Nor'west arch
Encyclopedia
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 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.
The northwesterly wind in the South Island is a foehn 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 rainforests 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 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 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
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. Once every few years, a nor'wester will approach hurricane strength and cause widespread damage.
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.
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.
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. Research by the environmental scientist Neil Cherry
indicated that the mechanism is a significant buildup of positive ions, strongest somewhat inland from the coast, and electric fields of this strength have been shown to upset the melatonin
/serotonin
balance in some people.
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
's South Island
South Island
The South Island is the larger of the two major islands of New Zealand, the other being the more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, to the south and east by the Pacific Ocean...
. For this reason, it is also often referred to as the Canterbury
Canterbury, New Zealand
The New Zealand region of Canterbury is mainly composed of the Canterbury Plains and the surrounding mountains. Its main city, Christchurch, hosts the main office of the Christchurch City Council, the Canterbury Regional Council - called Environment Canterbury - and the University of Canterbury.-...
arch. It is shown in an apparent arch of high white cloud in an otherwise clear blue sky over the Southern Alps
Southern Alps
The Southern Alps is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the island's western side...
, 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
Chinook wind
Chinook winds , often called chinooks, commonly refers to foehn winds in the interior West of North America, where the Canadian Prairies and Great Plains meet various mountain ranges, although the original usage is in reference to wet, warm coastal winds in the Pacific Northwest.Chinook is claimed...
seen in the Pacific regions of the United States and Canada.
The northwesterly wind in the South Island is a foehn wind. Warm moist air from over the Tasman Sea
Tasman Sea
The Tasman Sea is the large body of water between Australia and New Zealand, approximately across. It extends 2,800 km from north to south. It is a south-western segment of the South Pacific Ocean. The sea was named after the Dutch explorer Abel Janszoon Tasman, the first recorded European...
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
West Coast, New Zealand
The West Coast is one of the administrative regions of New Zealand, located on the west coast of the South Island, and is one of the more remote and most sparsely populated areas of the country. It is made up of three districts: Buller, Grey and Westland...
, and is responsible for the temperate rainforests 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 as far north as Amberley
Amberley, New Zealand
Amberley is a town located in the Hurunui District of north Canterbury, on the east coast of the South Island of New Zealand. It is located on State Highway 1 approximately 50 km north of Christchurch...
and as far south as Central Otago
Central Otago
Central Otago is the inland part of the New Zealand region of Otago in the South Island. The area commonly known as Central Otago includes both the Central Otago District and the Queenstown-Lakes District to the west....
, but it is at its most prominent on the Canterbury Plains
Canterbury Plains
The Canterbury Plains are an area in New Zealand centred 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 the south they merge into the plains of North Otago beyond the Waitaki...
, 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
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...
s. A front lying across the South Island will often extend from northwest to southeast, reaching northern parts of the West Coast
West Coast, New Zealand
The West Coast is one of the administrative regions of New Zealand, located on the west coast of the South Island, and is one of the more remote and most sparsely populated areas of the country. It is made up of three districts: Buller, Grey and Westland...
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
Gale
A gale is a very strong wind. There are conflicting definitions of how strong a wind must be to be considered a gale. The U.S. government's National Weather Service defines a gale as 34–47 knots of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are...
force and cause isolated damage to trees and buildings. Once every few years, a nor'wester will approach hurricane strength and cause widespread damage.
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.
The heat and lack of moisture characteristic of nor'westers play a major role in the intermittent drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
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. Research by the environmental scientist Neil Cherry
Neil Cherry
Neil Cherry ONZM was a New Zealand environmental scientist.Cherry specialised most recently in the effects of electromagnetic radiation on human health, following his earlier work in meteorology and wind energy...
indicated that the mechanism is a significant buildup of positive ions, strongest somewhat inland from the coast, and electric fields of this strength have been shown to upset the melatonin
Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...
/serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...
balance in some people.