New Zealand weather patterns
New Zealand is a narrow and mountainous country. Geographical isolation makes it very exposed to the anticyclones and low pressure troughs which continually move east across the Tasman Sea.
The moist westerly airstreams are forced over the mountain ranges losing their moisture, creating both snow and rain. New Zealand is also exposed to occasional air masses from Antarctica, the southern oceans and tropical regions moving across the country.
The mountains' effect on the weather
The Tongariro National Park volcanoes, the Kaimanawa Ranges to the east and the high country to the south, all influence the weather of the park.
There is no wet or dry season or month in the park. The weather pattern is highly unpredictable, and rain and snow can fall at any time of the year.
In general, air masses rise over mountains. However, because the volcanoes stand alone, winds tend to funnel around their upper faces bringing heavy rainfall to all sides of the summits.
This flow of air is accelerated through Tama Saddle, between Ruapehu and Ngauruhoe. As the north westerly winds gather speed through here, they have a significant effect on Rangipo Desert on the eastern side. Drying winds are a major factor in creating a desert environment.
While the prevailing wind is westerly, Ruapehu is exposed to all winds. Winds from the west to the north west bring the rain but carry only light falls of snow. Southerly and easterly winds tend to be colder and drier, bringing snow rather than rain.
When the prevailing westerlies and north westerlies are forced to rise over the volcanoes, rainfall is increased. As a result, at least half the days in the year bring rain to the western and northern parts of the park.
Most months have high levels of rainfall. The expected rainfall per year varies around the Central plateau. Whakapapa Village receives around 2,200 mm of rain per year. In the south at Ohakune about 1,250 mm fall, with over 1,500 mm falling on Rangipo annually.
Daily and monthly temperatures can vary greatly. At Whakapapa Village frosts can occur all year round. However, temperatures average 13°C with a maximum temperature of 25°C and a mid-winter minimum of -10°C.
Frost can have a major impact on the alpine environment and cause severe erosion on exposed soils. A useful measure of the growth season is the summer period between the last frost in spring and the first autumn frost. In the vicinity of Whakapapa Village, it is 120 days long. In comparison with a growth season of less than 90 days in the Desert Road area. Frost is a significant feature of the eastern 'desert' quarter of the park.
In winter the snow level extends down to an average of 1,500 metres, and often lower. Winter alpine conditions create popular recreational opportunities such as skiing, snowboarding and mountaineering, and the hiking tracks are still available to experienced and properly equipped people. Conditions can vary between soft snow and hard ice, with avalanche risk often present.
Often snow falls as early as March or as late as November, and sometimes during the summer months.
During summer, snow is sometimes still present in the craters of Ngauruhoe and Tongariro, and there are permanent glaciers and snowfields high on Ruapehu.