The big white

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Snow began falling over large areas of Otago and Canterbury on Wednesday, July 8. The heaviest falls were in the foothills, where snow accumulated to depths of one metre or more over several days. Roads were closed, power lines brought down and tens of thou­sands of cattle and sheep trapped.

The snow stayed on the ground for more than a week on the hill country farms, and stock rescue became a major operation involving hundreds of volunteers, including prison inmates, off-duty police and Lincoln Univer­sity students.

Tractors and bulldozers were used to reach stock in the more accessible areas, but helicopters had to be used to fly hay and shep­herds into more remote places. Sometimes the sheep had to be dragged or lifted bodily to safety, but often it was enough to make a path for the animals by walking ahead of them and stamping down the snow. This exhausting process, called “snow raking”, entailed the risk of triggering small but danger­ous avalanches if attempted on the steeper slopes. Hard work in the cold conditions could also cause exposure. Keeping track of all the volunteers and their welfare was an added strain for farmers.

What causes such heavy snow over farmland? It results from the meeting of cold air from the south with warm air from the north. Cold air is unable to contain much water vapour. Consequently, if the air through the entire depth of the atmosphere has come from near Antarc­tica then heavy snow will only fall in the mountains. There may well be some snow to very low levels, but there will not be much of it.

However, when a depression brings warm, humid air down from the north, and this is undercut by very cold air from the south, then a heavy snow­fall is possible.

The warm air rises over the cold air, and both are forced to rise by the hills. As the warm air rises, it cools because of expansion, and this cooling causes water vapour to change to tiny ice particles or cloud droplets. When the tem­perature of the rising air falls below about minus 10°C the ice crystals begin to grow rapidly at the expense of the liquid cloud droplets.

At first the falling snowflakes melt in the air near the ground. However, the act of melting takes heat from this air until it is cooled to near 0°C, and then the snow penetrates all the way to the ground. The more snow there is falling from high levels the faster this cooling takes place.

The amount of snow is also increased if it falls through low-level clouds, where the snowflakes can scavenge some of the cloud droplets and grow bigger.

Paradoxically, the snow acts as an insulator once it is on the ground. In the clear nights that followed the snowfall, temperatures dropped to minus 16 degrees at the top of the snow surface, but the ground temperature stayed near zero. Animals trapped under the snow are shel­tered from the wind, and their body heat can help form small snow caves. (There is even a record of corn germinating and growing to 7cm under snow cover in Devon in 1891.)

In mid winter the sunlight in Canterbury is not strong enough to melt the snow, especially since most of the light is re­flected away by the snow surface. Warm winds or rain are needed for a thaw.

This finally came on Sunday, July 19, when a warm nor’wester developed and the snow began to melt. The strong wind further damaged the forests where the heavy snow had already knocked some trees over and snapped branches off many others.

There was a fear that the thaw would lead to serious flooding, but this did not happen because the nor’wester is a very dry wind, and much of the snow and water evaporated directly into the air.

Disastrous floods have followed a number of heavy snowfalls in the past. In 1868 the Opihi River in South Canterbury ran 11km wide when swollen with melt water. The snowfall and flood of the previous year have been graphically described by Lady Barker in her account of the early days of farming in Canter­bury.

Thousands of animals died in the 1992 snowfall, but exact numbers will not be known until after the spring muster. The cost includes helicopter bills as high as $30,000 for some individual farmers.

Many of the surviving stock were in poor condi­tion from dehydration and lack of food, having had little more than bark or wool to eat. They were prey to illnesses such as sleepy sickness, caused by lack of sugar, and staggers, caused by lack of magne­sium. The remedies for these ailments include, remarkably, strong tea with plenty of sugar—administered with a drench gun—and a couple of jabs of calcium borogluconate next to the ribs.

Lack of feed for the animals reached crisis point in late July, and a plea for help elicited a quick response from other farmers. NZ Rail Ltd waived freight charges of $100,000 on a 95-wagon train bringing 20,000 bales of hay from the North Island, and allowed trucks with hay free passage on the Cook Strait ferries.

More warm nor’westers were needed to promote spring growth and alleviate the need for hay, and to dry low-lying paddocks that had turned to mud when the stock were concentrated on them. The mud attracted large num­bers of seagulls, and it was feared that they would peck out the eyes of any weak sheep that collapsed.

Storms of this severity exact a far greater death toll if they occur in the middle of lambing. Tragically, in the last week of August another severe snowstorm hit Canterbury, and over a million lambs died. This storm was caused by the same mechanism responsi­ble for the July storm, but the snow was heavier in some areas, particularly parts of Banks Peninsula, where drifts of six metres were reported.

Worldwide, the heaviest snowfalls at sea level probably occur on the west coast of Japan. Extremely cold air from the Asian continent picks up mois­ture and is destabilised as it crosses the relatively warm Sea of Japan.

Takada, on the coast of the island of Honshu, has a similar latitude to Auck­land, and is only 20 metres above sea level. It has an annual seasonal snowfall of about 7 metres, but had just under 10 metres in the month of January 1945. During that winter snow covered the ground for more than four months.

One of the worst snow­storms to hit the USA occurred in Buffalo in January 1977. Winds of 60 knots (115km/hr) caused blizzard conditions and snow drifts up to 10 metres deep. Twenty-nine people were killed, nine of whom froze in their vehicles. Total damage was esti­mated at $US250 million, and thousands of tons of snow and ice were railroaded out of town through fear of flooding when the thaw came.

When strong winds are blowing snow around, the deepest drifts form in hollows or sheltered places. During an exceptional snowstorm in the north­east of England in February 1941—not much reported at the time because of war­time censorship—six trains were buried by snow in a cutting near Newcastle.

There were about a thousand people on the trains, and they were only discovered when someone walking over the top heard their voices coming up through the snow.

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