General.

Such meteorological statistics as are available are not of much value for ecological plant-geography, nevertheless they give an indication of the general climate to which the plant-communities are exposed and they are of service for comparative purposes. But in a sparsely populated young country such accuracy or thoroughness cannot be expected as in the Old World.

Almost the whole of the data is derived from observations made in the lowlands and there are no statistics concerning the high mountains for altitudes exceeding about 600 m. But, apart altogether from instrumental observations, the members of a plant-community can tell a good deal concerning the various climatic factors on which, in part, its structure and activities depend. So, too, the behaviour of indigenous species abroad and of both indigenous and exotic species in New Zealand furnishes meteorological information of no small value. Finally, my studies of the vegetation carried out at all seasons for some 40 years have enabled me to gain some knowledge of the climate of many localities for which no other data are available.

In what follows only the climate of North Island, South Island and Stewart Island is dealt with, that of the Outlying Islands being discussed when treating of the vegetation and floras of each group. Also, the special details concerning the Botanical Districts are removed from this chapter to their more suitable position in Part IV, and climate is frequently referred to when dealing with the communities.

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New Zealand possesses, for the most part, a maritime climate, situated as it is remote from other lands in the widest ocean of the globe with no part of the area more than 128 km. from the sea. There are however marked differences in climate, owing firstly to the region extending through 25 degrees of latitude and secondly to the lofty mountain chains of the main islands lying athwart the prevailing winds. "Aspect" in a wide sense therefore has a remarkable influence both on rainfall and temperature, not only as to average annual amounts, but also in every atmospheric disturbance that passes over the land. Ecologically it is a primary cause of the wide-spread continuous formations — forest and tussock-grassland. Aspect in a narrow sense regulates climate or modifies its effects and so affects both plant-distribution and the composition and structure of communities.

With regard to rainfall, that of South Island, in the west, is extremely high, while, on the contrary, parts of the eastern districts are, in comparison, very dry. North Island has a maximum rainfall — almost a rainy season indeed — in the winter months, but South Island shows a remarkable evenness in its monthly averages. Periods of drought occur at times in the eastern districts, such being commonest in spring and summer, in the north, and in autumn and winter, in the south. Although the total average rainfall, especially in the east, decreases with increase of latitude, yet the number of rainy days is greater in the south than in the north. This arises through the frequent occurrence of atmospheric disturbances in the latitude of the "forties", but the northern districts are under the influence of occasional cyclonic disturbances of tropical origin which travel from north-west to south-east over North Island. Occasionally, extensive "Lows", decreasing northwards, account for much warm and moist weather but do not usually bring about a heavy precipitation. The cyclone track will often pass to the northward of New Zealand; sometimes it crosses as low as Cook Strait; occasionally it comes from the north-east to the East Cape and then passes down the east coast before taking an eastward route under the guiding and controlling influences of the prevailing westerly winds of these higher latitudes. The upper winds are almost invariably westerly and a divergence to a southwesterly direction from the west usually precedes a marked change in weather conditions, which the forecaster values as a guide to the subsequent swing of atmospheric pressure. High pressure, or the anticyclone, may be regarded as the controlling factor of weather conditions and on the edge and between these high pressure systems are found the "Lows".

The frequency of the above disturbances judging from the average of 9 years are as follows: (1) For the cyclone or monsoon of marked intensity — Spring 2; Summer 1.8; Autumn 3.3; Winter 5.3. (2) For the westerly or antarctic low — Spring 6.3; Summer 4.3; Autumn 5.2; Winter 4.7.