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(d) Air Temperature
The predominantly maritime climate of the North Auckland Peninsula and the protection of the Gulf by the mainland to the east, south and west greatly lessens the occurrence of extreme low temperatures. In fact, the geographic pattern of the coastline is followed closely by the mean annual isotherm of 14 4° C. (Garnier, 1950, p. 61, Fig. 4). This isotherm is 1.1° C. higher than the mean for the corresponding coastline west of the North Auckland Peninsula, which is exposed to the full force of prevailing westerlies. The average range of variation is between a mean lowest minimum of 2.8° C. and a mean highest maximum of 27.1° C. (Fig. 5). The extreme highest and lowest temperatures yet recorded
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Text-fig. 4.
Auckland. Average monthly relative humidity and number of hours sunshine.
at Auckland are 32.3° C. and 0.0° C. respectively. When prevailing winds are onshore, February may be a warmer month than January, but in most eastern districts January is the warmest month. Coldest temperatures occur in July. Ground frosts are exceptional in coastal parts of the Auckland district, rarely more than three occurring in a year. However, a heavy frost in winter, 1951. was sufficient to kill mangrove (Avicennia) plants at Henderson, in the Upper Waitemata Harbour (V. J. Chapman, personal communication).