The warmest year on record has put New Zealand's snow season off to a sluggish start - and today forced operators on Mt Ruapehu to push back ski area openings that were scheduled this weekend.

The delay at Whakapapa and Turoa was the latest of several postponements at skifields around the country following an unusually balmy season lead-up.

But a climate scientist says one big event was all that was needed to kick the slopes into action.

Weather-permitting, the beginner trails on both Ruapehu ski areas would be open in time for the school holidays.

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When the Herald visited Whakapapa today, there was a conspicuous absence of snow around the lower mountain as staff were busy making final preparations for the annual visitor influx.

Ruapehu Alpine Lifts chief executive Dave Mazey said the mountain hadn't received as much snow this year as it did on average, but he noted the 2015 season also began similarly before good amounts arrived in mid-July.

The forecast for the next week included a low pressure system coming through next Thursday night, which could bring up to 15cm of natural snow on our upper slopes.

This was expected to be followed by cold clear conditions through the weekend which would provide good snow-making conditions on the lower slopes.

"We are optimistic this weather pattern will enable us to open beginner trails at both ski areas next week and on time for the school holidays," Mr Mazey said.

"We've already had two reasonable snowfalls so we have a base on the upper mountain. We expect that with one or two more heavy snowfalls we will be able to open for intermediate and advanced skiing at both Whakapapa and Turoa.

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"We understand the delayed opening of ski trails is disappointing for our customers, but the teams at both ski areas are all trained and ready to go as soon as conditions allow."

Visitors to Whakapapa would still be able to ride the ski area's newest lift, the Rangatira Express, along with the Waterfall Express lift, which reached the country's highest cafe.

Ski Patrol members on the upper slopes of Whakapapa Ski Area on Mt Ruapehu. Photo / Alan Gibson
Ski Patrol members on the upper slopes of Whakapapa Ski Area on Mt Ruapehu. Photo / Alan Gibson

What makes a good snow season?

In ideal seasons, snowpacks are kept healthy by mountain temperatures that stay below freezing levels, along with frequent fresh falls that produce much-revered "powder days".

In bad seasons, unusually warm temperatures melt the snowpack and bring rain instead of the white stuff.

June had been a "perfect example" of the latter, said Dr Gregor Macara of the National Institute of Water and Atmosphere.

"There was some really good snowfall in late May and early June which had the ski areas, particularly in the South Island, looking great.

"But subsequent persistent warm temperatures have melted a considerable proportion of the snowpack and we've had numerous rain events up the ski areas as well, such that Coronet Peak has had to close, and others like Treble Cone and Porters, have had to delay their opening date."

The balmier weather down could be put down to a combination of factors - but namely lingering, warmer than normal sea surface temperatures surrounding the country.

The higher rate of northwesterly air flows seen throughout last month were also forecast to persist until at least the end of August.

"So there are indications which suggest the likelihood of a good snow season are reduced, but I'm not willing to write it off as heading for a poor season just yet.

"It only takes one big snow event to create a decent snowpack - we can't say for certain if or when that will happen, but history suggests invariably the snowfalls will arrive."

As to whether skiers get an extended season - previous late falls have prompted bonus "Snowvember"promotions - Dr Macara said this also looked unlikely.

"For a decent snowpack to last into November, ideally you want a deep snowpack to build over the course of winter and early spring, and cooler than normal temperatures to persist through spring."

In the bigger picture, climate change was continuing to threaten the ski industry.

Shrinking glaciers over recent decades suggested the warming climate had already had an impact on alpine areas.

Snowpack depths would still vary year by year, Dr Macara said, but the long-term trend was increasing temperatures and the freezing point climbing to higher elevations.

"As a result, we can expect an overall trend of reducing snowpack depths at our ski areas in the coming decades."

Higher freezing levels would hamper the ability to produce artificial snow at ski areas.

"While we'll still have good ski seasons, the likelihood is that these will become fewer and
farther between."

What makes snow?

Snowfall is frozen precipitation, and therefore, moisture is needed and temperatures must be at or below freezing for a snow crystal to form in the atmosphere, Dr Macara said.

Water vapour accumulates on the snow crystal causing it to grow.

When the snow crystal is heavy enough it will fall from the atmosphere towards the ground surface.

"Snow crystals collide and often attach to others which create the relatively large snowflakes we see falling in New Zealand towns and cities."

At New Zealand latitudes, most precipitation begins as snow in the upper troposphere.

If temperatures are warm enough lower down, these snow crystals melt and arrive at the surface in liquid form; in colder conditions and at higher altitudes, the crystals arrive as snowflakes.