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Weather forecasting

by  Erick Brenstrum

With its elongated shape, New Zealand can be subtropically warm in the north and bitterly cold in the south, on the same day. Forecasting such diverse weather conditions used to be done with the help of hundreds of volunteers. The process is largely automated now, although the weather stills holds a fascination for outdoors-loving Kiwis.


Early forecasting

Polynesian and Māori forecasting

When the Polynesian ancestors of Māori voyaged to New Zealand they sought and used knowledge about the weather. They would sail south to catch the westerly winds, and north to catch the south-easterly trade winds. Māori traditionally used the patterns and colours of clouds to predict the weather. They would observe cloud formations at sunset to tell whether and when it would rain the following day.

Getting wind of trouble

During the wars in south Taranaki in 1868, Ngā Ruanui leader Tītokowaru, who had only 80 men to the British army’s 1,000, planned an armed attack on a redoubt only 5 kilometres from the British camp. He chose to attack at a time when he predicted there would be strong westerly winds. His forecast was right, and the sound of gunfire was carried away from the camp by the winds. In the ambush only six of his men were wounded.

European observations

European explorers were also interested in the weather. On his 1642 voyage of discovery, for example, Abel Janszoon Tasman had been instructed by the Dutch East India Company to note the winds he encountered, as well as monsoons, rains and other seasonal variations. Captain James Cook’s second voyage in 1772–75 resulted in the compilation of extensive meteorological logs. Their publication left no doubt in the minds of those who would follow him that New Zealand’s coasts were subject to violent and frequent storms, and at times alarming and rapid changes in weather.

The first European missionaries and settlers who made their way to New Zealand in the early 19th century observed and recorded the temperature, barometric pressure, wind direction and other weather patterns out of curiosity about their new environment. They were quick to make comparisons with the climate in Britain. Favourable accounts were published in Britain to encourage immigration, providing a highly coloured picture of a warm and hospitable country.

A national weather service

Following earlier moves by the military and various harbour boards to collect and regularly report weather conditions, official arrangements for a weather service in New Zealand were made in 1861. The government established a network of nine reporting stations extending from the Bay of Islands to Dunedin. After a hesitant start, services proved to be far from adequate. This, together with concern over the number of ships wrecked during storms along the coast in the mid-1870s, eventually led to action by the Marine Department.

Having earlier established an experimental storm warning scheme, in January 1874 the Marine Department appointed Commander Robert Edwin as New Zealand’s first official weather forecaster. Within two years the weather reporting section was functioning along lines developed by Robert FitzRoy (a former governor of New Zealand and commander of the Beagle) in his influential Weather book (1863), published in England.

Printed forecasts

Newspapers began printing weather maps at the end of 1882. These were based on the reports of about 24 observation stations. Robert Edwin sent daily reports to the major papers, along with code numbers that corresponded to one of the 24 isobaric (air pressure) patterns commonly occurring over New Zealand. The number indicated which map to print.

Technological developments

After the First World War, advances in radio technology meant that weather reports could be received from ships at sea. Soon after, a group of Norwegian meteorologists researching mid-latitude cyclones introduced the idea of fronts (where two different air masses meet). New Zealand’s meteorological service soon used this research in their forecasts.

When regular air services began in New Zealand in 1935, the number of reporting stations and the frequency of reports greatly increased. Some stations began to take measurements in the upper atmosphere using weather balloons tracked by theodolite. In 1942, radiosondes were also introduced – attached to balloons, these transmitted pressure, temperature and humidity readings.

Service providers

Forecasting remained a marine service until 1926, when it became part of the newly formed Department of Scientific and Industrial Research. Responsibility shifted to the Royal New Zealand Air Force in 1939, and later the Department of Civil Aviation and the Ministry of Transport. Finally, during the 1980s, a combination of commercial competition arising from a deregulated market and a move to publicly funded science led to the establishment of two government bodies responsible for meteorology. MetService, a state-owned enterprise, is responsible for operational meteorology and weather forecasting, while the National Climate Centre at the National Institute of Water and Atmospheric Research is part of a Crown research institute charged with tracking global influences on New Zealand's climate, particularly the El Niño–La Niña weather patterns, and sea-surface temperatures.

New Zealand was one of the first countries to commercialise weather forecasting. While most people continue to rely on the government-run MetService, new companies such as Blue Skies Weather and Climate Services provide customised forecasts for their clients.


Recording information

Modern weather forecasting depends on detailed knowledge of current conditions.

Surface surveillance

In New Zealand, observations of weather conditions at the earth’s surface are made at about 100 places every hour, and at some places every 10 minutes. They are made mostly by machines, and some are made by people. Variables measured include air pressure, temperature and humidity, wind speed and direction, visibility, the level of the cloud base and the amount of cloud, as well as rainfall. Reports taken by people include rain visible in the distance, cloud type and the presence of snow or hail. In some places webcams are available.

Getting it right

People often complain that forecasters get their predictions wrong. This might be because forecasts need to be presented simply and clearly, underplaying any scientific complexity. It might surprise the critics to know that a five-day forecast in 2003 is as accurate as a one-day forecast was in the 1950s.

Weather balloons

Since the early 20th century, surface surveillance has been supplemented by weather-balloon observations. The balloons are tracked with radar to ascertain wind direction and speed at different altitudes, and temperature and humidity throughout the atmosphere. Their limited coverage over oceans is compensated for by satellite-borne instruments, which can accurately measure the air temperature at different altitudes.

Satellites

Different types of satellite images taken over the Tasman Sea and New Zealand are received hourly, 24 hours a day:

  • Visual images show reflected light and clouds as small as 1 kilometre in diameter
  • Infrared images show heat radiation emitted by clouds, land and sea surfaces
  • Microwave images of clouds indicate where rain is falling and help to locate the eyes of tropical cyclones
  • Water-vapour images highlight areas where cloud may form.

Some satellites are also able to measure the air temperature at different heights, or surface winds over the sea. This is especially beneficial for forecasting in the southern hemisphere, where there are large areas of open ocean. These satellites make tens of thousands of observations every day for regions that would otherwise have only a handful of ship reports and drifting buoys.

Other data gathering

Weather radar, operated by MetService, covers about 80% of New Zealand. It detects rain and hail and estimates their intensity, and can record wind speed and direction. A lightning-detection network helps keep track of thunderstorms. Some commercial aircraft provide high-altitude wind and temperature measurements.


Making forecasts

Computer modelling

Data collected from observations of weather conditions is entered into a computer modelling system, which performs billions of calculations to predict future atmospheric conditions. These are displayed as isobar maps at the earth’s surface and throughout the atmosphere. They show variations in temperature, humidity, wind, divergence and convergence of air streams, and derived variables such as vorticity (the rotation of air) and upward motion. Fine-scale models covering New Zealand and its immediate surroundings are embedded within global models and provide a better resolution of features over the country.

Beyond about four days, the computer model returns an increasing number of errors. To counter this, the model can be run many times for the same situation using slightly different initial conditions to create an ensemble of predictions.

Public forecasts

Forecasts for radio, television or newspapers are summaries of expected conditions, concentrating on temperature, rainfall, snow, storms and cloud.

Aviation forecasts

Forecasts for the aviation industry are more complex. Airport forecasts contain details of the expected amount and height of clouds, variations in visibility caused by rain, changes in wind speed and direction, and the likelihood of fog or thunderstorms.

Forecasts of conditions en route include wind speed and direction at various altitudes, and the probability of turbulence or conditions that would cause ice to build up on the aircraft.

Marine forecasts

Marine forecasts concentrate on wind speed and direction, wave height, and visibility. Wind speed over the sea can be stronger than over the land, and direction can also differ significantly. New Zealand meteorologists forecast conditions around the coastline and are responsible for a large part of the South Pacific Ocean, from longitude 150° east to 120° west between latitude 25° and 55° south.

Extreme weather

MetService issues warnings of extreme weather – for heavy rainfall (more than 100 millimetres in 24 hours, or 50 millimetres in six hours over an area of more than 1,000 sq km), severe gales over land, and snowfall greater than 25 centimetres in 24 hours. Forecasts are made on request for search and rescue operations and in the event of forest fires. There are specialised forecasts for mountain areas like the Homer Tunnel in the South Island, where the risk of an avalanche is high. Thunderstorm hazard maps are posted on the MetService website.


Using climate data

Gathering data

At some sites in New Zealand daily weather records have been collected since the 1840s. For a long time these observations were made by volunteers and government employees. By the mid-20th century many observers were setting off for their climate stations at 9 o’clock each morning to read the rain gauge, record the night’s minimum temperature and the previous day’s maximum, and replace the sunshine recorder card. Manual records were taken every hour at a few airports. At the peak of manual data collection in the early 1970s there were more than 1,300 people observing the weather in New Zealand. In the early 2000s about 750 manual observations were made every day, and about 130 automatic weather stations made recordings every hour.

Stevenson screens

Temperatures in New Zealand are measured using louvred wooden boxes called Stevenson screens. Air flows through the box, which contains thermometers sheltered from direct sunlight and rain. The boxes are mounted 1.3 metres off the ground.

Archives

Understanding our climate involves countless analyses of data, and the longer the data has been collected, the better the analysis. However, it needs to be stored and made accessible. There are two archives in New Zealand of national importance: the National Climate Database, housed at the National Institute of Water and Atmospheric Research (NIWA) in Wellington, and the National Water Resources Archive, housed at NIWA in Christchurch. These databases have over 270 million records from 5,900 land-based climate stations, and 10 million ship-based records.

Derived data

Apart from raw data, there are also derived data records such as summary statistics tables which list monthly and annual averages, extremes or totals of rain, air temperature, atmospheric pressure and wind. Information about temperatures above or below certain critical thresholds are important for growing crops or heating houses.

This information can be used to generate wind roses, which are diagrams showing prevailing wind direction. The data can also help anticipate heavy rainfall, and gauge weather in places between climate stations.


Hononga, rauemi nō waho

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How to cite this page: Erick Brenstrum, 'Weather forecasting', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/weather-forecasting/print (accessed 23 September 2019)

Story by Erick Brenstrum, published 12 Jun 2006, updated 2 Jun 2016