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Climatic.
The climate of the peninsula may be described as typically insular and warm-temperate. It differs somewhat from that of the Canterbury Plains, which is more continental in type.
Thus the plains are much colder on winter nights than the adjacent hills. The temperature of Cashmere Hills on frosty nights is from 3° C. to 5° C. higher than that of Christchurch, and on the hills at Redcliffs and Sumner it is still warmer. There ice is rarely seen. The lowest temperature that may be observed on the Cashmere Hills is about —8° C., but winters pass in which it does not fall below —4° C. or —5° C. Even in midwinter the days are often bright and warm, and the temperature in the lower valleys is as high as 15° C. in the shade. Frosts may occur on the plains in any month of the year, but on the neighbouring hills they are almost unknown between September and April. Thus in the beginning of November, 1918, an unusually severe frost (—7° C.) occurred in Christchurch. This was so slightly felt on the hills that tomatoes and potatoes remained unchecked and almost untouched by it. The conditions on the hilltops have been but little studied, though, of course, the temperatures there must be considerably lower both in summer and winter than those at the foot. The temperature range through the year on the lower portion of the hills is comparatively small, but reliable records are difficult to obtain. Probably the difference of average temperature is not more than 8° C. or 9° C. between a month of winter and one of summer, though the maximum temperature in summer is often comparatively high, and the thermometer may even rise above 33° C. In the warm, sheltered valleys on the north side of the peninsula the cond tions must approach the warm-temperate, and this is shown in the fact that many typical North Island plants find here their southernmost limit. (A list is appended, pp. 369–70.)
The rainfall varies in different localities, but as comparatively few records have been taken on the peninsula it is impossible to give detailed results. However, the average probably varies from about 25 in. (Convalescent Home, Cashmere Hills) to about 50 in. or 60 in. (on the top of Mount Herbert). A series of careful observations extending over nineteen years (1899–1918) at Pigeon Bay, taken by Mr. E. Hay, give an average of 29.5 in., with a maximum of 39.5 in in 1913 and a minimum of 16 in. in 1915. The rainfall in the outer portions of the area between Pigeon Bay and Akaroa is no doubt higher than this. In the latter place the records show an average of 45 in. Between Timutimu Head and Birdling's Flat there is again a reduction in the rainfall, for the Akaroa Hills cut off
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much of the easterly rain. This district therefore reproduces largely the flora of the Lyttelton Hills, but, having a southerly aspect, no doubt is neither quite so hot nor so dry as they are.
Snow falls every year on the hilltops, and on Mount Herbert usually lies for some weeks, but at the base of the hills it does not often fall, and usually does not lie for more than a few hours. The winter of 1918 was exceptional, when snow fell to a depth of 10 in. oven at the foot of the hills, and the conditions above 1,000 ft. were truly alpine. Such falls do much damage in the forest in breaking down trees and branches, though but little permanent harm results if frosts do not follow.
Next to the rainfall and temperature the most important climatic factor is the wind. The prevailing wind is from the north-east. This in summertime is usually a sea-breeze, and then brings no rain, but when part of a cyclonic system it frequently brings continuous though often light rains lasting over many hours. The north-east wind passes into the north-west wind, which is much less frequent, and is hot and dry. Only on the rarest occasions does it bring a scanty shower of rain. It has, as will be seen later, a most important effect on the distribution of the plants within the area. The south-wester is a cold, wet wind, bringing much rain, and determines largely the vegetation on the cliff-faces exposed to the south. The south-easter is, on the northern side of the peninsula, a somewhat rare wind, of a gusty character—often stormy and occasionally bringing heavy rains. It is often deflected as an easterly or north-east wind. The higher rainfall of the outer portion of the peninsula is largely due to it.
The rainfall is very irregular in its distribution over the year. The months of December to March are usually dry, but exceptions occur. During this period droughts are not uncommon, particularly on the north-west faces of the peninsula, and often affect the vegetation. This has tended to produce a distinctly xerophytic type of vegetation on the Lyttelton Hills, though elsewhere it tends to the mesophytic.