Canterbury’s damaging nor’wester

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The northwest wind in Canterbury drives people crazy. Suicide rates go up and domestic violence increases, and when this hot, dry wind reaches gale force physical destruction becomes widespread.

On October 15, 1988, Canterbury farmers watched in despair as thousands of tonnes of topsoil, already bone-dry from drought, were blown out to sea by a northwest gale. Hundreds of hectares of crops were destroyed, either through windburn or the sandblasting effect of the dust-laden wind shearing plants off at ground level.

Arcing power lines started many fires, roofing iron was torn from buildings and a couple of large power pylons were toppled. At Christchurch airport the peak gust recorded was 90km/hr while inland the wind was estimated to have reached 140km/hr.

Comparisons were made with the disastrous storm of August 1975 in which even stronger winds were recorded: sustained winds of 115km/hr and peak gusts of around 160km/hr. Road, rail, power and telephone links were cut in many places from Wairarapa to Southland by falling trees and power poles. Vast tracts of forest were laid waste, buildings were demolished, boats sank at their moorings and a number of chickens were blown out to sea.

To understand why these spring-time gales occur it is necessary to look at the origin of the wind itself. Wind is caused by the difference in temperature between the equator and the poles. Because the earth is spherical the tropics receive far more sunlight than the poles. This heats up the tropical air and causes it to expand, so much so that the part of the atmosphere where the weather takes place is almost twice as thick at the equator as it is at the South Pole.

As a result some of the air spills out from equatorial latitudes and moves towards the poles. As it does so it moves closer to the earth’s axis of rotation (the line through the poles around which the earth spins) and speeds up. This wind starts off as a northerly, but because the earth is spinning from west to east it becomes a westerly wind by the time it reaches New Zealand.

The principle behind the speeding up of the wind is easily demonstrated by spinning in a swivel chair with your legs extended horizontally in front and your arms behind. When you pull your arms in and bend your knees your rate of spin increases dramatically in order to conserve angular momentum. It is the same principle ice skaters use to execute high speed pirouettes.

In spring the South Pole is at its coldest, so the temperature contrast between the pole and the equator is at its greatest. Consequently the strength of the westerly wind reaches its peak at this time of year. In addition, when northwest gales blow across the Alps, wave motions can be set up in the atmosphere which cause the winds to be much stronger downwind of the mountains. Far from sheltering the Plains, the Alps can actually increase the wind’s force.

Why is the wind so hot and dry when it reaches the Canterbury Plains?

Again, the answer lies 100km away at the Southern Alps and the effect they have on the wind as it passes over them.

As air rises it expands because atmospheric pressure decreases with altitude. (This is why it is so hard to breathe on top of Mount Everest. At that altitude one lung-full of air contains about one third the oxygen that it does at sea level.) Conversely, as air sinks it is compressed.

Anyone familiar with a bicycle pump knows that air gets hotter when it is com pressed. Fridges work on the opposite principle of expanding gases cooling down.

So, as air ascends on the Hokitika side of the Alps it cools due to expansion. This causes water gas to condense as tiny droplets of liquid water, which we see as clouds, and from which rain is formed when the droplets coalesce.

The process of condensation releases heat and this effectively reduces the extent to which the air is cooling. Everyone has experienced the reverse phenomenon at the beach when getting out of the water. In order for water droplets on the skin to evaporate they rob heat from the body, making you feel cold.

As the air descends on the Christchurch side it becomes compressed, warms up and the cloud evaporates. But the cloud base is much higher on the descending side because most of the moisture in the air has fallen out as rain. Therefore the descending dry air heats up at a much faster rate than it cooled down when rising inside cloud on the West Coast.

The result? Air reaching sea level at Christchurch is much hotter and drier than it was when it commenced its ascent at Hokitika.

The 1988 spring has been windier than most. In the months from July to October most reporting stations from Wellington southwards have had considerably more days than normal when gusts exceeded gale force. In fact, in October Wellington had 29 such days, which is a record. West of the Alps Milford had 1967mm of rain which is three and a half times the average and a record for October.

The reason for such a windy spring may well be connected with lower than usual sea surface temperatures near Antarctica, but as yet climatologists have not unravelled the mechanisms involved.

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