Editing Nor'west arch (section)

==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.