The prime movers of our air are fire and ice. The fire of the sun updates the warmth of the tropics daily, but each pole has six continuous months of darkness each year to grow icy cold.

New Zealand is half-way between the warmth of the tropics and the chill of the polar regions. Warm moist air from the tropics does not directly mix with cold dry air from the poles. Instead, tropical air and polar air twist around each other (causing low pressure areas or depressions) and bump into each other (causing fronts).

Air arrives in New Zealand after blowing over long stretches of ocean. It picks up moisture from the sea surface; since no place in New Zealand is more than 90 miles from the sea, most of the country has reliable rainfall.

But some of New Zealand's mountains are long and tall enough to interfere with air flowing over them by removing its moisture as rain, so that what arrives on the other side is drier and less cloudy.