Amazing light pillars
DAVID HAMBLING makes the most of his winter by searching out some chilly weather phenomena
This winter has seen some extraordinarily low temperatures in the United States, and that has led to some extraordinary phenomena. In some cases, what was happening was reasonably easy to explain. In other instances, the happenings were – initially at least – baffling.
The case of the falling lizards was straightforward, if weird. Iguanas are a common invasive species in Florida, and in the recent cold spell they were literally dropping off their perches and lying on the ground, legs in the air. Iguanas are cold-blooded creatures unable to generate their own body heat. Below about 10˚C (50˚F) they get increasingly sluggish, and at 4˚C (39˚F) they are not able to function at all and just lie comatose. They recover when left out in the sun, though the local wildlife commission urged people to use the opportunity to capture them.
The booming sounds heard on 27 January in Ozaukee County, Wisconsin, were more puzzling. Some people reported that their houses shook and windows rattled in what appeared to be a miniature earthquake; one resident told the authorities that they heard five separate explosions. While mystery booms in California have been blamed on secret military aircraft tests and nicknamed ‘sky quakes’, the cause here appears to have been a cryoseism or ‘frost quake’. This is a rare but well-established effect that occurs during sharp cold spells. Normally when rock saturated with water freezes, the expanding ice slowly causes small cracks in the surface, known as freeze-thaw weathering. This breaks up bedrock over a period of many years.
A cryoseism is far more dramatic. The surface layer of water freezes, capping the water below. The expanding ice exerts more and more pressure until an entire section of rock or frozen dirt gives way with a tremendous boom or crack. One of the first documented frost quakes occurred in Deerfield, Massachusetts, in 1819. Witnesses reported a loud ‘bang’ in the early hours, and geologist Edward Hitchcock located a large crack in the frozen ground. Since then, many cryoseisms have been recorded, though they rarely show up on seismographs because their area of effect is so much smaller than earthquakes. They can damage plaster, and move small objects and furniture, but there have never been reported casualties.
The spectacular ‘ice pillars’ seen over many parts of North America were easy to explain, though startling to some. They look exactly like pillars of light descending from (or rising into) the sky; Washington Post reporter Kathryn Prociv thought it looked “as if a Canadian town is being abducted by aliens.”
Also known as light pillars, ice pillars are common inside the Arctic Circle, but the cold snap brought them much further south than usual. An ice pillar is an optical illusion, like a rainbow, but caused by reflection rather than refraction. In the cold, moisture in the air crystallises out as tiny flakes. These crystals tend to be hexagonal and flat, and align themselves roughly horizontally as they slowly drift down. The crystals in the air above a light source reflect it back to an observer, giving the illusion of a pillar of light above the source. The effect is most dramatic at night, when different coloured artificial lights – orange, blue, white – send up rods of light.
The low temperature may also cause the sea to freeze over, sometimes with surprising results. Seawater normally freezes at about minus 4˚C, and the ice forms as small needle-like crystals, each 3mm or 4mm long, known as frazil ice. Where the ocean is smooth, the crystals join together in a single unbroken sheet, but in rough water they form into rough, slushy discs known as pancake ice. These are the curious ‘ice circles’ in the 1914 postcard published in the last issue [ FT363:71]. In a rough sea the slushy ice can remain for some time before freezing over, and this gives rise to what are known as ‘Slurpee waves’ after a frozen soft drink. In January, surfers visiting the beach at Nantucket, Massachusetts, were surprised to find waves made of slush where frazil ice had reached the shore. The most eerie aspect of the scene was the silence: rather than the roar of breakers, the slush waves folded over soundlessly. They lasted for several hours, giving local photographers a chance to take spectacular pictures before the sea froze over completely.
Lake Erie also witnessed an unusual phenomenon in January: ice shove. This occurs when an onshore wind pushes a sheet of ice up the beach, causing it to fracture and pile up. The result was a ridge of ice blocks along the shoreline more than 10m (33ft) high and several hundred metres long. Similar ice shoves have been known to destroy lakefront properties, and in this case West Lakeshore Drive was closed for some time as the blocks of ice spilled over it like an icy landslide.
While Britain was not lucky (or unlucky) enough to experience these extreme effects, at least one unusual phenomenon was spotted. Snow rollers are giant, cylindrical snowballs, often hollow in the middle [see FT260:6-7]. They are formed when there is strong wind and a covering of snow damp enough to stick together. The wind lifts up the edge of a sheet of snow and rolls it over; as in the process of building a snowman, the roller picks up snow from the ground and can grow to considerable size.
In January, hundreds of snow rollers were photographed in Lanarkshire, each with a trail behind it. Again, rollers are rare because they require the right balance of conditions. If it is too cold, the snow is dry and does not stick together. The wind has to be strong enough to start rollers going, but not so strong it breaks them up and blows the snow away.
Climate change scientists suggest that severe winters may become more common. In which case ice quakes and Slurpee waves may become the new normal, and the conditions that give rise to them will become better understood.