Wind.

Wind, especially in certain botanical districts, is a most important ecological factor. Generally speaking its effect becomes more intense the further south one proceeds or the higher one ascends. It is also of great moment on the coast especially on the west and on small islands.

The westerly winds of South Island are of special moment. Striking the western mountain wall, the wind loses its moisture in passing over the high lands and descends on the east as a hot wind sweeping through the river valleys and over the gravel plains, raising transpiration to its maximum. Though this hot north-west wind occurs to the east of the Southern Alps as a whole, in places its effect is greatly lessened through interception by ranges to the east of the Divide, so that its maximum strength is limited to the valleys and plains of the North-eastern and Eastern districts, where it even reaches the coast-line, and to the North Otago district. Perhaps its greatest intensity is experienced on the Canterbury Plain and the montane valleys extending westwards from its upper part to the Divide. Such a "Canterbury nor-wester", as it is called, sweeping through the river-gorges page 59bursts with all its fury upon the plain — a hot, dry wind, its progress marked by clouds of sand and silt rising out of the wide beds of the glacial rivers. In the west clouds hang over the distant mountains indicating the rainstorm that is raging there, but over the plain is a clear blue sky while a burning sun strikes down. On plants the leaves hang flaccid, in orchards the trees are stripped of their fruit, everywhere the surface of the ground if unprotected by vegetation becomes dry as dust and the soil of ploughed fields may be blown away. On the dunes sand in clouds is carried back to the sea, sandhills are bodily removed and the rope-like entangled stems of Desmoschoenus, metres in length, laid bare. On mountain-passes and exposed ridges the fury of the storm reaches its height, it is impossible to stand upright, small stones are hurled through the air.

The south-west wind — of far wider range — is of equal ecologic importance and frequently brings with it squalls of a subantarctic character, leading to snow on the mountains, or even to a heavy downpour, but at other times rain is wanting while a furious gale extending to the southern parts of North Island rages for one or two days at a time. The change from north-west to south-west is quite sudden the temperature dropping many degrees and conditions approximating to those of midwinter may occur in the middle of summer. Obviously such sudden changes are of great physiological importance.

The average velocity of the wind in the following tables is from records of the Robinson anemometer in kilometers per day.