Wānaka Sun       

Those bloody nor’westers!

Posted at 6:00am Thursday 08 Apr, 2021 | By Pat Deavoll editor@thewanakasun.co.nz

No one can not have experienced the gale-force nor' westers that happened over the weekend. I went out to the Wheels at Wanaka on Saturday and although I enjoyed watching the Clydesdale horses towing ancient machinery, the steam engines and the giant grader weighing 65 tonnes, the wind was a pain. It picked up the dust and my hair. It blew away my event schedule, never to be seen again. It annoyed the heck out of me as I sat in the stadium. It was distracting, to say the least.

So what's the deal with this wind? I have to think back to fifth form geography for the details, which believe me, was a long time ago.

Apparently, the nor'wester can blow at any time of year but is less common in winter. Many of the strongest northwesterly winds blow ahead of cold fronts. 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. This is according to Chris Brandolino, chief meteorologist at NIWA, with whom I am intermittently in contact.

The steep air pressure gradient ahead of the cyclonic system associated with a front gives these nor'westers their strength. Brandolino says.  They will commonly reach gale force, like this past weekend, 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. In mid-winter, a nor'wester is often followed by a brief but intense cold snap, frequently bringing thunder, hail, or snow, he says.

The heat and lack of moisture characteristic of nor'westers play a major role in the intermittent droughts experienced by Canterbury, Otago and other regions on New Zealand's eastern coasts. We haven't had a drought this year, but it has been dry.

What's more, 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. About 10 per cent of people affected by the nor'wester feel elated and wonderful. But the rest of us feel depressed, irritable, and lacking energy. People feel they can't cope with everyday things. ... there is irrational anxiety and a sense of foreboding. I think I'm one of these people, I genuinely hate nor'westesters and feel quite blah when they are happening.

The Nor'wester is a föhn wind,says Brandolino - moist air is driven over the Southern Alps, falling as condensation on the western side, leaving the eastern plains in the rain shadow.

It 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. 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 is a weather pattern peculiar to the east coast of the South Island, says Brandolino. We don't get so many of them here in Wānaka because we are so close to the main divide.- it's more just a mass of cloud to the west. For this reason, the nor'west arch is also often referred to as the Canterbury arch, although it is visible in both Otago and Marlborough as well as in the Canterbury Region. It is shown in an arch of high white cloud in an otherwise clear blue sky over the Southern Alps. Closer to the Canterbury coast, some distance from the mountains, it appears as a clear area of blue above the mountains, with white clouds 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, says Brandolino.

So that's what I know (with help from Chris Brandolino)  about the nor'wester. I only hope we don't get too many of them because I don't cope too well.

Read edition 1021 of the Wānaka Sun here. 

 

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