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Weather

by  Erick Brenstrum

New Zealand’s weather is shaped by its mountains, the ocean surrounding the country and the winds from the roaring forties.


Factors determining weather

The weather is the particular atmospheric conditions and events at any one time. The climate is the average and the range of conditions over several decades.

New Zealand’s weather patterns are determined by three factors: wind, sea and land.

Wind

With much of the country lying in the roaring forties weather system, New Zealand is a windy place. The winds bring New Zealand’s weather from all directions:

  • Westerly winds are the most common. They carry wet weather to areas upwind of the main mountain chains, and dry, warm weather along the east coasts. Westerly airstreams often extend for thousands of kilometres upstream of New Zealand.
  • Northerlies can bring tropical air, heavily laden with water vapour, and are responsible for most of the flooding.
  • Southerlies can bring cold Antarctic air, which can cause snow to fall at sea level.
  • Easterlies are usually airstreams that have changed direction only a few hundred kilometres upwind of the land. A northerly airstream from the tropics can turn east towards the land near the end of its journey, bringing heavy rain to eastern areas. A southerly airstream that turns easterly will often be dry, or bring just a few showers.

Sea

Air blowing towards New Zealand crosses thousands of kilometres of ocean before reaching the coast. The air’s contact with the sea surface has two effects:

  • It moderates the temperature. Warm air leaving the tropics is cooled as it moves over the cold ocean surface nearer New Zealand. Cold air from Antarctica is heated as it crosses progressively warmer water on its journey towards New Zealand.
  • The air picks up moisture. Evaporation of water from the sea surface means that the air has high relative humidity by the time it arrives. However, the absolute humidity of the air from the north is generally two or three times higher than the air from the south. This is because the amount of water vapour that warm air can contain is much higher than cold air.

Land

On any day the weather is determined by the direction of the wind, and the impact the mountain ranges have on this.

Air that blows in from the sea is forced to rise as it moves over higher ground such as a mountain range. As the air rises, it encounters lower air pressure, which causes it to expand. This in turn lowers its temperature. As the air cools, some of its water vapour condenses to form the tiny, liquid droplets that make up clouds. If rising and cooling continue, enough cloud droplets develop so that some of them combine to form raindrops (precipitation).

How windy is Wellington?

On 173 days a year, New Zealand’s breezy capital is buffeted by winds of over 60 km an hour. Gusts regularly reach 140 km an hour. The city’s strongest winds were an incredible 248 km an hour, at Hawkins Hill on 6 November 1959 and 4 July 1962. (This was just under the national record: on 18 April 1970, gusts of 250 km an hour hit Mt John in Canterbury.)

Once the air begins descending on the other side, the higher air pressure at lower altitudes compresses it. Its temperature rises and any cloud droplets evaporate. So a north-west airstream crossing New Zealand will typically bring a period of heavy rain about the main ranges and west of them. It then moves towards the east coast, bringing dry warm weather.

In the South Island, the prevailing winds off the Tasman Sea meet the Southern Alps. The resultant precipitation makes the West Coast the wettest area of New Zealand. Annual rainfall at Milford Sound is over 6,000 millimetres, and 10,000 millimetres just below the divide of the Southern Alps. But just 100 kilometres or so to the east is the driest region, Central Otago, with annual totals of around 400 millimetres or less.


Highs – anticyclones

The fluctuations in New Zealand’s weather are caused by the passage of anticyclones (centres of high air pressure) separated by troughs of low pressure.

Fronts

Within the low-pressure troughs there are usually several bands of cloud and rain known as fronts. These form at the boundaries of air masses of different origin and temperature. Centres of low pressure, known as depressions, develop on some of the fronts.

Anticyclone weather

Anticyclones form when air currents in the high and middle atmosphere converge, piling up air in one area. The air inside these areas sinks from high levels towards the earth’s surface. The sinking air is warmed by compression, and this evaporates any cloud. This is why high pressure is generally – but not always – associated with good weather.

A cool story

A spectacular frost occurred in Otago in early July 1991 when overnight air temperatures dropped below -15°C – for days in some places. The effect was chaos. Beer froze in pubs, water pipes burst, diesel turned to sludge, and rabbits with frostbitten ears were accused of cannibalism on national television. Exposed metal (such as gates) became so cold that skin froze to it on contact.

However, a layer of colder air is sometimes trapped below the sinking air, typically as cloud, and in winter this frequently produces drizzle. In summer, although the cloud usually evaporates before midday, its moisture often feeds the showers or thunderstorms that develop over certain locations – such as the Canterbury Plains.

Fine weather generally occurs in areas sheltered by a mountain range. The cloudiest areas are those where the wind blows from the sea onto the land, for instance Christchurch in a north-east wind.

Winds

Near the centre of an anticyclone the winds are typically very light. However, air in an anticyclone is generally stable and resists upward motion. Consequently, the air tends to dam up against large features, such as mountain ranges, and pour around the ends, sometimes causing gale force winds over the adjacent sea.

This effect contributes to typical wind patterns, including:

  • frequent north-west gales over the sea south and west of Puysegur Point in Fiordland
  • regular northerly and southerly gales in Cook Strait
  • westerly and south-east gales near East Cape
  • south-west gales off Kahurangi Point on the north-west of the South Island.

Frosts and fog

During long winter nights, frosts or fogs are likely to develop over land that the anticyclone leaves free of cloud. As the land becomes colder, it cools the air next to it. In the absence of any wind to mix the air, the surface air will rapidly cool below zero and frost will form. However, if the surface air contains enough water vapour, fog may result.

Lost in fog

During the Second World War, heavy fog over Auckland on the morning of 24 May 1942 served a double purpose: it hid ships in the harbour from a Japanese reconnaissance plane operating from a submarine waiting offshore, and New Zealand’s ground observers could not see the plane. Pilot Susumo Ito regained his bearings only after Auckland Airport’s lights were turned on – probably in response to the sounds of his plane circling.

In high inland valleys, fog can form after the surface temperature falls below zero. This fog is composed of liquid droplets. When the fog droplets touch the surface of trees, fences or buildings they become rime ice, forming spectacular landscapes that can last for days. This can be seen in South Island inland areas. This phenomenon is called a hoar frost.

Pollution

In urban areas the light winds and stable conditions in anticyclones also favour the build-up of air pollution from car exhausts and industrial emissions. This occurs particularly in winter when open fires are being used.


Lows – depressions

Where the air pressure at the surface is low, air rises and becomes cooler. The water vapour in it begins to condense into tiny drops of water or, if it is cold enough, into tiny ice crystals. If there is enough water or ice, rain or snow begins to fall. This is why areas of low pressure (sometimes called depressions) are associated with bad weather.

Fronts

The trough of low pressure that moves over New Zealand when an anticyclone moves away to the east will often contain several bands of cloud and rain, known as fronts. Ahead of the first front, north or north-west winds will often bring air from the tropics or sub-tropics. The tropical air will have a large amount of water vapour and consequently produces heavy rain when it rises over the hills and mountains.

If the front moves slowly, then the rain may last long enough to cause flooding. In these situations, on the West Coast of the South Island, rainfall in excess of 200 millimetres in 24 hours or less is often recorded on the slopes of the mountains.

Behind the first front there will usually be westerly winds with scattered rain. The next front in the trough will typically have much less rain. Behind this second front the wind flow is likely to be south-west, originating over the cold ocean near Antarctica. As this cold air moves over the relatively warmer seas near New Zealand, thunderstorms are likely to develop, with hail and possibly snow to low levels. Occasionally there will be a sequence of fronts embedded in the south-west flow, each with its own burst of cold air and thunderstorms.

When the trough is deep (the air pressure is much lower than either side of it), the winds around the trough will be strong or gale force.

Low-pressure centres

Lows form when the air currents in the middle and upper atmosphere of a trough diverge, so that more air leaves one place than arrives to replace it, and the air pressure falls. This often happens east of Australia. There are several reasons for this:

  • There is a warm ocean current running down the coast, which feeds heat and moisture into the air.
  • Air in the westerly flow crosses a mountain chain just before it reaches the Australian coast, and this favours divergence aloft.
  • During the winter, some of the air rising in the northern hemisphere monsoon spills across into the southern hemisphere. This increases the high-level winds, known as the jet stream, where wind speeds can exceed 300 kilometres an hour. In turn, this increases the high-level divergence in the trough and so accelerates the pressure fall at sea level.

Unsettled weather

Many of the lows that form in the Tasman Sea bring unsettled weather to New Zealand. When the lows are deep, they have strong easterly winds on their southern flank. Where these blow from sea to land, the air is lifted as it rises over the hills. This adds to the upward motion already taking place within the low, thereby causing heavy rain. Such lows are able to bring rain to eastern areas that are sheltered from the prevailing westerlies.

In winter, if very cold air feeds into the low centre then heavy snow can fall in eastern areas. This happened over Canterbury during the winter of 1992.

New Zealand’s worst 20th-century snowstorm

Late July 1939 saw widespread snow when a deep trough lay east of New Zealand, allowing cold south-westerlies to bring Antarctic air over the country. It snowed from Cape Maria van Diemen in the far north to Southland, where flooding occurred when the thaw set in. Dunedin was worst affected. There was snow a metre deep in some of the hill suburbs, which ran short of food. In Auckland on 27 July, 5 centimetres of snow fell on the summit of Mt Eden, and the Bombay Hills shone white for most of the morning.

In Gisborne, snow fell for nearly three hours, and in Masterton, the snow lay 15 centimetres deep. Snow fell to sea level at Castlepoint, and drifts in the hills closed the road inland.

Snow was not the only problem during the devastating cold snap. Overnight frosts caused water pipes to burst in Palmerston North and Hastings. At Paremata, just north of Wellington, eight hectares of the harbour froze over. Tidal waters also froze at Ōpōtiki, in the Bay of Plenty.


Tropical cyclones

From about October to April, intense storms known as tropical cyclones occasionally form in the tropics to the north of New Zealand. Generally much smaller than a typical low in New Zealand’s latitudes, they can have much lower pressures as well as hurricane-force winds and extremely heavy rain.

Normally, these cyclones rapidly weaken as they move towards New Zealand over cooler seas. However, if they meet a cold front, they can be transformed into very large and severe mid-latitude depressions.

Severe cyclones

Occasionally, one of these storms will pass over New Zealand bringing some of the most destructive weather experienced in the country, including widespread flooding and damaging winds.

In April 1968, Cyclone Giselle brought flooding and destructive winds to many parts of New Zealand. The inter-island ferry Wahine sank in Wellington Harbour, with the loss of 51 lives.

Twenty years later, Cyclone Bola caused severe flooding in parts of Hawke’s Bay, Gisborne and Northland, as well as bringing damaging winds to central and western parts of the North Island.

Worst storm of the 20th century

The cyclone of February 1936 was probably the most destructive storm to hit New Zealand in the 20th century. The depression that crossed the North Island on the second and third of February 1936 brought widespread heavy rain, causing every major river in the North Island to flood. The Mangakāhia River in Northland rose by 19 metres.

Roads and railway lines were cut in hundreds of places by slips, washouts and inundation. One slip near Stratford was 500 metres long. Along the east coast, large waves on top of elevated sea levels caused widespread erosion. The wind blew in windows and destroyed buildings from Picton to Kaitāia and brought down hundreds of thousands of trees, cutting power, telephone and telegraph lines. A train was derailed near Makerua (now known as Ōpiki), just south of Palmerston North, when three carriages were blown down a bank.

Two people died of hypothermia in the Tararua Range north of Wellington where, at the height of the storm, trees were uprooted from ridges and thrown into valleys. In Auckland 40 boats were sunk or driven ashore in the Waitematā Harbour, and several more sank in the Manukau Harbour.

Disaster was narrowly averted when the interisland ferry Rangatira steamed onto rocks on Wellington’s south coast. After being stuck fast for 20 minutes, the Rangatira was able to reverse off, then turn and back slowly up the harbour.


Thunderstorms

Thunderstorms occur when the atmosphere is unstable and there is rapid upward movement of air inside a cloud. There are three conditions for a thunderstorm:

  • moist warm air at lower levels of the atmosphere
  • very cold air at middle levels
  • a low-level trigger such as a cold front which forces the warm air upwards.

As the warm air rises, it condenses into heavy droplets of rain which may freeze. The condensation of water releases further heat which causes more uplift and increasing turbulence.

Seasonal variations

  • In winter the air is heated when very cold air from near Antarctica moves north over the relatively warm sea around New Zealand.
  • In summer the land is heated by the sun which, in turn, heats the surface air.
  • Spring thunderstorms are often a combination of both patterns.

Summer and spring thunderstorms are more likely to occur in the afternoon and evening. In autumn and winter they can occur at any time.

Lightning

The strong updraughts inside thunderstorm clouds separate ice particles and cloud droplets according to size, carrying the smaller ones higher in the cloud. Since the smaller particles tend to carry a negative electric charge and the larger particles a positive charge, a strong electric field builds up inside the cloud. The electricity is ultimately discharged in a flash of lightning. Monitoring equipment has shown that over 50,000 lighting strikes hit New Zealand each year.

Thunder power

In a thunderstorm with rain at a rate of 50 millimetres an hour, the energy released in one hour is equivalent to the power demands of an average household for about 8,000 years.

Where to find lightning

Lightning strikes are more frequent about and west of the main ranges, especially in the west of the South Island. This is because airstreams from the westerly quarter are far more common over New Zealand than those from other directions. Storm clouds approaching from the west usually drop all their rain as they rise over the mountains. Little remains of the cloud when the air descends on the eastern side.

Some thunderstorms occur just ahead of cold fronts and others are in the deep pools of cold air that usually follow several hundred kilometres behind the fronts. Often, these fronts weaken as they approach the North Island, and the cold pools more often track across the South Island than the North.

Lightning strikes are more common in summer than in winter, but the winter strikes usually have a stronger current because of the greater instability of the cold Antarctic air in which they develop.

Bolt from the blue

In 2002 Paul Martin was running to avoid heavy rain. Suddenly he heard a loud bang and then saw a bright flash. Thinking he was on fire, he dropped into a gutter full of water. Lightning had entered through his back and gone out through his eye. He experienced brief heart complications, a graze on his back and some damage to his left retina, but survived the ordeal.

Deaths from lightning

In New Zealand, lightning kills someone about once every 5–10 years. Lightning is not only hazardous as a direct strike; it can also travel a long way down electricity supply lines, telephone lines and fence lines. It has even travelled through water pipes and into metal sinks.

Hail

The very strong updraughts inside thunderstorms carry liquid cloud droplets rapidly up to levels where the temperature is well below zero, and hailstones can form. The updraughts can lift the hailstones higher until they grow so big that they begin to fall.

The most frequent hailstorms occur in western areas in winter and spring. But if only severe storms with large hailstones are considered, then the greatest frequency is in eastern districts in spring and summer. Storms that produce small hailstones often affect large areas, but the most severe storms have a narrow path of destruction.

Hail damage

In New Zealand, hailstones as large as cricket balls have been reported, although they are rare. Golf-ball sized hail is more common and can do millions of dollars worth of damage to crops and fruit trees. Although hail is often spherically shaped, it sometimes has sharp knifelike projections. Such hail has been known to slice through apples, leaving half the fruit on the tree.

Hail has killed many different types of animals in New Zealand including dogs, rabbits, sheep, chickens, ducks and seagulls, but so far no people have died, although some have received head wounds requiring stitches.


Flash floods, down bursts and tornadoes

Flash floods

Thunderstorms can produce torrential rain over a small area in a short space of time, causing streams and rivers to rise rapidly in what is known as a flash flood. The most lethal in New Zealand occurred in February 1938 in the Kopuawhara stream, between Gisborne and Napier. In the middle of a Saturday night, the stream rose rapidly by 5 metres after thunderstorms dropped an estimated 130 millimetres of rain in an hour in the coastal hills. Carrying logs and rolling boulders, it became an 80-metre-wide torrent, inundating 47 miners’ huts in a railway construction camp and drowning 21 people. In another stream, north of Gisborne, the flood rose to almost 20 metres, leaving debris tangled in the top of a telephone pole.

Down bursts

As air surges up inside a cumulonimbus cloud, some of the surrounding air is drawn inside the cloud. This air is often much drier than the air inside the cloud. Consequently, cloud droplets evaporate into the air, making it colder and denser. The dense air then begins to fall inside the cloud and may become a large-scale down burst of wind that hits the ground and spreads out in all directions, much like a cup of water poured onto the ground from a height.

In New Zealand, these winds can briefly reach gale force and have destroyed small buildings and fences, flipped over small aircraft parked on runways and snapped off trees as thick as 40 centimetres in diameter. Down bursts cause squalls that are hazardous for small boats. They are also dangerous for aircraft approaching a runway.

Tornadoes

As air is drawn into the base of a large thunderstorm cloud and accelerated upwards, any rotation in the air is concentrated – much the same as when a skater or dancer spins faster when they pull their arms in towards their torso. In certain circumstances, the rotation inside the cloud produces a narrow, tightly spinning funnel of air known as a tornado, which extends below the cloud. With wind speeds as high as 300 kilometres per hour, a tornado can be very destructive when it reaches the ground.

An uplifting story

Just after finishing the morning milking in September 1990, Manukau dairy farmer Laurie Coe drove off on his four-wheel bike to see what was worrying his cows, who refused to leave the milking shed. Unable to see the approaching tornado because of heavy rain, he was suddenly thrown into a fence, then lifted into the sky. He was carried 100 metres before being slammed down into another paddock. He suffered only a stiff neck, sore back and a few minor bruises.

In August 2004 a tornado swept across Taranaki uprooting trees and destroying a house near Waitara. Two of the occupants were killed. The worst tornado in New Zealand history struck Frankton and Hamilton in 1948. Almost 150 houses were wrecked, three people were killed and dozens injured. However, New Zealand tornadoes are neither as common nor as destructive as those that occur over the plains of the United States, where in 1974 over 300 people were killed and over 6,000 were injured.


Extreme weather

‘My overwhelming feelings at the time were partly fear and panic, but mainly helplessness and powerlessness. For a time there was nowhere to hide from the forces of nature …’ Brian Hollis remembers the Wahine storm of 1968.

When we asked people in the community to send us in their personal stories of surviving a natural disaster, we expected the accounts of earthquakes and floods. We were surprised at the number who told of battling wind and rain. Here are five of the stories.


External links and sources

More suggestions and sources


How to cite this page: Erick Brenstrum, 'Weather', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/weather/print (accessed 17 October 2019)

Story by Erick Brenstrum, published 12 Jun 2006