The sunniest areas are to be found locally near Blenheim, Nelson, and Whakatane, where the average duration of bright sunshine exceeds 2,400 hours per annum. Napier and the rest of the Bay of Plenty are only slightly less sunny. A large portion of the country is favoured with at least 2,000 hours. Even Westland, despite its high rainfall, has 1,800 hours. Southland, where sunshine drops sharply to 1,700 hours per annum, lies on the northern fringe of a broad zone of increasing cloudiness. Four hundred miles further to the south, at Campbell Island, the sunshine has the extremely low value of 650 hours per annum. A pleasant feature of the New Zealand climate is the high proportion of sunshine during the winter months. To eliminate the effect of varying day length, the summer and winter sunshine at a few selected stations have been expressed as a percentage of the possible sunshine.

As these figures indicate, there is a marked increase in cloudiness in the North Island in winter, but little seasonal change in the South Island, except in Southland.

by Neil George Robertson, M.SC., Assistant Director (Climatology), Meteorological Service, Department of Civil Aviation, Wellington.

• New Zealand Journal of Science and Technology, Vol. 12 (1930), “The occurrence of Thunderstorms in New Zealand”, Kidson, E., and Thomson, A.
• Ibid, Vol. 13 (1931), “Mean Temperatures in New Zealand”, Kidson, E.
• Ibid, Vol. 14 (1932), “The Frequency of Frost, Snow and Hail in New Zealand”, and “The Canterbury Northwester”, Kidson, E.
• Ibid, Vol. 22B (1940), “The Variations of Annual Rainfall in New Zealand”, Seelye, C. J.
• Ibid, Vol. 27B (1946), “Variations in Monthly Rainfall in New Zealand”, Seelye, C. J.
• Ibid, Vol. 32B (1950), “Droughts in New Zealand”, Bondy, F.
• New Zealand Geographical Society Miscellaneous Series No. 1 (1950), “The Climates of New Zealand according to Thornthwaite's Classification”, Garnier, B. J.
• New Zealand Meteorological Service Note No. 33 (1947), “The Frequency of Heavy Daily Rainfalls in New Zealand”, Seelye, C. J.
• Ibid, No. 43 (1956), “The Estimation of Natural Evaporation and Water Need”, Gabites, J. F.
• New Zealand Journal of Science, Vol. 4 (1961), “Estimation of Open Water Evaporation in New Zealand”, Finkelstein, J.

Humidity is commonly between 70 and 80 per cent in coastal areas and is about 10 per cent lower inland. It varies inversely as the temperature, falling to a minimum in the early afternoon when temperature is highest and frequently lying between 90 and 100 per cent during clear nights. As the following table shows, the diurnal variation is greater than the difference between summer and winter.

Very low humidities – between 20 and 30 per cent or lower – occur at times in the lee of the Southern Alps where the Fö effect is often very marked. In summer the hot, dry “Canterbury Norwester” is generally a most unpleasant wind. Cool south-westerlies are also at times very dry when they reach eastern districts. In Northland the humid midsummer conditions are inclined to be rather oppressive, though temperatures there rarely reach 85°. Dull, humid spells are generally not prolonged anywhere, but their frequency shows a marked increase in the south.

The majority of New Zealanders seldom see snow, except on the mountains. The North Island has a small permanent snow field above about 8,000 ft on Mt. Ruapehu (9,175 ft), but the snow line rarely descends below 2,000 ft, 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. The snow line on the Southern Alps is around 7,000 ft in summer, being slightly lower on the western side where the Franz Josef and Fox Glaciers descend through heavy bush to within 1,000 ft of sea level. In inland Canterbury and Otago, where there are considerable areas of grazing lands above 1,000 ft, snowfalls are heavier and more persistent and have caused serious losses of sheep during severe winters in the past. Even there the snow line rarely stays below 3,000 ft through the winter.

It is well known that local variations in frostiness are considerable, even within quite small areas. On a calm, clear night the cold air in contact with sloping ground gravitates slowly downhill to collect in valleys and depressions, and it is these “Katabatic” drifts which are mainly responsible for local temperature variations at night. 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 (4 ft. above the ground) has registered below 32° F only once in nearly 50 years, yet across the harbour at Whenuapai Aerodrome there are eight screen frosts per annum on the average. In the South Island screen frosts occur on most clear nights from May to September. Excluding the uninhabited mountainous areas, the most severe winter conditions are experienced in Central Otago, on the Mackenzie Plains of inland Canterbury, and on the central plateau of the North Island. Even in these areas night temperatures as low as 10° are rarely recorded. Elsewhere over the North Island the winters are mild and pastures maintain continuous growth. In both Islands sheep and cattle remain in the open all the year round.

Mean temperatures at sea level decrease steadily southwards from 59° F in the far north to 54° about Cook Strait, then to 49° in the south. With increasing altitude, temperatures drop about 3° per 1,000 ft. January and February, with approximately the same mean temperature, are the warmest months of the year; July is the coldest. Highest temperatures are recorded east of the main ranges, where they rise to the nineties on a few afternoons in most summers, usually in association with a north-westerly Föhn wind. The extremes for New Zealand (measured in a standard thermometer screen) are 101° at Ashburton and – 3° at Ophir (Central Otago).

As is to be expected, there is a small annual range of temperature (difference between mean temperature of the warmest and coldest months). In Northland and in western districts of both Islands the annual range is about 15° F. For the remainder of the North Island, and east coast districts of the South Island, it is 17°–19. Further inland it exceeds 20° in places, reaching a maximum of 25° in Central Otago where there is an approach to a continental type of climate.

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 five 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.

Thunderstorms are not numerous. Their frequency is greatest in the north and west where thunder is heard on 15–20 days a year; east of the ranges the figure is five or less.

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 rainfall figures range from as little as 13 in. in a small area of Central Otago to over 300 in. in the Southern Alps. The average for the whole country is high, but for the greater part it lies between 25 and 60 in., a range regarded as favourable for plant growth in the temperate zone. The only areas with under 25 in. are found in the South Island, to the east of the main ranges. These include most of Central and North Otago, and South Canterbury. In the North Island the driest areas are southern Hawke's Bay, Wairarapa, and Manawatu, where the average rainfall is 30–40 in. a year. Of the remainder, much valuable farm land, chiefly in northern Taranaki and Northland, has upwards of 60 in. Over a sizable area of both Islands rainfall exceeds 100 in. 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, although its effectiveness in summer is, of course, much reduced. 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. The depletion of soil moisture through evaporation and transpiration is such that vigorous pasture growth is not maintained during summer unless the rainfall is at least 4 or 5 in. a month. Summer rainfall in most farming districts is less than this, and east of the South Island ranges it amounts to less than 3 in. in half the summer months. There, the dry summer conditions favour the ripening of grain and fruit crops, but farm production of all types is considerably increased by irrigation.

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 rain can be measured on at least 150 days a year, except to the east of the ranges where there are, in places, fewer than 125 rain days. Those areas of the South Island with annual rainfall under 25 in. generally have about 100 rain days a year. In the far south the frequency of rain increases sharply; in Stewart Island and Fiordland rain days exceed 200 a year. Over most of the country between 55 and 65 per cent of the rain days also qualify as wet days (at least 0.10 in. of rain). The percentage increases to over 70 in Westland, but in the lowrainfall area of inland Otago there are only 40 wet days a year compared with 100 rain days. 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 22 in., which occurred at Milford Sound where the mean annual rainfall is 250 in. 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 north-eastern districts of the Auckland Province. By contrast, in the Manawatu district and in Otago and Southland daily falls reaching 3 in. are very rare.

A century of annual rainfall totals at the four main cities is represented in the diagram above. Some adjustments have been made, where necessary, to take account of local changes in the site or exposure of the rain gauge. It may readily be seen that the pattern of variation from year to year is very erratic and shows no regular cycles or general upward or downward trend; and that abnormally wet (or dry) years do not usually occur simultaneously at all places.

Winds from a westerly quarter prevail in all seasons, with a general tendency to increase in strength from north to south; however, considerable local modifications to the general air flow occur during its passage across the mountainous terrain. Approaching the main ranges the westerly flow turns towards the north-east and, on descending on the eastern side, swings towards the south-east. This results in an increased number of south-westerlies in Westland and a predominance of north-westerlies in inland districts of Otago and Canterbury, where strong gales from this quarter occur at times in the late spring and summer. Daytime sea breezes usually extend from the coast inland for 20 miles or more during periods of settled weather in summer. On the Canterbury coast north-easterlies are the most frequent winds but south of Dunedin south-westerlies prevail. Cook Strait, the only substantial gap in the main mountain chain, acts as a natural funnel for the air flow and is a particularly windy locality afflicted by gales from the south-east as well as the north-west. This “funnel” effect is also in evidence about Foveaux Strait. North of Taranaki the general air flow is more south-westerly and there is a noticeable reduction in windiness in the summer.

In the Australasian region there is no semi-permanent anticyclone, as exists in subtropical latitudes in the Indian and eastern Pacific Oceans. Instead, a continual eastward migration of anticyclones takes place, roughly at weekly intervals. Most of the centres pass over or to the north of the North Island. The low-pressure troughs which separate successive anticyclones are associated with deep depressions centred far to the south. A period of disturbed weather accompanies the trough, with a change to cold southerly or south-westerly winds as it advances north-eastwards over New Zealand. Conditions improve again with the approach of the next anticyclone from the west. While this simple progression dominates the day-to-day weather, the situation frequently becomes much more complex. The troughs are unstable systems where depressions commonly form, some of which develop into vigorous storms that travel south-eastwards across New Zealand.

The anticyclones themselves continually vary in size, intensity, and rate of movement. Their tracks are furthest north in the spring, on the average, and reach their southern limit in late summer or early autumn when most of the centres cross central or southern New Zealand. At this time of the year, too, northern and eastern districts of the North Island occasionally come under the influence of deep cyclones of tropical origin.