Nor'west arch

[1] 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, and is responsible for the temperate rainforests found there.

The nor'west arch can be seen as far north as 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 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.

The heat and lack of moisture characteristic of nor'westers play a major role in the intermittent droughts experienced by Canterbury and other regions on New Zealand's eastern coasts.

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'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 .
A typical Canterbury nor'west arch at sunset