New Zealand Weather

New Zealand Weather

Introduction

New Zealand lies in the mid-latitude zone of westerly winds, in the path of an irregular succession of anticyclones, which migrate eastwards every six to seven days. The centres of these anticyclones generally track across the North Island, more northerly paths being followed in spring, and southerly paths in autumn and winter. Anticyclones are areas of descending air, and settled weather, with little or no rain, which may bring clear skies, or low cloud and fog.

Between the anticyclones are troughs of low pressure, which extend northwards from low pressure depressions moving eastwards far to the south of New Zealand. Within these troughs there are often cold fronts, orientated northwest to southeast, which produce one of the commonest types of weather sequence over the country: as the front approaches from the west, northwesterly winds become stronger and cloud increases, followed by a period of rain for several hours as the front passes over, and then a change to cold showery southwesterly winds.

The presence of a mountain chain extending the length of the country has a major effect on the climate of the various regions, and produces much sharper climatic contrasts from west to east, than from north to south. In some inland areas of the South Island, just east of the mountains, the climate is distinctly 'continental' in character -- with large daily and seasonal temperature extremes -- despite the fact that no part of the country is more than 130km from the sea. Ophir in Central Otago has the greatest temperature range of 55°C.

Winds

The prevailing wind direction is westerly, although in individual months easterlies may predominate, and north of Taranaki the general flow is southwesterly. In the North Island winds generally decrease for a period in the summer or early autumn, but in many parts of the South Island July and August are the least windy months.

The blocking effect of the mountain ranges modifies the westerly wind pattern. Wind strength decreases on the western side, but increases through Cook Strait, Foveaux Strait, and about the Manawatu Gorge. Air is also forced upwards over the ranges, which results in a warm drying (föhn) wind in the lee areas to the east of both islands.

Wellington averages 173 days a year with wind gusts greater than about 60 km/h, compared with 30 for Rotorua, 31 for Timaru, and 35 for Nelson.

Sea breezes are the predominant winds in summer in many coastal places, such as Canterbury where the northeasterlies are almost as frequent as the predominant southwesterlies.

Rainfall

The distribution of rainfall is mainly controlled by mountain features, and the highest rainfalls occur where the mountains are exposed to the direct sweep of the westerly and northwesterly winds. The mean annual rainfall ranges from as little as 300mm in a small area of Central Otago to over 8000mm in the Southern Alps. The average for the whole country is high, but for the greater part lies between 600 and 1500mm. The only areas with average rainfalls under 600mm are found in the South Island to the east of the main ranges, and include most of Central and North Otago, and South Canterbury. In the North Island, the driest areas are central and southern Hawkes Bay, Wairarapa, and Manawatu, where the average rainfall is 700-1000mm a year. Of the remainder, much valuable farmland, chiefly in northern Taranaki and Northland, has upwards of 1500mm. Over a considerable area of both islands rainfall exceeds 2500mm a year.

For a large part of the country the rainfall is spread evenly through the year. The greatest contrast is found in the north, where winter has almost twice as much rain as summer. However predominance of winter rainfall diminishes southwards: it is still discernible over the northern part of the South Island, but over the southern half, winter is the season with least rainfall, and a definite summer maximum is found inland due to the effect of convectional showers. Rainfall is also influenced by seasonal variations in the strength of the westerly winds. Spring rainfall is increased west of, and in, the ranges as the westerlies rise to their maximum about October, with a complementary decrease of rainfall in the lee of the ranges.

Areas that are exposed to the west and southwest experience much showery weather, and rain falls on roughly half the days of the year. Over most of the North Island there are at least 130 rain days a year, (days with at least 1.0mm of rain) -- except to the east of the ranges where in places there are fewer than 110 rain days. These areas of the South Island with annual rainfall under 600mm generally have about 80 rain days a year. In the far south the frequency of rain increases sharply, rain-days exceeding 200 a year in Stewart Island and Fiordland.
On the whole the seasonal rainfall does not vary greatly from year to year, the reliability in spring being particularly advantageous for agricultural purposes. It is least reliable in late summer and autumn, when very dry conditions may develop east of the ranges, particularly in Hawkes Bay.

The highest daily rainfall on record is 682mm which occurred at Colliers Creek, Hokitika, where the mean annual rainfall exceeds 6000mm. Areas with a marked lower annual rainfall can be subject to very heavy daily falls: such areas are found in northern Hawkes Bay and northeastern districts in the Auckland province. By contrast, in the Manawatu district, Otago, and Southland, daily falls reaching 80mm are very rare.

Temperature

Mean temperatures at sea level decrease steadily southwards from about 15°C in the far north to about 10°C in the south of the South Island. Temperatures also drop, by about 2°C per 300m, with altitude.

January and February, with approximately the same mean temperature, are the warmest months of the year, and July is the coldest. Highest temperatures are recorded east of the main ranges, where they exceed 30°C on a few afternoons in most summers.

The extremes for New Zealand are 42°C. which has been recorded in three places, in the Awatere Valley (Marlborough), Christchurch, and Rangiora (Canterbury); and -22°C at Ophir (Central Otago). The annual range of mean temperature (the difference between the mean temperature of the warmest and coldest months) is small. In Northland and in western districts of both Islands it is about 8°C and for the remainder of the North Island and east coast districts of the South Island it is 9°C to 10°C. Further inland the annual range exceeds 11°C in places, reaching a maximum of 14°C in Central Otago where there is an approach to a 'continental' type of climate.

Sunshine

The sunniest places are near Blenheim, the Nelson-Motueka area, and Whakatane, where the average duration of bright sunshine exceeds 2350 hours a year. The rest of the Bay of Plenty, and Napier are only slightly less sunny. A large portion of the country has at least 2000 hours, and even Westland, despite its high rainfall, has 1800 hours. Southland and coastal Otago, where sunshine drops sharply to about 1700 hours a year, lie on the northern fringe of a broad zone of increasing cloudiness.

A pleasant feature of the New Zealand climate is the high proportion of sunshine during the winter months, although there is a marked increase in cloudiness in the North Island in winter -- but little seasonal change in the South Island, except in Southland.

Hail, Thunderstorms and Tornadoes

The number of severe hailstorms reported annually over the whole country averages 9, but this figure varies yearly from 4 to 20. Severe hailstorms occur widely throughout the country, but the areas most affected are Canterbury, the low country of central Hawkes Bay, and a small area south and west of Nelson. Most of the hail stones are small, but occasionally larger stones cause local damage to glasshouses and orchards.

Thunderstorms are not numerous. Their frequency is greatest in the north and western side of the country, where thunder is heard on 15 to 20 days a year. On the east coast of the South Island the average is commonly less than 5.

Tornadoes show a similar pattern to thunderstorms, apart from a maximum in the Waikato and Bay of Plenty. An average of about 20 tornadoes and water-spouts is reported each year, but most of these are small.

Frost and Snow

Local variations in frostiness are considerable, even within quite small areas. For example, at Albert Park, Auckland, the screen minimum thermometer has registered below 0°C only once in 65 years, while further up the harbour at Whenuapai aerodrome there are on average eight screen frosts per annum. Favourable sites in coastal areas of Northland are free of frost, although further inland light frosts occur frequently in the winter months. Excluding the uninhabited mountainous areas, the coldest winter conditions are experienced in Central Otago and the Mackenzie Plains of inland Canterbury, and on the central plateau of the North Island, but even in these areas night temperatures as low as -12°C are rarely recorded. Elsewhere over the North Island the winters are very mild, and in both islands sheep and cattle remain in the open all year round.

The North Island has a small permanent snowfield above 2500m on the central plateau, but the snow line rarely descends below 600m even for brief periods in winter. In the South Island snow falls on a few days a year in eastern coastal districts, where in some years it may lie for a day or two even at sea level, but in Westland it does not lie at sea level. The snow line on the Southern Alps is about 2000m in summer, being slightly lower on the western side where the Franz Josef and Fox Glaciers descend through heavy bush to within 300m of sea level. In inland Canterbury and Otago, where there are considerable areas of grazing lands above 300m, snowfalls are heavier and more persistent and have caused serious sheep losses during severe winters. In that area, however it is rare for the winter snow line to remain below 1000m for extended periods.

Humidity

Mean relative humidity is often between 65 and 85 percent. However, much lower values (from 30 percent down to 5 percent) occur at times in the lee of the Southern Alps, where the föhn wind (the Canterbury nor-wester) is often very marked. Cool southwesterlies are also at times very dry when they reach eastern districts. In Northland the humid mid-summer conditions are inclined to be oppressive, although temperatures rarely reach 30C. Dull, humid spells are generally not prolonged anywhere, but their frequency shows a marked increase in the south.

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