The size, rugged relief, latitude, and isolated maritime location of South Island all affect the conditions of the air above it. With no considerable body of land nearer than Tasmania, each air mass which moves in over it must have been subjected to oceanic influences in its lower strata for considerable periods. The excessively high rainfall of the west coast, and the advanced rate of accumulation in the snow fields which feed the island's glaciers, are attributable to the universally moist and potentially unstable character of these strata. The significance of the slow oceanic drifts along each coast, with a general direction to the northeast, is not well established beyond a possible lowering of the mean air temperatures.
The oceanic aspects of the climate are, however, much obscured by the effects of the rugged relief. Not only do we have marked rain-shadow effects in the eastern lee of the mountains, but in the same locations moderately high temperature ranges occur, a characteristic usually associated with continentality. Above all, uneven terrain creates a sharply varied pattern of climates in limited contiguous areas, contributing to vivid local contrasts in habitat character. Despite this obvious limitation, generalizations must be made, but the implicit qualification should always be recognized.
Since each part of the earth's surface has a unique location, and the controls of climate are precisely repeated in no two places, it becomes a matter of some difficulty to identify "homoclimes" (areas of closely similar but not precisely duplicated climates) of South Island in other parts of the world. Yet there is no more useful way of suggesting the essential character of the regional climate. Although classifications of climate are legion, the system developed by Vladimir Köppen and his associates is both reasonably precise and widely understood, and is particularly useful for climatic comparisons. In South Island all stations with usable records fall within, his Cfb type; translated from the arbitrary symbols, this indicates a climate of mild to moderately cold winters, warm but not hot summers, and a precipitation regime which has a limited month-to-month variability. Traditionally this is called a "West Coast Marine" climate, and the type areas of the climatologist include coastal northwestern Europe, the state of Washington in the United States, the British Columbia coast, and southernmost Chile. There are areas of the same classification, however, in the northeastern United States, the western Caucasus, northern Honshu, and southeastern Australia.
In a search for similar climatic areas, South Island should be divided along the line of the major watershed. Its excessively humid west coast (the perhumid of Thornthwaite's 4 early classification) has its only analog in Chile's southern archipelago. Washington and Oregon have too uneven a distribution of precipitation throughout the year, and the British Columbia littoral too great an annual temperature range, for either to be comparable. More acceptable homoclimes exist for the drier eastern slopes of South Island with their greater extremes of temperature, yet there, too, we can quickly eliminate many possibilities. The northern Appalachians have too great a temperature range, the Biscayan coasts too warm a summer, northern Honshu too severe a winter. Apart from Tasmania, and little-known parts of southern Patagonia, the most nearly comparable area in the world, climatically, with eastern occupied South Island is found in the British Isles. This has been a matter of very great significance to the importation of British people, plants, and animals.
At a much higher latitude than South Island, the United Kingdom and Eire experience days much longer in summer and much shorter in winter. A lower total of sun energy received there directly each year is, however, apparently balanced by the great, poleward-moving oceanic drift of the northeast Atlantic which carries relatively warm water past northwestern coastal Europe. The greatest differences between the climates of South Island and Great Britain seem to be the greater cloudiness and humidity of the latter.
Of the individual climatic elements, little can be said of air pressure in a brief and nontechnical discussion. Pressure fluctuates with the passage of air masses of different character over the island, the fluctuations being associated with differences in the character of each mass and of the "fronts" which form between them. Most rising of air, condensation, and precipitation are associated with these fronts, and general changes in air temperature, as well as pressure, with the transition from one mass to another.
Careful study of the data of wind direction reveals only an expectable variation from one place to another in an area of such highly broken surface, except for a more than haphazard tendency toward winds from the northwest. Mean velocities appear to be rather lower than in other areas of similar (Cfb) climate, a conclusion which contradicts a belief widespread in New Zealand and which is undoubtedly fostered by the existence of certain local wind "funnels" where rather high wind speeds may be reached. Attention should perhaps be called to the phenomenon of the "Nor'wester" of Canterbury, a desiccating wind, in nature comparable to the chinook of the Rocky Mountains or the föhn of the Alpine valleys, which blows hot, dry, and sometimes with considerable force across the plains.
The accompanying isothermal maps do not show actual air temperatures, but rather these reduced to sea-level equivalents. Maps of actual isotherms would be most complicated, due to the normal decrease in temperature of 3 degrees F. for each thousand feet of increased elevation in an air mass, and would become, essentially, relief maps indicating little about the character of the atmosphere. The maps do indicate that the interior and east coast are much warmer in summer (January) and that the southeast, particularly Central Otago, is decidedly the coldest place in winter (July).
it is important to both plants and animals that the critical winter isotherm of 43 degrees F., and the somewhat less significant summer isotherm of 60 degrees F. both run through the center of the island. The latter isotherm (more precisely that of 57 degrees F. for the three summer months) has long been considered the critical limiting isotherm for the growing of most commercial varieties of wheat. The importance of the 43 degrees F. isotherm lies in the fact that if the average temperature of the coldest month in any place falls below 43 degrees F., a very large number of useful perennials which have no protective devices against frost cannot be grown. It is true that the actual isotherms, rather than those shown, would be the significant lines in these respects, but the actual isotherms would almost coincide with the sea-level lines in the chief, low-lying, farming areas.
An examination of actual monthly means for the several stations indicates a very moderate regime with respect to both annual ranges and extremes of temperature. Except in a few interior stations, the actual mean of the coldest month is uniformly above 40 degrees F., and only one known station has a mean monthly winter temperature below freezing. Whereas July is the coldest month throughout the island, the highest temperatures are unlikely to occur before late January, and February is the warmest month in several places. Similar "lags" are noticeable in Norway, for example, and indeed are characteristic of maritime places in general. The warmest station on the island has a summer mean of less than 65 degrees F., yet the lowest summer mean is above 54 degrees F. Briefly, the winters tend to be mild with occasional frosts, and the summers relatively cool.