Tag: snow

After looking at the beach over the past week, which for most of us brings vision of warm Summer days, I thought it might be interesting to look at something that is a wonder of a different season: Snow!

Snow is definitely not among the favorite forms of precipitation for everyone, as, in even small amounts, it can hinder travel, unless one is prepared for it.  For me, snow is a contributor to amazing landscapes, so I always look forward to a beautiful snowfall that gives a complete different expression to the landscape that we see every day.

Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft, white, and fluffy structure, unless subjected to external pressure.  Snowflakes come in a variety of sizes and shapes.  Types that fall in the form of a ball due to melting and refreezing, rather than a flake, are hail, ice pellets or snow grains.

To connect back to last week, I present an image that contains both beach and snow…

 Snowflakes

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (0 °F), because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. Then the droplet freezes around this “nucleus”. Experiments show that this “homogeneous” nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F).  In warmer clouds an aerosol particle or “ice nucleus” must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.

Once a droplet has frozen, it grows in the supersaturated environment—one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets by a process known as the Wegner-Bergeron-Findeison process. The corresponding depletion of water vapor causes the ice crystals to grow at the droplets’ expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground. Guinness World Records list the world’s largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm (15 in) wide. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of the whole spectrum of light by the small ice particles.

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.  The most common snow particles are visibly irregular. Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the shape reverts to plate-like, often with branched or dendritic features. At temperatures below −22 °C (−8 °F), the crystal development becomes column-like, although many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. If a crystal has started forming in a column growth regime, at around −5 °C (23 °F), and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called “capped columns”.

A snowflake consists of roughly 10¹⁹ water molecules, which are added to its core at different rates and in different patterns, depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, it is extremely difficult to encounter two identical snowflakes. Initial attempts to find identical snowflakes by photographing thousands their images under a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.  It is more likely that two snowflakes could become virtually identical if their environments were similar enough. Matching snow crystals were discovered in Wisconsin in 1988. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms.

Types of Snow

Types of snow can be designated by the shape of the flakes, the rate of accumulation, and the way the snow collects on the ground. Types that fall in the form of a ball due to melting and refreezing cycles, rather than a flake, are known as graupel, with ice pellets and snow pellets as types of graupel associated with wintry precipitation.  Once on the ground, snow can be categorized as powdery when fluffy, granular when it begins the cycle of melting and refreezing, and eventually ice once it packs down into a dense drift after multiple melting and refreezing cycles. When powdery, snow drifts with the wind from the location where it originally fell,  forming deposits with a depth of several meters in isolated locations.  Snow fences are constructed in order to help control snow drifting in the vicinity of roads, to improve highway safety.  After attaching to hillsides, blown snow can evolve into a snow slab, which is an avalanche hazard on steep slopes. A frozen equivalent of dew known as hoar frost forms on a snow pack when winds are light and there is ample low-level moisture over the snow pack.

Snowfall’s intensity is determined by visibility. When the visibility is over 1 kilometer (0.62 mi), snow is considered light. Moderate snow describes snowfall with visibility restrictions between 0.5 and 1 km. Heavy snowfall describes conditions when visibility is less than 0.5 km.  Steady snows of significant intensity are often referred to as “snowstorms”.  When snow is of variable intensity and short duration, it is described as a “snow shower”.  The term snow flurry is used to describe the lightest form of a snow shower.

A blizzard is a weather condition involving snow and has varying definitions in different parts of the world. In the United States, a blizzard occurs when two conditions are met for a period of three hours or more: A sustained wind or frequent gusts to 35 miles per hour (56 km/h), and sufficient snow in the air to reduce visibility to less than 0.4 kilometers (0.25 mi).  In Canada and the United Kingdom, the criteria are similar.  While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as blowing snow can create a ground blizzard

Technical Details

This image was captured with a Canon EOS 1D MkII with an EF 24-105mm f/4L lens attached.  Exposure settings were at 1/50 second at f/20 with 400 IS