Anticyclones form when air currents in the high and middle atmosphere converge, piling up air in one area. The air inside these areas sinks from high levels towards the earth’s surface. The sinking air is warmed by compression, and this evaporates any cloud. This is why high pressure is generally – but not always – associated with good weather.

However, a layer of colder air is sometimes trapped below the sinking air, typically as cloud, and in winter this frequently produces drizzle. In summer, although the cloud usually evaporates before midday, its moisture often feeds the showers or thunderstorms that develop over certain locations – such as the Canterbury Plains.

Fine weather generally occurs in areas sheltered by a mountain range. The cloudiest areas are those where the wind blows from the sea onto the land, for instance Christchurch in a north-east wind.