The Nor'west arch is a weather pattern peculiar to the east coast of New Zealand's South Island. For this reason, it is also often referred to as the Canterbury arch. It is shown in an apparent arch of high white cloud in an otherwise clear blue sky over the Southern Alps, and is accompanied by a strong hot northwesterly wind simply known as The Nor'wester.

The northwesterly wind in the South Island is a föhn wind. Warm moist air from over the Tasman Sea is pushed up by the presence of the Southern Alps, causing it to cool rapidly. Much of the moisture is dumped on the West Coast, and is responsible for the temperate rain forests 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.

The Nor'west arch can be seen from 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 Nor'wester, when it hits, is not only responsible for Canterbury's intermittent droughts, but also has a deep psychological effect on people subjected to its hot, dry nature. It has been statistically linked to increases in suicide and domestic violence.

Research has indicated that the mechanism is a significant buildup of positive ions, strongest at the coast, and electric fields of this strength have been shown to upset the melatonin/seratonin balance in some people.

See also

• lee waves

External links

• Dr Neil Cherry's research
• Meteorological explanations of the Nor'wester
• New Zealand Meteorological Service Learning Centre