Amaz­ing light pil­lars

DAVID HAMBLING makes the most of his win­ter by search­ing out some chilly weather phe­nom­ena

Fortean Times - - Contents -

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.

LEFT: A snow roller in La­nark­shire.

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