plates and columns develop

It must be noted the development shown above is the initial formation of the crystal. If there is little updraft to keep them afloat, or they become so large that they fall from the cloud, they will remain in these shapes. However in the vast majority of cases the shape will change as it will be affected by local air pressure, crystal size prior to accretion, amount of vapour present, etc. and the amount of time spent an area of the cloud,


Precipitation types.

The International Association of Cyrospheric Sciences (IACS) classification system is widely used and consists of seven snow types with 3 other ice *precipitations. Each type can be sub-divided into different types, e.g. Columns and hollow columns, Plates and sectored plates, etc

Each type can be sub-divided into different types, e.g. Columns and hollow columns, Plates and sectored plates, etc. attract Rime which is (are) frozen droplets which attach to the surface of an ice crystal but does not (like Grauple)  obscure the original shape

In the outdoor environment virtually all crystals are Rimed.

Plates Stellar Crystals Columns Needles Spatial Dendrites
The original shape of the crystal is maintained as it enlarges. Formed when corners extend into an area of higher supersaturation The result of the end (basel) facets enlarging and extending Once the corners are rounded the ends continue to attract deposition. The same as Stellar Crystals but with the arms on different planes.

Capped Columns Irregular * Sleet * Hail  * Grauple
Columns moving into warmer areas will form end Plates. The vast majority of snow is, eventually, irregular. Ice which has melted and partly refrozen Ice which melts and re- freezes, possibly many times Formed by frozen droplets obscuring the original crystal’s shape.



Ice may form in clouds and if it descends to the ground in its crystalline form it is called Snow
Note – snow is not frozen rain (that would be sleet).