The Southern Alps create a general rain-shadow effect over Central Otago so that the area has the driest and most striking of all climates in New Zealand. The rainfall is usually under 20 in. and Alexandra records an average annual rainfall of 13·2 in., and only 100 days of rain. The sunshine hours are correspondingly high. A considerable portion of the rainfall is, however, in the warmer months, reducing its effectiveness and accentuating the dryness of the area. As is usual in dry climates, the rainfall regime is highly variable from year to year. Extremes of temperature, especially diurnal, are associated with radiation frosts and the drainage of cold air at night followed by clear, sunny days. Ophir has the lowest recorded temperature for New Zealand, –3°F. Alexandra records a mean daily maximum of 73·6F in January and 44·7F in July, and a mean daily minimum of 51·16F in January and 27·9F in July.
It is the tussock grasslands which gives much of Central Otago its superb tawny colour, set off by the glowing autumnal colours of the deciduous trees and by the green of irrigated pastures. The dry climate is the essential factor in accounting for the tussock vegetation of the most central parts, but there is strong evidence to suggest that the tussock grasslands, through the use of fire, had been extended well beyond their original limits even before the arrival of the European pastoralists. Their constant burnings severely disrupted the tussock ecology, leading to a deterioration of pasture and the acceleration of soil erosion so that in the worst-treated areas semi-desert conditions were produced.