On the afternoon of Monday August 13, a cold southerly change swept up the South Island putting paid to the campaign to boost electricity savings to 10 per cent.
Snow fell to about 300 m above sea level, but was not particularly heavy, with amounts generally less than 5 cm for places below 500 m and a bit more higher up. Skifields received between 20 and 40 cm.
The cold wind, combined with rain or snow, was stressful for farm animals coping with a long drought, but the worst of it was over in a day or less and stock losses were minimal. In fact, many farmers were grateful for the water, which will give a valuable boost to spring growth.
All the highways remained open, although chains were required on some higher stretches such as the Lindis Pass, and there was a spate of minor accidents.
On Tuesday, the southerly spread over the North Island. Initially, conditions were similar to those in the south, although the snow level was higher. Overnight, however, the storm intensified over the central North Island and heavy snow fell to levels where it had not been seen for 20 years or more. Snow blocked the roads through the central North Island as well as the Napier Taupo highway.
The main trunk railway line was also blocked—the first time in over a decade that snow forced the cancellation of the Northerner express.
Fourteen trucks were abandoned in snow on either side of Waiouru. Soldiers using the army's Unimog trucks rescued the drivers and took them to a makeshift soup kitchen set up in the Waiouru Camp cinema. Snowdrifts in the camp were 1.5 m deep.
The weight of the snow broke branches off many trees and brought down power lines, cutting electricity to 8000 homes between Marton and Turangi. The deep snow hampered the movement of repair crews, who had to use helicopters to gain access to some backcountry areas, where power was not restored for several days.
Snow or slips blocking access roads closed many country schools. A large slip of trees, rocks and mud cut State Highway 1 at the top of Vinegar Hill, 6 km north of Hunterville.
Con Vasil, at the Taihape Motel, described the fall as the heaviest in 34 years.
On the other side of the ranges, snow fell to places as low as Norsewood and Ormondville, and heavy rain in Dannevirke caused surface flooding.
In the middle of the night, swiftly rising floodwaters from the Manawatu River captured a mail truck near the Opiki Bridge. The driver climbed on to the roof of the cab holding a torch. After one and a half hours in the cold wind he was rescued by helicopter. It was too dangerous to lower a line, so the helicopter hovered above the truck while the driver climbed on to its skids.
Farmers in southern Wairarapa suffered the largest stock losses on account of the wind and rain. Some lost hundreds of lambs, but most farmers were happy to get the rain. Happiest of all were skifield operators on Mount Ruapehu, where between half and one metre of snow fell.
The wind was at its strongest around Cook Strait. The mean speed was over 110 km/h at The Brothers, with gusts to 130 km/h. Waves were frequently over four metres in height, and a few were observed as high as nine metres. Sixteen ferry railings were cancelled over a period of 30 hours.
Wellington Airport stayed open, but air traffic controllers increased the separation between flights because of turbulence. Electric trains were also delayed when salt spray affected overhead wires.
Trees were blown down in many suburbs, as well as one power pole in Petone, and a number of buildings were damaged. The fire service answered more than 40 calls for assistance because of floods, downed power lines or roofs lifting off houses. Blowing sand was a problem at Lyall Bay, where it drifted half a metre deep on the landward side of the seawall and was 10 cm deep outside some houses.
Although it was the first day of the whitebait season, the conditions in Wairarapa prevented anyone going fishing at Lake Ferry, while at Castlepoint breaking waves were reported to be reaching as high as the lighthouse.
The storm was much worse over the North Island than the South Island for a number of reasons. The low that formed east of Hawkes Bay by Wednesday morning (see map) caused a tongue of relatively warm, moist air from north of New Zealand to rotate southwards around the low centre, and so ride up over the cold air that had arrived from south of the country. The upward motion caused by the warm air rising over the colder, denser air was further increased by the air being driven up and over the central North Island ranges by the easterly winds on the poleward side of the low.
Warm air is able to carry much more water vapour than cold air. When the southerly spread up the South Island, the air was very cold throughout the depth of the atmosphere, as all the air was coming from well south of New Zealand. This air contained a relatively small amount of water vapour, and so could produce only a small amount of snow.
Aside from its effect of combining warm and cold air, the low east of Hawkes Bay increased the intensity of the rain and snow over the North Island by directing gale force winds at right angles towards the mountains. This caused very strong upward motion, thereby increasing the rate at which the air cooled and so speeding up the rate at which water vapour condensed to form snow and rain inside the storm. Over the South Island, by contrast, the southerly wind struck the mountains at a more oblique angle.
The fact that the strong winds around the low were blowing perpendicular to the line of the mountains also increased the funnelling effect of the wind through Cook Strait.
It seems paradoxical that for heavy snow to fall to low levels over New Zealand a tongue of relatively warm air usually needs to feed down into the storm from the north. The same meteorological principle explains why climate warming may produce heavier snowfalls over some parts of the earth.
For example, despite the fact that most alpine glaciers around the world have been observed to be in rapid or steady retreat, it is possible that in some very cold areas, such as Greenland or Antarctica, heavier snowfalls will occur in the future, increasing the thickness of the ice sheets in places. A warming of air temperature by several degrees in these areas would still leave the ambient temperature well below freezing, but if the storms coming to these places from mid latitudes contain warmer air in the future, because of global warming, then that air will contain more water vapour and hence deliver more snow.
As the August storm subsided, attention turned to the cost of the damage. Chris Ryan from the Insurance Council warned that total claims from this winter's weather could reach tens of millions of dollars. Not all of the damage was caused by storms: July's long-lasting anticyclone produced severe frosts in inland South Island areas, causing burst water pipes and extensive damage to some houses once the pipes unfroze again.
As the economic value of property and possessions continues to rise in many countries, the cost of patching things up after the weather has had a rampage also rises. It is sobering to realise that the eastern seaboard of the United States alone has hundreds of trillions of dollars' worth of real estate in areas vulnerable to hurricanes.
Expect insurance premiums to match the movement in temperatures: upwards.