When does spring officially start in NZ? It's complicated

Monday, 2 September 2019

MetService National Forecast Monday 2 September 2019.

Whether you're a September 1 or 21 kind of person when it comes to the official start of spring, rest assured the weather won't necessarily go along with either.

The seasons will unfold as they will, paying no attention to our efforts to constrain them for convenience.

One of the best-known seasonal rules is we have three months of each – with a sudden change, not far short of miraculous, to the next season on the first days of September, December, March and June.

The blooming of the first daffodils in Christchurch's Hagley Park is not always the best sign of the arrival of spring.

But we all know that doesn't work and that spring is often the most tempestuous of the seasons.

* Predicting how low snow will fall remains a difficult job for Kiwi forecasters

* Forecasting the weather when you don't know - precisely - what's happening now**

Heavy hail and snow proved difficult for drivers on the road to Sumner on this spring September day in 2016.

The start of spring, and of all the seasons, is more to do with the length of daylight and average temperatures than just the weather.

As a long and skinny country – more than 1600 kilometres from end-to-end in a straight line or 2033km along State Highway 1 – New Zealand straddles more than 12 degrees of latitude, which affects the length of daylight at different times of the year.

Lambs relaxing in the early spring sun at Rangiora. Or is it late winter still (file photo)?

That New Zealand also lies roughly northeast-southwest means we also cover more than 12 degrees of longitude, markedly affecting sunrise and sunset times between the northernmost and southern parts of the country at North Cape and Southland's Slope Point respectively, and between the most eastern and western points at East Cape and near Dusky Sound.

However, what is consistent across New Zealand is the increase in the length of daylight each day reaches a peak at the spring equinox (when daylight and night are of equal length).

High winds and heavy rain battering central Christchurch on a spring day in September 2018.

In Christchurch, Monday will have two minutes 49 seconds more daylight than Sunday. In Auckland, Monday's daylight lasts two minutes 13 seconds longer than Sunday while in Invercargill it is three minutes and seven seconds longer.

That rate of change flattens out at the equinox – September 23 this year – and then the daily increase in daylight hours starts reducing towards the summer solstice and longest day, December 22.

Many northern hemisphere countries choose March 21 as the start of their spring, around the equinox and a later date that probably reflects their longer winters.

But WeatherWatch managing director Philip Duncan suggests as many as four starting dates for the New Zealand spring:

Astronomical dates – based around the equinox and solstice, so nominally September 21.
Meteorological dates – the tidy three-month division of the year into seasons, so September 1.
The solar winter – the three 'darkest' months with the June 21-22 winter solstice in the middle, so about August 8.
Nature – hard to pin down.

MetService meteorologist April Clark said using 'meteorological dates' meant scientists could compare apples with apples.

'The meteorological seasons have been used for a long time, since the 1780s, so any weather data recorded about the seasons from then on can be easily compared with recent data.'

Daylight saving time begins on September 29. Winding clocks ahead by one hour has no effect on daylight length.

Given expected stormy weather this month and possible cold outbreaks, we might end up viewing September 1 as a false start.

Forget September 21 too, as it is not actually the equinox this year.

The 23rd anybody?