Editing Nor'west arch

{{Short description|Föhn cloud formation in New Zealand}}
{{Use dmy dates|date=September 2024}}
{{Use New Zealand English|date=September 2024}}

[[File:Nor'west arch2.JPG|thumb|right|250px|The nor'west arch, viewed from [[Christchurch]], appears as a blue belt of clear sky to the west from which high white cloud streams.]]
The '''nor'west arch''' is a band of high white [[lenticular cloud]] that forms on the eastern side of New Zealand's [[South Island]] and which looks like an arch in an otherwise clear blue sky over the [[Southern Alps]]. It is accompanied by a strong hot northwesterly or northerly wind known as a "nor'wester". In [[Canterbury Region|Canterbury]], where it is a well known feature, it is also called the '''Canterbury arch'''. It also occurs in [[Otago]] and [[Marlborough District|Marlborough]] and east of mountain ranges along the east coast of the [[North Island]].<ref name=NIWA_clouds/>

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 wind|Chinook arch]] seen in the Pacific regions of the United States and Canada.

==Formation==
[[File:Foehn effect mechanisms.png|thumb|The nor'wester is a [[föhn wind]] – moist air is driven over the [[Southern Alps]], falling as condensation on the western side, leaving the eastern plains in the [[rain shadow]].]]
[[File:Northwest arch new zealand canterbury.jpg|thumb|A typical Canterbury nor'west arch at sunset]]
The nor'west arch is a [[föhn cloud]] or a [[lenticular cloud]].<ref name=NIWA_clouds>{{cite web |title=Clouds |url= https://niwa.co.nz/climate-and-weather/clouds |publisher=[[National Institute of Water and Atmospheric Research|NIWA]] |access-date=22 September 2024}}</ref> The northwesterly wind drives warm moist air from over the [[Tasman Sea]], 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]] of the Alps; much of the moisture is dumped on the [[West Coast, New Zealand|West Coast]], and is responsible for the [[temperate rainforest]]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 rain and sometimes snow which may settle to sea level.

==Occurrence==
The nor'west arch can be seen as far north as [[Amberley, New Zealand|Amberley]] and as far south as [[Central Otago]], but it is at its most prominent on the [[Canterbury Plains]], 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]]s. A front lying across the South Island will often extend from northwest to southeast, reaching northern parts of the [[West Coast, New Zealand|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]] force and cause isolated damage to trees and buildings.<ref name="RNZ_64745">{{cite news |url=http://www.radionz.co.nz/news/national/64745/dunedin-homes-without-water-as-wind-topples-trees |title=Dunedin homes without water as wind topples trees |date=21 December 2010 |work=[[Radio New Zealand]] |accessdate=1 November 2011}}</ref> Once every few years, a nor'wester will approach [[hurricane]] strength and cause widespread damage.

==Effects==
The heat and lack of moisture characteristic of nor'westers play a major role in the intermittent [[drought]]s experienced by Canterbury and other regions on New Zealand's eastern coasts.{{citation needed|date=August 2020}} 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.<ref>Erick Brenstrum, [https://www.nzgeo.com/stories/weather/ Canterbury's damaging nor'wester], New Zealand Geographic. Reprinted from ''[[New Zealand Geographic]]'' No.1 (Jan–Mar 1989). Accessed 2007-06-17.</ref>

{{quote|About 10 percent of people affected by the nor’wester feel elated and wonderful. But the rest feel depressed, irritable, and lacking energy. People feel they can’t cope with everyday things. ... There is irrational anxiety and a sense of foreboding.|[[Neil Cherry]]<ref>[http://www.prairiepublic.org/programs/plainsfolk/transcripts/Chinooks.jsp Chinooks], Plains Folk, Prairie Public Broadcasting. Accessed 2007-06-17.</ref>}}

==See also==
*[[Lee waves]]
*[[Southeast Australian foehn]]
*[[Roaring forties]]
*[[Taieri Pet]]

==References==
{{reflist}}

==External links==
{{Commons category|Nor'west arch}}

[[Category:Winds]]
[[Category:Climate of New Zealand]]
[[Category:Föhn effect]]