Current statistics appear monthly in the New Zealand Gazette. Special reports are published from time to time to supplement the regular data publications. Most weather data are now held in computer-accessible data files, and several kinds of output are available to provide an alternative means of dissemination of information to the printed publications.
The daily weather patterns are dominated by eastward-moving anticyclones and troughs of low pressure whose frequencies and intensities vary substantially. The troughs normally have a north-west to south-east orientation and are associated with deep depressions centred far to the south. A typical weather sequence commences with a low-pressure trough approaching from the west. Freshening north-westerly winds prevail with increasing cloud, followed by rain for a period during which winds may reach gale force.
The passage of the trough, with its associated cold front, is accompanied by a change to cold south-westerly or southerly winds and showery weather, occasionally with some hail and thunder. The barometer then rises with the approach of the next anticyclone from the west. Winds moderate and fair weather prevails for a few days as the anticyclone moves across the country.
While the sequence just described is very common the situation is frequently much more complex. The troughs are very unstable systems where depressions readily form. Some of these develop into vigorous storms that may pass over New Zealand at any time of the year. Occasionally in summer a cyclonic storm of tropical origin accompanied by gales and heavy rain passes over or near New Zealand, affecting mainly northern and eastern districts of the North Island. The anticyclones vary in size, intensity, and rate of movement. Their centres, on the average, follow a track across the North Island but individual centres may pass either north or south of the country, the more northerly tracks being favoured in spring and the southerly tracks in autumn and winter.
At times when little development occurs within the troughs the anticyclones follow each other at intervals of about 6—7 days. The other main factors which influence the climate of New Zealand are, first, its position in the midst of a vast ocean, and second, the shape and topography of the country itself. Hot air masses from the interior of Australia in summer or freezing air masses from the Antarctic, which occasionally reach New Zealand, retain little of their original characters after their long ocean passages.
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Since abundant supplies of moisture are supplied by evaporation from the ocean, and depressions are frequent and vigorous, the average precipitation is high. The chain of high mountains, which extends from south-west to north-east through the length of the country, rises as a formidable barrier in the path of the prevailing westerly winds. The effect is to produce much sharper climatic contrasts from west to east than in the north-south direction. In some inland areas of the South Island just east of the mountains the climate is distinctly continental in character, despite the fact that no part of New Zealand is more than km from the sea.
Winds —Winds from a westerly quarter prevail in all seasons although in individual months easterlies may predominate. In the North Island winds generally decrease for a period in the summer or early autumn. However, in the South Island, July and August are the least windy months. Important modifications to the wind pattern are caused by mountain ranges and by the heating or cooling contrasts between land and sea. The blocking effect of the mountain ranges decreases wind strength on the upwind side but increases it in the mountain passes and in Cook and Foveaux Straits and about the Manawatu Gorge.
Sea breezes are frequent and in many parts of New Zealand are almost certainly coupled with the mountain winds. North of Taranaki the general air flow is from the south-west and there is a noticeable reduction of windiness in the summer. An indication of the variation in the frequency of strong winds from summer to winter, and in different parts of the country, is given in the next table.
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These figures were all obtained by the use of anemographs at airports. 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 north-westerly winds. The mean annual rainfall see map ranges from as little as mm in a small area of Central Otago to over mm in the Southern Alps.
The average for the whole country is high, but for the greater part lies between and mm, a range regarded as favourable for plant growth in the temperate zone. The only areas with under mm are found in the South Island to the east of the main ranges. These include most of central and north Otago, and South Canterbury.
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In the North Island, the driest areas are central and southern Hawke's Bay, Wairarapa, and Manawatu where the average rainfall is — mm a year. Of the remainder, much valuable farm land, chiefly in northern Taranaki and Northland, has upwards of mm. Over a considerable area of both Islands rainfall exceeds mm a year but, with the exception of Westland, this is mountainous and unoccupied, much of it being forest-covered. 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.
This 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. The rainfall is also influenced by seasonal variations in the strength of the westerly winds. Spring rainfall is increased in and west of the ranges as the westerlies rise to their maximum about October, while a complementary decrease occurs at the same time in the lee of the ranges.
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Areas which are exposed to the west and south-west 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 rain days days with at least 1. Those areas of the South Island with annual rainfall under mm generally have about 80 rain days a year. In the far south the frequency of rain increases sharply; in Stewart Island and Fiordland rain days exceed a year.
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 Hawke's Bay. The highest daily rainfall on record is mm which occurred at Rapid Creek, Hokitika Catchment, where the mean annual rainfall exceeds mm.
Other areas with considerably lower rainfall are also subject to very heavy daily falls; such areas are to be found in northern Hawke's Bay and in northeastern districts of the Auckland Province. By contrast, in the Manawatu district and in Otago and Southland daily falls reaching 80 mm are very rare. Thunderstorms —Thunderstorms are not numerous.
Their frequency is greatest in the north and west where thunder is heard on 15 to 20 days a year; east of the ranges except in Gisborne the figure is five or less. Hail —Hail is most frequent in the south-west where it is reported on about 20 days a year, but for the greater part of the country it occurs on about 5 days a year or less. Most of the hailstones are small, but occasionally large stones cause local damage to glasshouses, and to orchards and market gardens, chiefly in Canterbury and Hawke's Bay.
January and February, with approximately the same mean temperature are the warmest months of the year; July is the coldest. Some temperature statistics for selected places are included in the table on climatological averages. As is to be expected, there is a small annual range of temperature difference between mean temperature of the warmest and coldest months. Frost —It is well known that local variations in frostiness are considerable, even within quite small areas.
Gently sloping ground with a northerly aspect tends to be least affected by frost. Favourable sites in coastal areas of Northland are free of frost, although further inland light frosts occur frequently in the winter months. At Albert Park, Auckland, the screen minimum thermometer 1.
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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. Elsewhere over the North Island the winters are very mild and pastures maintain continuous growth.
In both Islands sheep and cattle remain in the open all the year round. Snow —The majority of New Zealanders rarely see snow except on the mountains. The North Island has a small permanent snow field above m on the central plateau, but the snow line rarely descends below m even for brief periods in winter.
In the South Island snow falls on a few days a year in eastern coastal districts, and in some years may lie for a day or two even at sea level. In Westland it does not lie at sea level.