Prepare for the worst up the mountain

By COLIN MOORE

Here's something to think about: any snow on a slope steeper than 25 deg could avalanche. Here's something else: a snow slide never knows that you're an avalanche expert. The late Dave McNulty, New Zealand's foremost avalanche guru, used to write that on the blackboard at his avalanche courses. It was tragically prophetic.

McNulty was killed in an avalanche while checking the stability of a snow slope. But his pioneering work in teaching about the dangers of snowpack stability lives on in a range of courses.

And as more skiers and boarders escape from the madding crowd and head into the alpine backcountry they need to think seriously about beefing up their knowledge of alpine conditions. Avalanches have already claimed the lives of touring snowboarders in Australia and Japan.

Avalanches most often happen on slopes of around 37 deg. There is a danger of an avalanche whenever there is a poor bond between snow layers, so one layer can slide on another.

Wind and sun tend to be the major causes. Wind blowing over a ridge leads to a build-up of snow on a leeward slope, so a good rule of thumb in the backcountry is to avoid lee slopes after storms. A heavy dump of fresh snow will settle and stabilise more rapidly on the sunny, north-facing slopes while the shady, south-facing slopes will remain unstable for long periods.

Because of the sun factor, wet, loose snow slides are more common on north-facing slopes and deep, slab avalanches on south-facing slopes.

All backcountry skiers and boarders should wear an avalanche transceiver, or trail an avalanche cord, so they can be more easily found if they are buried by a slide. They should also avoid gullies and steep, open lee slopes and cross a potentially dangerous area only one at a time.

Hollywood is responsible for some misconceptions about avalanches. A fallen person can be trapped by a slide just a few centimetres deep in the same way that babies can drown in just a few centimetres of water. When a snow slide stops, the avalanche debris sets hard, making it difficult for a trapped person to break free.

Another misconception is that avalanches happen only in the high mountains of the South Island - the backcountry slopes of Tongariro National Park are just as prone to avalanche.

Mark Woods, head of the Mt Ruapehu ski patrol, says the park creates distinct safety problems because its backcountry is so readily accessible. He warns against the temptation to be adventurous and go outside patrolled areas without some alpine education.

You should be particularly wary when there has been 30cm or more of new of wind-blown snow; when there is a hollow feeling or sound under the snow; when fresh snow is followed by warm weather or rain; and when there have been rapid changes in temperature.

A daily report on backcountry snow conditions is available from the Mt Ruapehu ski patrol or from the Department of Conservation ranger station at Whakapapa.

Probably the greatest danger in the New Zealand backcountry is from weather extremes. There is no high land mass between Australia and South America other than our mountains and they will cop whatever weather patterns are flung across the southern oceans.

The usual weather pattern in New Zealand is from west to east. High-pressure systems (anticyclones or ridges) usually bring fine weather, but as they move away and are followed by troughs and depressions (low pressure) there is often bad weather.

New Zealand's mountain chains - including the North Island trio in Tongariro National Park - lie abreast the prevailing westerly air flows, so air is forced to rise when it hits them, which increases the severity of mountain weather.

Bad weather also starts earlier and usually goes on later in the mountains than on the plains, and will be felt on both sides of ridge tops even though there may be fine weather in the lowlands on the leeward side.

The Met Service regularly updates mountain weather forecasts, which are also available from the Department of Conservation ranger stations. No one should venture into the alpine backcountry without having first checked the weather forecast. But most important of all, you must be prepared for the worst.