Snow Crystals
The size and types of snow crystals influence the amount of water in the snow pack and the stability of the snow pack. Observations of snow crystal size and shape should be made for each snow layer. Crystal size is determined by placing crystals on the 1 mm and 2 mm grids. These grids provide a reference to determine the size of the average crystals. At least 20 crystals should be examined and an estimate made of their average size. Use the loup when examining the crystals on the grids. You should be able to estimates crystal size to the nearest 0.5 mm. When the crystals are bonded together they must be broken apart before the measurements can be made. In the case of extreme bonding (e.g. ice layer) or in very wet snow, crystal size and shape lose their meaning and need not be recorded. Record the size of the crystals in each layer. |
Types of Snow Crystals:
Snow crystals are initially formed in the atmosphere. After falling,
crystals change through processes of sublimation, melting and
refreezing. The type of crystal found in the snow pack determines the
stability of the snow, water content, depth of the snow pack and
density of the snow layers. Identifying the types of crystals in each
layer helps in understanding the characteristics of the layer and snow
pack. The following types of crystals may be identified.
Type ‘a’ crystals Type ‘a’ refers to new snow composed mainly of whole crystals or parts of broken ones. Snow such as ice pellets and hail do not belong in this class. Type ‘a’ snow is usually very soft. These crystals are classified on the data sheets as:
New snow (symbol +) is represented by the classic six sided crystal and by plates of hexagonal crystals There are more than 100 classified types of new snow crystals.
New snow, rimed (symbol r) new snow that is made of droplets that freeze onto a snow crystal called rime. New snow rimed are crystals that have additional deposits of ice on the outer surfaces of the crystals.
Graupel (symbol X) When a snowflake turns into just a ball of rime, which is called graupel.
Type ‘b’ crystals Type ‘b’ snow refers to snow during the initial stage of settling. It is composed mainly of fragments of crystals but has not yet reached the stage of very fine fragments that is the end of the process of settling. Although is has lost a great deal of its crystaline character, some crystalline features can be observed; it still sparkles. Type ‘b’ snow is usually fairly soft. These crystals are classified on the data sheets as:
Partially settled (symbol /\) These are fine fragments of snow that have not yet changed through metamorphism.
Type ‘c’ crystals When snow is transformed by melting or melting followed by freezing, it completely losses all crystalline features and its grains become irregular and more or less rounded. This is type ‘c’ snow. It has no sparkle effect even in bright sunlight and can be readily recognized by its dull appearance. It is usually fairly soft when wet, but can be very hard when frozen. Type ‘c’ snow grains may be any size from very fine to very coarse. These crystals are classified on the data sheets as:
Rounded grains (symbol )
Melt-freeze particles (symbol )
Type ‘d’ crystals At temperatures well below freezing and without any apparent melting, snow is transformed into Type ‘d’ by deposition of water vapour removed from crystals deeper in the profile by the process of sublimation. Deposition of this vapour higher in the profile produces irregular grains with flat facets. These layered facets are visible with a lens and give the snow a distinct sparkle effect in the bright sunlight. Type ‘d’ snow is usually fairly hard. These crystals are classified on the data sheets as:
Faceted crystals (symbol )
Hoar Hoar is characterized by its hollow cup-shaped crystals. These crystals are produced by a very low
rate of deposition of sublimed vapour during a long uninterrupted cold period. Depth hoar is most frequently found directly below a more or less impermeable crust in the lower part of the snow cover. The strength of a layer of depth hoar is very low and the structure of the layer falls at the slightest touch. These crystals are classified on the data sheets as:
Surface hoar (symbol )
Depth hoar (symbol )
Photographing snow crystals:
For good pictures you need some good subjects, and not all crystals are ideal for snow crystal photography. The first step is to use a small magnifying glass to look at the crystals the next time it snows to see if there’s anything worth photographing. Not every snow fall produces great crystals. You will need to make your camera into a macro camera. Snow crystal photography is somewhat beyond what’s normally called macro photography, but at the same time it’s not as demanding (or expensive) as full-blown photomicroscopy. Take a 10 x to 12 x loup and fix it to an adaptor made to screw into the front of the camera. You can do this with digital cameras that have a manual focus. Selecting the best crystals requires care. A hands-off technique is to simply catch crystals on a cold glass plate and photograph them in place, but this is not always practical. In particular, one usually wants to select the best specimens, and then transport them to a photographic stage. Utensils for this task include small sticks, feathers, etc., and of course a steady hand is useful. Sublimation will rapidly cause the sharp crystal facets to become rounded. Although rounded edges do produce a pleasing prism-like effect, it is usually desirable to minimize the effects of sublimation by photographing crystals as they are selected. Snow crystal photographs should be taken quickly. Other techniques include catching snow crystals in a shallow beaker of cold hexane, which largely eliminates the sublimation problem.
Type ‘a’ crystals Type ‘a’ refers to new snow composed mainly of whole crystals or parts of broken ones. Snow such as ice pellets and hail do not belong in this class. Type ‘a’ snow is usually very soft. These crystals are classified on the data sheets as:
New snow (symbol +) is represented by the classic six sided crystal and by plates of hexagonal crystals There are more than 100 classified types of new snow crystals.
New snow, rimed (symbol r) new snow that is made of droplets that freeze onto a snow crystal called rime. New snow rimed are crystals that have additional deposits of ice on the outer surfaces of the crystals.
Graupel (symbol X) When a snowflake turns into just a ball of rime, which is called graupel.
Type ‘b’ crystals Type ‘b’ snow refers to snow during the initial stage of settling. It is composed mainly of fragments of crystals but has not yet reached the stage of very fine fragments that is the end of the process of settling. Although is has lost a great deal of its crystaline character, some crystalline features can be observed; it still sparkles. Type ‘b’ snow is usually fairly soft. These crystals are classified on the data sheets as:
Partially settled (symbol /\) These are fine fragments of snow that have not yet changed through metamorphism.
Type ‘c’ crystals When snow is transformed by melting or melting followed by freezing, it completely losses all crystalline features and its grains become irregular and more or less rounded. This is type ‘c’ snow. It has no sparkle effect even in bright sunlight and can be readily recognized by its dull appearance. It is usually fairly soft when wet, but can be very hard when frozen. Type ‘c’ snow grains may be any size from very fine to very coarse. These crystals are classified on the data sheets as:
Rounded grains (symbol )
Melt-freeze particles (symbol )
Type ‘d’ crystals At temperatures well below freezing and without any apparent melting, snow is transformed into Type ‘d’ by deposition of water vapour removed from crystals deeper in the profile by the process of sublimation. Deposition of this vapour higher in the profile produces irregular grains with flat facets. These layered facets are visible with a lens and give the snow a distinct sparkle effect in the bright sunlight. Type ‘d’ snow is usually fairly hard. These crystals are classified on the data sheets as:
Faceted crystals (symbol )
Hoar Hoar is characterized by its hollow cup-shaped crystals. These crystals are produced by a very low
rate of deposition of sublimed vapour during a long uninterrupted cold period. Depth hoar is most frequently found directly below a more or less impermeable crust in the lower part of the snow cover. The strength of a layer of depth hoar is very low and the structure of the layer falls at the slightest touch. These crystals are classified on the data sheets as:
Surface hoar (symbol )
Depth hoar (symbol )
Photographing snow crystals:
For good pictures you need some good subjects, and not all crystals are ideal for snow crystal photography. The first step is to use a small magnifying glass to look at the crystals the next time it snows to see if there’s anything worth photographing. Not every snow fall produces great crystals. You will need to make your camera into a macro camera. Snow crystal photography is somewhat beyond what’s normally called macro photography, but at the same time it’s not as demanding (or expensive) as full-blown photomicroscopy. Take a 10 x to 12 x loup and fix it to an adaptor made to screw into the front of the camera. You can do this with digital cameras that have a manual focus. Selecting the best crystals requires care. A hands-off technique is to simply catch crystals on a cold glass plate and photograph them in place, but this is not always practical. In particular, one usually wants to select the best specimens, and then transport them to a photographic stage. Utensils for this task include small sticks, feathers, etc., and of course a steady hand is useful. Sublimation will rapidly cause the sharp crystal facets to become rounded. Although rounded edges do produce a pleasing prism-like effect, it is usually desirable to minimize the effects of sublimation by photographing crystals as they are selected. Snow crystal photographs should be taken quickly. Other techniques include catching snow crystals in a shallow beaker of cold hexane, which largely eliminates the sublimation problem.