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What climate change?

THURSDAY SEPTEMBER 20, 2012

For a small country, the variation in amount of rainfall is actually surprisingly very large. Clyde gets about 350mm per year, but 240kms away, Puysegur Point in a typical year can receive over 5000mm.  In the North Island there is copious rain in the north of Auckland. Hokianga farmers jokingly claim they only get two seasons, wet and wetter.

In all districts north of Auckland the annual rain total is over 1250mm, and occasionally closer to 1750mm.  This high amount is caused by hilly country exposed to moist westerlies. When these winds strike hills they are forced to rise, which lowers their temperatures. No longer able to hold onto accumulated moisture, this drops as rain.

Coromandel Peninsular is another wet district because the land is high and the lowlands of the Auckland isthmus have not robbed the rain from prevailing westerlies. To the south of Auckland rainfall is less, because the high western hills incorporating Mt Pirongia and Mt Karioi border the Waikato and rob the winds of their moisture before they reach flatter farmland.

All the long west coast has sufficient rain, most abundant where land is highest. For example, near Wanganui the hills are lower and yearly rain seldom exceeds 750mm.  But nearer Mt Egmont-Taranaki the amount may be 2500mm, with about 7000mm on the mountain's northern slopes.

The South Island has a wider variation of rain patterns because the high land that borders the ocean all along the west coast causes all the moisture to fall immediately as rain.  With the exception of the far northwest of the South Island all yearly rainfall on the West Coast exceeds 2500mm, which is at least double that of Christchurch.

The area getting least rain in the whole country is a fat strip that takes in mid-Canterbury from Amberley southwards (except for the city of Christchurch), South Canterbury and all of Otago except for the city of Dunedin.  Another area also often overlooked for rain is coastal Marlborough. These two areas can expect only 200mm-600mm per year and are the most prone to droughts.

Cyclones form above Australia just south of the equator in the Timor, Aratura and Coral Seas and move left or right before descending south. From the Coral Sea they may drift down into the Pacific, already somewhat decaying.  In short, Austraila is our cyclone shield.

The approach of a cyclone for NZ is marked by a NE change coupled with a gradually falling barometer.  The cyclonic system veers to the NW when the barometer has fallen to 1009mbs.  Howling and blowing from the NW, as the centre of the cyclone passes over us there is a momentary lull.

The temperature, which is high as the cyclone approaches, does not fall until winds switch to SWs, when the barometer rises, the wind increases to gale-force, and rain falls.  On the West Coast thunder and lightning may mark the change, and in the far south; snow, hail or fierce driving rain and/or biting cold. Then the sky clears as the barometer rises to reach 1022mbs.

There are variations, as in double-centre cyclones, and systems that pass over us from NNW - SSE, and these may skip the final SW stage and go straight to the barometric rise once the cyclone has moved on.  But in the main, this NE-NW-SW sequence for NZ’s extreme weather conforms to pattern.

A country's shape and position determines its climate. Whether or not it rains depends on wind direction. The amount of rain depends on the sun's heat and angle.  The timing of rain is a function of the moon's air-tide cycle.  Temperatures are determined by the angle the sun's rays make as they strike the ground. This angle is a function of latitude, the distance from the equator.

In fact the dictionary definition of "climate" is the temperature regime at any given latitude.  Take an Australia example.  Perth in the west, Broken Hill near the centre, and Taree on the east coast a tenth of the world away from Perth have averages for yearly maximums of 24degC, and for minimums 12degC.  It is no coincidence that all three places are around 31degS latitude. The fact that Broken Hill is 1000 feet above sea-level whilst the other two are coastal makes little difference to their climates.

If there were never any humans in New Zealand the weather and climate would still be exactly the same, year in and year out, and over many centuries.  Nothing that Man has done has changed wind direction or sun angle.  To bring about climate change we would have to remove our hills and drag the whole country closer to the equator.

If we could find a way of pushing Australia further to the west we would get less westerly winds, more tropical cyclones and less earthquakes.  Until that day, this is the climate of NZ.  It is regular, predictable and only of surprise, shock and bewilderment to those who know little about it. The same conditions come and go with day to day variance, conforming to seasonal cycles.

Climatologists say they don’t understand climate change. I happen to agree with them.
They don't.

 


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