Nature’s icy artistry
The fragile appearance of ice crystals belies a hidden strength, as these dainty creations of winter hold the power to transform entire landscapes
THE DAY DAWNS bitterly cold; the frost-encrusted, snow-mantled landscape shrouded in an icy fog. The long December night has radiated away the meagre daytime heat of winter. Strange noises emanate from a frozen lake: eerie twangs and curious, high-pitched reverberations; the result of sound waves as the iced surface flexes and heaves.
The delicate ice crystals of a hoar frost, the feathery beauty of rime and the myriad unique patterns of snowflakes, are part of the magic of a winter’s day. These forms of ice are quite different to the strong and powerful ice sheets that still overwhelm much of Antarctica and a large number of the world’s highest mountains, and even engulfed much of the British Isles for long periods in the past. But all varieties of ice develop at or below the critical point of 0°C, the freezing point of water.
A snow crystal forms high in the atmosphere on ever-present nuclei originating from dust or airborne salt spray. Although the air temperature at that height may be many degrees below freezing, it can begin as a minute water droplet, remaining unfrozen because of its tiny size until the air temperature reaches below -40°C. At this point, these water droplets will quickly freeze, forming crystals. As each crystal gathers more water droplets onto its surface, these in turn freeze, enabling it to grow larger.
Ice in many forms
Ice can also arise directly on the crystal itself from vapour, through the process of sublimation. A snowflake is an aggregate of such crystals as they fall within the cloud. In the colder parts of the cloud, snow is made up of ice crystal columns, but as the temperature rises, they can become plate-like. In the UK, where winters are often close to freezing, and there is more moisture available, we are familiar with snowflakes that are quite large in size.
Sleet, hail, ice prisms or diamond dust, snow pellets and snow grains are some of the other forms of ice within our atmosphere. Frost presents itself when ice is deposited from the cooling of air, usually at times of quiet weather.
It occurs in a variety of forms, such as the common hoar frost, with its white, often needle-shaped crystals, which are formed from frozen dew or deposited direct from the air as a vapour. Rime frost, with its more granular white crystals, arises from water droplets in fog that are below freezing, or supercooled. Ice crystals initiate when the droplets touch objects such as twigs or lattice fences, leading to feathery formations on the upwind side. Another variety of frost can occur in very cold weather, especially on single-pane windows, where supercooled water gels slowly to produce beautiful fern-like patterns.
There are more unusual types of ice that may
be encountered. In winter, water can remain unfrozen within the soil, yet in the air immediately above it, the temperature may drop below freezing. Strands of ice can then form at the surface and rise upwards, added to by capillary action in the soil, creating ice needles.
A rare form is hair ice; a most subtle, delicate phenomenon, resembling candyfloss in appearance. The conditions for its formation occur when the temperature falls just below freezing overnight, following a wet spell, when small twigs and branches become saturated with water. The air surrounding the branch or twig is just a little drier, causing a slight difference in the vapour pressure and allowing the formation of ice crystals through the vascular openings in the wood. This process seems to be associated with the presence of the fungus Exidiopsis effusa. It can be very transient, with the relationship between temperature, humidity and wind speed critical to its formation. A slight temperature rise above 0°C can initiate a quick demise.
Ice spikes are also rather unusual, although they can occur in a water-filled garden bird bath or a bucket of water, as well as in puddles and small ponds. The formation of ice spikes is bound up with the varied and interesting properties of water. Unlike most substances, water actually reaches its greatest density when cooled to
“Glitt’ring with ice here hoary hills are seen” Alexander Pope, ‘The Dunciad’
4°C and then expands if it cools further. Normally, the side of the container would freeze first, being in contact with colder air. The warmer subsurface water towards the middle expands. As the birdbath or container freezes over, the subsurface water is under pressure and forces itself out through a crack in the ice. The hole is often triangular-shaped, due to the hexagonal crystalline structure of the ice, generating sides that are at angles of 60°. Being under pressure, it prevents the water from freezing but solidifies quickly once exposed on the surface away from the fissure. Further pressurised water spurts upwards, helping to continue the process. The temperature should not be too far below freezing, and there should be a light breeze for evaporative cooling of the spike’s sides.
Icy spears
Looking like large stalactites, icicles are the embodiment of winter chill. These shards of ice are often thickest at the inverted base and hang from thick snow-covered roofs and gutters. They can form when sunshine melts the surface of a sloping snow cover that has built up on a roof or a shed. This allows drops of water to dribble from its edge, and if the air itself is below freezing, icicles may form. They can also sometimes be seen when water streams out from cracks in rocks or tumbles over a cliff face or ledge, and if it is particularly cold, a cascade of icicles can develop. England’s highest waterfall, High Force, in County Durham, was transformed into huge spires of ice during the severe winter of 1963.
The process of icicle formation begins when the outer surface of the drops ice over in contact with the sub-freezing air, but the inner core is insulated by the ice and remains liquid and continues to fall further. As more heat is lost at the tip compared to the base, it freezes more quickly here while still maintaining a core of unfrozen water. At the other end, there is more extensive water, being nearer the source, and it ices over more slowly due to a greater release of latent heat from the freezing process. The heat tends to rise towards the base, and the water remains liquid here for longer on its exterior, making the base thicker and giving the icicle its typical shape.
Icicles can grow to several feet in length, posing a danger to those below. In February 1991, several people were injured at Dartford in Kent, as huge icicles crashed down from buildings onto pedestrians out shopping. In 1776, a young man was killed by an icicle that broke off from the church tower at Bampton in Devon.
Slippery surfaces
Black ice is a term used when light rain or drizzle, or fog droplets fall onto a road or other surface that has a temperature below freezing. It often forms a transparent covering on the surface, so is not immediately obvious, making it a hazard to road users in particular.
Another dangerous form of ice to both pedestrians
“The children hail the happy day – As if the snow were April’s grass, And pleas’d, as ‘neath the warmth of May, Sport o’er the water froze as glass” John Clare, ‘The Shepherd’s Calendar: December’
and motorists is known as glaze. Like black ice, it may not be apparent at first, causing an unfortunate person to end up in an undignified sprawl on the ground, while vehicles can be at grave risk of collision with no grip at all on the icy surface. This phenomenon is colourless and is caused by a layer of warm air aloft riding over cold conditions below. Any snow falling through the warmer layer turns to rain, and if the cold surface air is reluctant to move, a glaze can develop. On 30 December 1995, such a situation arose across southern England, halting motorway traffic, freezing locks, windows and doors, and coating trees and hedges with a veneer of ice. Many hospitals had their busiest ever day, with hundreds of emergency calls for fractures, cuts and concussion. On January 27 1940, some areas, including the Cotswolds, suffered a remarkable spectacle, when a thick coating of glaze ice was deposited on trees and shrubs, power lines and telegraph wires. Some were covered with 10 tons of ice between posts, and when the wind blew, it caused a clattering sound like a symphony of ice and sent shards shimmering to the ground. Ponies on Dartmoor, in Devon, even became encased with ice.
Curious sounds
Ice on lakes and ponds can make noises. These ghostly pitches and tones result from the sound waves being separated as they travel through the different mediums of ice, air and water. This effect will be familiar to anyone who has skipped stones across the surface of a frozen lake. Also, sound travels further in cold air, as the sound waves are refracted downwards from a layer of warmer air above, instead of being dispersed in all directions. This stratum of warmer air produces what is known as an inversion, trapping the cold, icy air near the ground.
Expanding water
As water cools below 4°C, it begins to take up a greater space, becoming less dense, so by the time it freezes and becomes ice at 0°C it has expanded by some nine per cent. Water molecules, made up of groups of hydrogen and oxygen atoms that bond together, take up a
hexagonal pattern when changing to ice, which is less tightly bound. It exerts a remarkable pressure that can be as much as 5 tons per cm2 in the process. Any cracks and imperfections in a rock surface that water can penetrate will be subject to this force when the temperature reaches 0°C, and it starts to freeze. If the water is severely constrained as it freezes, ice can break apart the toughest of rocks and in a winter cold spell can also make a mess of a road surface or poorly constructed buildings. Beneath steep slopes of bare rock in hilly or mountainous areas, there is often a jumbled pile of rocks, called scree, that is caused by this process, also known as frost shattering. Our winter climate is often a rollercoaster ride in temperature, with rain being followed by frost, then back to rain again. Constant freeze and thaw ultimately reduces the rocks to sediments that can be both part of our soils or, in a much longer time scale, will eventually form new rocks to start the cycle again.
Another significant aspect about water reaching its greatest density at 4°C is that it prevents ponds and lakes from freezing to the bottom, unless they are very shallow. It averts polar sea ice from freezing too deeply and from the seabed upwards. Water, when it reaches this critical temperature, tends to sink, while the colder, less dense water in contact with the air at the surface, will cool and freeze. As the layer of ice slowly increases, it insulates the water beneath from the sub-freezing air. This is vital, as it ensures that even in a very cold winter, aquatic life will not be encased in ice at the bottom of a pond. It explains how life teems beneath the polar sea ice. If less dense ice did not float, shallow seas would freeze solid: there would be more ice, which would cool the surface even more and reflect the sun’s heat, and much flora and fauna would perish.
Water reaching its greatest density at 4°C also helps the Gulf Stream or North Atlantic Drift to function and keep the British Isles from having much colder winters than our latitude would suggest. Water sinks as it cools in the North Atlantic and returns south in a giant gyre formation, having first warmed our shores. This prevents us being plunged into an ice age similar to that of 20,000 years ago.