New Zealand is roughly midway between Antarctica and the tropics, lying between latitudes 34° and 47° south. It is on the edge of the ‘roaring forties’, a zone of high winds and stormy seas. The northern outlying islands are subtropical, while those in the south are subantarctic. Between these extremes, New Zealand’s climate is cool to temperate, but can vary widely, even within one day. The saying ‘raining at seven, fine by eleven’ often rings true for weather forecasters.
The weather pattern
At New Zealand’s latitude, warm moist air from the tropics meets cold dry air from Antarctica. The two don’t mix: they twist around and bump into each other. These swirling air masses sweep over New Zealand from the west.
Large volumes of stable, dry, descending air are characteristic of high-pressure systems, often termed ‘highs’, which bring settled weather. Eastward-travelling highs cross the country about once a week, on average. Between the highs are areas of unstable, ascending air known as low-pressure systems, or ‘lows’.
Within the air systems there are often fronts – the boundary between a cold and warm air mass. Fronts produce one of the most common weather sequences. As a cold front approaches from the south-west, north-westerly winds bringing warmer air across the Tasman Sea strengthen and cloud increases. It rains for several hours as the front passes over, with a subsequent change to cold showery south-westerly or southerly winds.
New Zealand’s weather is largely produced by this endless procession of highs and lows.
New Zealand’s seasons roughly follow this pattern:
- Spring arrives in September or October, as the days lengthen and yellow kōwhai blooms mark the end of the coldest weather. Springtime is often windy and it is the season with the most variable weather. National average temperature: 12.1°C.
- Summer is cool to mild in the south, and mild to warm in the north. Although Christmas Day is just after the summer solstice, the weather is often still unpredictable. As the days start to shorten in January and February, there are often long fine spells. Average temperature: 16.6°C.
- Autumn is the most settled season. In the far north an extended ‘Indian summer’ is a very real prospect. Average temperature: 13.3°C.
- Winter is cold, especially in the South Island and inland areas. Southerly blasts coat the mountains on both islands with snow. Average temperature: 8.3°C.
At any time of year, it is quite possible to experience four seasons in one day.
The wind comes to New Zealand mainly from the west. In the country's southern latitudes there is no other land apart from the tip of South America. Nothing gets in the way of these ‘brave west winds’, as mariners call them; they hit New Zealand after travelling 10,000 km over the ocean.
An ill wind
The nor’wester dumps metres of rain on the West Coast, then rises over the Southern Alps and descends on the Canterbury Plains in hot, dry gusts. When Canterbury’s dust-laden nor’wester blows, suicide rates go up and people get headaches or become grouchy. At gale force, it does serious damage to farms and buildings. Known to Māori as parera, it is one of many regional seasonal winds, including France’s mistral, California’s Santa Ana, and the Chinook of western Canada. In winter, Canterbury’s ‘mad dog’ is tamed, bringing welcome mild weather.
The early European explorers who ventured onto upland areas were blown off their feet. Māori would not have been surprised – they had more than a dozen terms for different winds.
One of the most readily recognisable regional winds is Canterbury’s nor’wester – a hot, dry wind. And everyone in the east and south knows the southerly, which blasts up from the subantarctic.
The distribution of New Zealand’s usually ample rainfall is greatly affected by the mountains. The general rule is that east is drier, west is wetter. In populated areas, the mean annual rainfall varies from 350 mm in Central Otago to over 6,000 mm at Milford Sound, on the south-west coast. The highest recorded annual rainfall is over 18,000 mm, at Cropp River (Hokitika catchment) on the West Coast, where the average annual rainfall is a pluvial 11,500 mm. Most areas receive 600–1,500 mm a year, and large areas of both islands over 2,500 mm.
As New Zealand lies just west of the International Date Line, Chatham Islanders are the first people to see the sun rise each day. The sun beams down on Blenheim and Nelson (in the South Island), and Whakatāne (in the North Island), where average annual sunshine hours exceed 2,350. Other coastal areas of Bay of Plenty and Napier also bask in the sun. Many retired people move to the sunnier northern and eastern areas. But the New Zealand sun is harsh, and Auckland has one of the highest rates of melanoma (skin cancer) in the world.
Much of the country gets at least 2,000 sunshine hours a year, and even rainy Westland, which tourists dub ‘Wetland’, has 1,800 hours. Annual sunshine hours drop to around 1,700 a year in Southland and coastal Otago.
A nice cold beer
In Central Otago they are used to the chill, but in July 1991 even hardy locals were tested. When a high-pressure system brought clear skies and intense frosts, the temperature dropped to -15°C for days on end. For the first time in at least 100 years the Shotover River froze over. Power lines snapped, weighed down by sausages of ice. Sheep’s coats froze to the ground, water pipes burst, and diesel fuel in engines turned to sludge. The greatest challenge came when the beer inside pubs froze.
Northland’s reputation as the ‘winterless north’ is well earned. By contrast, in parts of the deep south the winter chill approaches that of regions in similar latitudes in Europe. Average coastal temperatures range from about 15°C in Northland to about 10°C in Southland. January and February are the warmest months, and July is the coldest.
The highest recorded temperature of 42.4°C occurred in the South Island simultaneously at Rangiora (Canterbury) and Jordan (Marlborough) on 7 February 1973. In 1903, a numbing -25.6°C was recorded at Ranfurly in Central Otago.