Factors determining weather
The weather is the particular atmospheric conditions and events at any one time. The climate is the average and the range of conditions over several decades.
New Zealand’s weather patterns are determined by three factors: wind, sea and land.
Wind
With much of the country lying in the roaring forties weather system, New Zealand is a windy place. The winds bring New Zealand’s weather from all directions:
- Westerly winds are the most common. They carry wet weather to areas upwind of the main mountain chains, and dry, warm weather along the east coasts. Westerly airstreams often extend for thousands of kilometres upstream of New Zealand.
- Northerlies can bring tropical air, heavily laden with water vapour, and are responsible for most of the flooding.
- Southerlies can bring cold Antarctic air, which can cause snow to fall at sea level.
- Easterlies are usually airstreams that have changed direction only a few hundred kilometres upwind of the land. A northerly airstream from the tropics can turn east towards the land near the end of its journey, bringing heavy rain to eastern areas. A southerly airstream that turns easterly will often be dry, or bring just a few showers.
Sea
Air blowing towards New Zealand crosses thousands of kilometres of ocean before reaching the coast. The air’s contact with the sea surface has two effects:
- It moderates the temperature. Warm air leaving the tropics is cooled as it moves over the cold ocean surface nearer New Zealand. Cold air from Antarctica is heated as it crosses progressively warmer water on its journey towards New Zealand.
- The air picks up moisture. Evaporation of water from the sea surface means that the air has high relative humidity by the time it arrives. However, the absolute humidity of the air from the north is generally two or three times higher than the air from the south. This is because the amount of water vapour that warm air can contain is much higher than cold air.
Land
On any day the weather is determined by the direction of the wind, and the impact the mountain ranges have on this.
Air that blows in from the sea is forced to rise as it moves over higher ground such as a mountain range. As the air rises, it encounters lower air pressure, which causes it to expand. This in turn lowers its temperature. As the air cools, some of its water vapour condenses to form the tiny, liquid droplets that make up clouds. If rising and cooling continue, enough cloud droplets develop so that some of them combine to form raindrops (precipitation).
How windy is Wellington?
On 173 days a year, New Zealand’s breezy capital is buffeted by winds of over 60 km an hour. Gusts regularly reach 140 km an hour. The city’s strongest winds were an incredible 248 km an hour, at Hawkins Hill on 6 November 1959 and 4 July 1962. (This was just under the national record: on 18 April 1970, gusts of 250 km an hour hit Mt John in Canterbury.)
Once the air begins descending on the other side, the higher air pressure at lower altitudes compresses it. Its temperature rises and any cloud droplets evaporate. So a north-west airstream crossing New Zealand will typically bring a period of heavy rain about the main ranges and west of them. It then moves towards the east coast, bringing dry warm weather.
In the South Island, the prevailing winds off the Tasman Sea meet the Southern Alps. The resultant precipitation makes the West Coast the wettest area of New Zealand. Annual rainfall at Milford Sound is over 6,000 millimetres, and 10,000 millimetres just below the divide of the Southern Alps. But just 100 kilometres or so to the east is the driest region, Central Otago, with annual totals of around 400 millimetres or less.
