Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snow crystals form when tiny supercooled cloud droplets freeze. Experiments show that this 'homogeneous' nucleation of cloud droplets only occurs at temperatures colder than -35 °C. In warmer clouds an aerosol particle or 'ice nucleus' must be present in (or in contact with) the droplet to act as a nucleus. Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is supersaturated with respect to ice when the temperature is below the freezing point) and grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are deposited. 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 (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground.The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a 'sticky' liquid-like layer on the crystal surface.
Snowflakes by Wilson Bentley 1902
Ice crystals formed in the appropriate conditions can often be thin and flat. The shape of the snowflake is determined broadly by the temperature, and humidity at which it forms. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.
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 habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei. Interestingly, if a crystal has started forming in a column growth regime, say at around −5 °C (23 °F), and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called 'capped columns'.
There is a widely held belief that no two snowflakes are alike. it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance.
The water equivalent of a given amount of snow is the depth of a layer of water having the same mass and upper area. For example, if the snow covering a given area has a water equivalent of 50 centimetres (20 in), then it will melt into a pool of water 50 centimetres (20 in) deep covering the same area. Snow that falls in maritime climates is usually denser than snow that falls in mid-continent locations because of the higher average clouds over oceans than over land masses. Cloud temperatures and physical processes in the cloud affect the shape of individual snow crystals. Highly branched or dendritic crystals tend to have more space between the arms of ice that form the snow flake and this snow will therefore have a lower density, often referred to as "dry" snow. Conditions that create columnar or platelike crystals will have much less air space within the crystal and will therefore be denser and feel "wetter".
Once the snow is on the ground, it will settle under its own weight (largely due to differential evaporation) until its density is approximately 30% of water. Increases in density above this initial compression occur primarily by melting and refreezing, caused by temperatures above freezing or by direct solar radiation. By late spring, snow densities typically reach a maximum of 50% of water.
Many rivers originating in mountainous or high-latitude regions have a significant portion of their flow from snowmelt. The energy balance of the snowpack is dictated by several heat exchange processes. The snowpack absorbs solar shortwave radiation that is partially blocked by cloud cover and reflected by snow surface. A longwave heat exchange takes place between the snowpack and its surrounding environment that includes overlying air mass, tree cover and clouds. Moisture exchange between the snowpack and the overlying air mass is accompanied with latent heat transfer that is influenced by vapor pressure gradient and air wind. Rain on snow can add significant amounts of thermal energy to the snowpack. The small temperature change from before to after a snowfall is a result of the heat transfer between the snowpack and the air.
Substantial snowfall can disrupt public infrastructure and services, slowing human activity even in regions that are accustomed to such weather. Air and ground transport may be greatly inhibited or shut down entirely. In addition, snow can make roads much harder to travel and vehicles attempting to use them can easily become stuck. In areas that normally have very little or no snow, a snow day may occur when there is only light accumulation or even the threat of snowfall, since those areas are ill-prepared to handle any amount of snow.
Some agricultural areas depend on an accumulation of snow during winter that will melt gradually in spring, providing water for crop growth. In areas near mountains, people have harvested snow and stored it as layers of ice. This allowed the ice to be used in summer for refrigeration or medical uses.
When snow falls at night, several things happen in urban areas that cause light levels to increase. This makes it much brighter even though the sun has gone down. First of all, the snow that has fallen reflects nearly all light cast upon it since it is very close to white. This causes the light from city lights to be reflected off the snow and back into the atmosphere.
- Blizzard: A long-lasting snow storm with intense snowfall and usually high winds. A class of snow flakes that is shaped like a six sided column. One of the 4 classes of snow flakes.
- Dendrites: A class of snow flakes that has 6 points, making it somewhat star shaped. The classic snow flake shape. One of the 4 classes of snow flakes.
- Flurry: A period of light snow with usually little accumulation with occasional moderate snowfall. Graupel: Precipitation formed when freezing fog condenses on a snowflake, forming a ball of rime ice. Also known as snow pellets.
- Ground blizzard: Occurs when a strong wind drives already fallen snow to create drifts and whiteouts.
- Lake-effect snow: Produced when cold winds move across long expanses of warmer lake water, picking up water vapor which freezes and is deposited on the lake's shores.
- Needles: A class of snow flakes that are acicular in shape (their length is much longer than their diameter, like a needle). One of the 4 classes of snow flakes.
- Rimed snow: Snow flakes that are partially or completely coated in tiny frozen water droplets called rime. Rime forms on a snow flake when it passes through a super-cooled cloud. One of the 4 classes of snow flakes.
- Snowsquall: A brief, very intense snowstorm.
Snow blowing from a roof in Ottawa, Ontario, Canada (submitted by Jacques Richard Brunet)
Chopped powder: Powder snow that has been cut up by previous skiers.
- Corn: Coarse, granular wet snow. Most commonly used by skiers describing good spring snow. Corn is the result of diurnal cycle of melting and refreezing.
- Cornice: An overhanging formation of windblown snow. Important in skiing and alpine climbing because the overhang can be unstable and hard to see from the leeward side.
- Crud: This covers varieties of snow that all but advanced skiers find impassable. Subtypes are (a) windblown powder with irregularly shaped crust patches and ridges, (b) heavy tracked spring snow re-frozen to leave a deeply rutted surface strewn with loose blocks, (c) a deep layer of heavy snow saturated by rain (although this may go by another term). Crud is negotiated with a even weighting along the length of the skis, and smooth radius turns started, if necessary, with a pop or jump. When an advanced skier falls over on crud, it is probably because it is 'heavy crud',
- Crust: A layer of snow on the surface of the snowpack that is stronger than the snow below, which may be powder snow. Depending on their thickness and resulting strength, crusts can be termed "supportable," meaning that they will support the weight of a human, "breakable," meaning that they will not, or "zipper," meaning that a skier can break and ski through the crust. Crusts often result from partial melting of the snow surface by direct sunlight or warm air followed by re-freezing.
- Depth hoar: Faceted snow crystals, usually poorly or completely unbonded (unsintered) to adjacent crystals, creating a weak zone in the snowpack. Depth hoar forms from metamorphism of the snowpack in response to a large temperature gradient between the warmer ground beneath the snowpack and the surface. The relatively high porosity (percentage of air space), relatively warm temperature (usually near freezing point), and unbonded weak snow in this layer can allow various organisms to live in it.
- Finger Drift: A narrow snow drift (30 cm to 1 metre in width) crossing a roadway. Several finger drifts in succession resemble the fingers of a hand.
- Ice: Densely packed material formed from snow that doesn't contain air bubbles. Depending on the snow accumulation rate, the air temperature, and the weight of the snow in the upper layers, it can take snow a few hours or a few decades to form into ice.
- Firn: Snow which has been lying for at least a year but which has not yet consolidated into glacier ice. It is granular.
- Packed Powder: The most common snow cover on ski slopes, consisting of powder snow that has lain on the ground long enough to become compressed, but is still loose.
- Packing snow: Snow that is at or near the melting point, so that it can easily be packed into snowballs and hurled at other people or objects. This is perfect for snow fights and other winter fun, such as making a snowman, or a snow fort.
- Penitentes: Tall blades of snow found at high altitudes.
- Pillow Drift: A snow drift crossing a roadway and usually 3 to 4.5 metres (10-15 feet) in width and 30 cm to 90 cm (1-3 feet) in depth.
- Powder: Freshly fallen, uncompacted snow. The density and moisture content of powder snow can vary widely; snowfall in coastal regions and areas with higher humidity is usually heavier than a similar depth of snowfall in an arid or continental region. Light, dry (low moisture content, typically 4-7% water content) powder snow is prized by skiers and snowboarders.
Slush: Snow which partially melts upon reaching the ground, to the point that it accumulates in puddles of partially-frozen water.
- Snirt: Snow covered with dirt, which occurs most often in Spring, in Prairie States like North Dakota, where strong winds pick up black topsoil from uncovered farm fields and blow it into nearby towns where the melt rate is slower. The phenomenon is almost magical; one goes to sleep with white snow outside and awakens to black snow. Also, snow that is dirty, often seen by the side of roads and parking lots near areas that have been plowed.
- Snowdrift: Large piles of snow which occur near walls and curbs, as the wind tends to push the snow up toward the vertical surfaces.
- Surface Hoar: Faceted, corn-flake shaped snow crystals that are a type of frost that forms on the surface of the snow pack on cold, clear, calm nights. Subsequent snowfall can bury layers of surface hoar, incorporating them into the snowpack where they can form a weak layer.
- Water-melon snow: reddish/pink-colored snow caused by a red-colored algae called Chlamydomonas nivalis.
- Wind slab: A layer of relatively stiff, hard snow formed by deposition of wind blown snow on the leeward side of a ridge or other sheltered area. Wind slabs can form over weaker, softer freshly fallen powder snow, creating an avalanche hazard on steep slopes.
This terminology doesn't appear to have reached North Norfolk so TaTa is thinking of going to Canada pretty soon.
and the church hopes a nesting pair will arrive. Even though barn owls have up to seven babies there are only 5000 breeding pairs in the UK. Barn owls have declined in Britain, due in part to the reduction of derelict old buildings, barn conversions, and due to the use of persistent pesticides, such as DDT, which prey consume, and eventually quantities build up in the predator, causing weak egg-shells. 
