Weather and climate

Although weather and climate are closely related, they are not the same thing. The chief difference between weather and climate is time.

Weather

Weather is the result of atmospheric conditions over a short period of time. When we think of weather, we think of temperature, precipitation, humidity, cloudiness, wind and atmospheric pressure. Weather conditions can change in a manner of minutes – for example, a warm sunny day can be interspersed with sudden showers.

Climate

Climate is defined as the weather conditions of an area averaged over a series of years, usually 30 years or more. Climate is the weather conditions we expect at certain times of the year based on long-term observations from instrumental monitoring, generational observation and mātauranga Māori. For example, we expect late summer to early autumn to be the driest time of the year for much of Aotearoa.

Influences on weather

Often, westerly winds move weather systems eastwards over the country. Due to the country’s long, slender north-to-south orientation, most of the weather in Aotearoa comes from these westerly flows. However, weather systems can also bring air from tropical or polar regions along with heavy rainfalls or cold showery precipitation.

Influences on climate

The climate is influenced by several factors. The first is geographic location. The country is located across latitudes 34° to 47° south – about midway from the equator, which influences the concentration of sunlight we receive. The second factor is the sea. The relatively mild sea-surface temperatures moderate the temperatures on the land. The third factor is topography. The mountains and highlands act as barriers to westerly weather systems. This effect is most obvious in the South Island’s West Coast – the wettest region – while over the divide is inland Otago – one of the driest regions. Topography also affects temperatures – generally, the higher the altitude, the cooler the climate.

Humans also have an effect on climate. By adding greenhouse gases to the atmosphere, we are changing the delicate balance of energy entering and leaving the Earth’s atmosphere. This extra energy trapped in the atmosphere is leading to changes in local and global climates.

Local and regional differences

Weather and climate vary from region to region and within regions. For example, the Bay of Plenty is sheltered by high country on three sides, so the region tends to be sunny and calm. The opposite occurs in the Taranaki region – it bears the brunt of westerlies coming in from the Tasman Sea and is one of the windiest places in the country. The Canterbury region has five different climate zones due to the influences of the Southern Alps to the west and the Pacific Ocean on the east.

Variations in climate

There are variations in climate caused by natural oscillations. A climate oscillation is a recurring climate pattern. There are three natural oscillations that influence New Zealand weather and climate. Each oscillation lasts for a different period of time.

The oscillation we hear about most often is the El Niño Southern Oscillation (ENSO). Surface waters in the central and eastern Pacific Ocean become warmer than usual, and this affects atmospheric conditions. During an El Niño event, Aotearoa usually experiences stronger westerly winds in summer, which leads to rain in the west and drier conditions in the east. During winter, southerly winds cause colder temperatures. El Niño events occur every 2–7 years and last around a year. During La Niña events, the country experiences warmer air temperatures, more rain in the northeast and reduced rainfall in the southwest.

The Interdecadal Pacific Oscillation (IPO) has phases that last around 20–30 years. Its phases affect the strength and frequency of El Niño events. The Southern Annular Mode (SAM) can bring unsettled stormy weather to New Zealand. SAM phases are short – lasting for a few days to a few weeks.