The Nor'west arch is a weather pattern peculiar to the east coast of New Zealand's

South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...

. For this reason, it is also often referred to as the

Canterbury Canterbury (, ) is a cathedral city and UNESCO World Heritage Site, situated in the heart of the City of Canterbury local government district of Kent, England. It lies on the River Stour. The Archbishop of Canterbury is the primate of t ...

arch, although it is visible in both

Otago Otago (, ; mi, Ōtākou ) is a region of New Zealand located in the southern half of the South Island administered by the Otago Regional Council. It has an area of approximately , making it the country's second largest local government reg ...

and Marlborough as well as in the Canterbury Region. It is shown in an apparent arch of high white cloud in an otherwise clear blue sky over the

Southern Alps The Southern Alps (; officially Southern Alps / Kā Tiritiri o te Moana) is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the range's western side. The name "Southern ...

, and is accompanied by a strong hot northwesterly or northerly wind simply known as The Nor'wester. Closer to the Canterbury coast, some distance from the mountains of the Southern Alps, it appears as a clear area of blue above the mountains, with white cloud streaming to the east from it. The phenomenon is similar to the Chinook arch seen in the Pacific regions of the United States and Canada.

Formation

The Nor'west arch is a föhn cloud. The northwesterly wind drives warm moist air from over the

Tasman Sea The Tasman Sea (Māori: ''Te Tai-o-Rēhua'', ) is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer ...

, and it is pushed up by the presence of the Southern Alps, causing it to cool rapidly. The area to the east of the divide is in the

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is ca ...

of the Alps; much of the moisture is dumped on the West Coast, and is responsible for the

temperate rainforest Temperate rainforests are coniferous or broadleaf forests that occur in the temperate zone and receive heavy rain. Temperate rain forests occur in oceanic moist regions around the world: the Pacific temperate rain forests of North American ...

s found there. As the air passes over the alps, the water vapour remaining becomes visible in a band of cloud over the mountains at the top of each wave of air. From the perspective of a viewer on the eastern side, this appears as an 'arch' of cloud. The standing wave or arch is caused by the moisture condensing and becoming visible towards the top of the wave and then evaporating again as the air descends to the trough of the wave. Nor'westers caused by cold fronts will often change within a day or two to a cool southerly wind accompanied by rain showers, as the front passes through. In mid-winter, a Nor'wester is often followed by a brief but intense cold snap, frequently bringing thunder, hail, or snow and sometimes rain which may settle to sea level.

Occurrence

The Nor'west arch can be seen as far north as

Amberley Amberley may refer to: Places Australia *Amberley, Queensland, near Ipswich, Australia *RAAF Base Amberley, a Royal Australian Air Force military airbase United Kingdom * Amberley, Gloucestershire, England * Amberley, Herefordshire, Englan ...

and as far south as

Central Otago Central Otago is located in the inland part of the Otago region in the South Island of New Zealand. The motto for the area is "A World of Difference". The area is dominated by mountain ranges and the upper reaches of the Clutha River and tribu ...

, but it is at its most prominent on the

Canterbury Plains The Canterbury Plains () are an area in New Zealand centred in the Mid Canterbury, to the south of the city of Christchurch in the Canterbury region. Their northern extremes are at the foot of the Hundalee Hills in the Hurunui District, and in t ...

, due to the flat and low-lying nature of the land to the east of the mountains. The Nor'wester can blow at any time of year, but is less common in winter. Many of the strongest northerly and northwesterly winds blow ahead of

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

s. A front lying across the South Island will often extend from northwest to southeast, reaching northern parts of the West Coast before it reaches corresponding areas on the east coast. The steep air pressure gradient ahead of the cyclonic system associated with a front gives these nor'westers their strength; they will commonly reach

gale A gale is a strong wind; the word is typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as sustained surface winds moving at a speed of between 34 and 47 knots (, or ). Once every few years, a nor'wester will approach

hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...

strength and cause widespread damage.

Effects

The heat and lack of moisture characteristic of nor'westers play a major role in the intermittent

drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...

s experienced by Canterbury and other regions on New Zealand's eastern coasts. The Nor'wester has a deep psychological effect on many people subjected to its hot, dry nature. It has been statistically linked to increases in suicide and domestic violence.Erick Brenstrum
Canterbury's damaging nor'wester
New Zealand Geographic. Reprinted from '' New Zealand Geographic'' No.1 (Jan-Mar 1989). Accessed 2007-06-17.

References

{{Commons category, Nor'west arch

External links

Meteorological explanations of the Nor'wester
Winds Climate of New Zealand Anomalous weather

Formation

Occurrence

Effects

See also

References

External links