The big freeze

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Although the cannibal rabbits of Central Otago, on close scrutiny, turned out to be only suffering from frost-bitten ears, the big freeze in July still provided plenty of the stuff of legend. Beer froze in some pubs, sheep froze to the ground, hands froze on metal gates, diesel turned to sludge, water pipes burst and icy roads caused dozens of accidents throughout Canterbury and Otago. One car skidded into Otago Harbour at Blanket Bay, and the occupant was rescued by two teenagers who swam out to the car.

At a trouble spot near Wanaka, Traffic Officer Brent Rissman attended three accidents in the space of a few minutes. As he helped one driver whose car had rolled down a steep bank, another car left the road and came down the bank towards them, stop­ping just in time. It had swerved to avoid two other cars that had collided head-on. Miraculously, none of the seven people involved was seriously injured.

At Mount Ida station, poledale sheep became stuck to the ground during the night, when their urine froze, and they had to be pulled free in the morning, leaving tufts of wool sticking to the ground. Metal surfaces such as gates became so cold that skin would freeze on contact and be torn off if the hand was pulled away. In many places, drinking water had to he taken from streams—after their covering of ice had been broken.

Then, freezing fog came and filled the valleys for days, covering everything in such beautiful thick rime ice that there was a minor tourist boom.

How (lid all this come about? Not through a storm of the type needed to produce snow, but through the fine weather and clear skies of a large, slow-moving anticyclone.

Because of its warmth, the Earth radiates heat in (It would only stop doing this if it were cooled to minus 273 degrees Celsius —absolute zero.) The Earth radiates heat both day and night, but we only notice the loss of heat at night is continually being mixed with air from close to the ground. However, the ground itself becomes cold—and so do parked cars, which may be hard to start.

In winter, if an anticy­clone is slow-moving and lasts for many days, the warming during the brief daytime will not make up for the cooling during the long night. Then each day becomes progressively colder. This is even more likely to happen if the ground is covered in snow or ice, as this reflects a lot of the sunlight straight back to space. (Incidentally, this is the reason why people on ski-fields can get sun­burnt under their chins.)

Once the ground has become colder due to heat loss by radiation, it cools the air next to it by conduc­tion. In the absence of a wind strong enough to mix this air with air above, a thin skin of cold air develops over the land. Because this cold air is now heavier than the air just above it, it will slide down slopes and collect in valleys. The temperature at the base of one of these ponds of cold air can sometimes be 15 degrees Celsius colder than a nearby ridge top.

Although the air in Canterbury and Otago was initially dry, with very low humidity, eventually it became so cold in the valleys that the relative humidity rose to 100 per cent, and fog formed. Even though the temperature of the fog was well below zero, it was still made of tiny droplets of liquid water. This supercooled water freezes as soon as it touches something. This means that even the lightest wind blowing the fog past an object such as a fence-wire or twig will cause ice to build up on the surface facing the wind.

The ice can be built out many centimetres, and often has the appearance of having been blown out by a wind in the opposite direction. The build-up of ice on structures such as power lines can be enough to cause their collapse. Ice from supercooled water is often encountered by aircraft in clouds, and can cause them to crash if they are not equipped for it.

After nearly two weeks of exceptionally cold weather, a warm northerly airstream finally put an end to the cold snap, but not before some rain had fallen. Since the ground was already so cold, the rain froze as it landed, and many roads were coated in a sheet of clear ice, result­ing in another spate of accidents.

For those who endured these conditions and dread their return next year, there is this consolation: a recent international survey has found that cold weather is associated with a reduced crime rate. Ice makes a great policeman!

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