Winters much shorter than they once were, says NIWA

New Zealand winters are a month shorter than they were 80 years ago, NIWA scientists say.

Spring came early in 2017, with August unusually warm. [Photo Dave Allen,NIWA]

Principal climate scientist Dr Brett Mullan has examined official temperature records from NIWA’s Seven Station Series which began in 1909. The series uses climate data from seven geographically representative locations around the country.

Dr Mullan defined a winter’s day as one in which the daily average temperature was less than a selected threshold and then compared the number of days this occurred for two 30-year periods, the first from 1909 to 1938 and the second from 1987 to 2016.

Climate scientists consider 30 years to be the minimum amount of time to come up with a statistically meaningful long-term average.

The graphs showed that, if a threshold of 9°C was chosen, then there was an average of 100 days per year between 1909 and 1938 when the temperature was less than 9°C, compared to only 70 days per year between 1987 and 2016. Winter has contracted about equally from both ends.

Comparing winter temperatures for 1909-1938 (blue curve) and 1987-2016 (red curve). In 1909-1938, the June 1 temperature averaged 8.6°C, but by 1987-2016 temperatures this low did not occur, on average, until June 16. Similarly, winter ‘ended’ in 1909-1938 with a daily temperature of 8.8°C, but this is now reached much earlier on August 14.

Winter is conventionally regarded as occurring between June 1 and August 31 in the Southern Hemisphere – a total of 92 days. If defined using temperature, however, winter lengths will vary from year to year. For example, in 2016 winter was essentially confined to July and August because June was unusually warm and August unusually cold. This year was more the reverse, with August unusually warm.

Dr Mullan said the number of frosts in New Zealand was also reducing in many locations, especially higher altitude inland regions of the country.

The statistics are mirroring trends worldwide. The US has also reported a shortening of winter, with the first frost of the year arriving more than a month later than it did 100 years ago.

“This is a consequence of rising temperatures around the globe, and such trends in colder temperatures and frosts will influence the life cycle of plants and animals”, said Dr Mullan.

Trends in colder temperatures and frosts will influence the life cycle of plants and animals. Ice forms on apricot trees in Central Otago. [Photo: Steve Le Gal]

Table of temperature thresholds

Number of days per year, on average, when daily temperatures in NIWA’s 7-station record are below a specified threshold

Threshold# days, 1909-1938# days, 1987-2016
8.5 C8451
8.8 C9362
9.0 C10070
9.5 C11385

A smooth annual cycle of daily temperatures fitted to the monthly temperatures from NIWA’s 7-station series, for the period 1987-2016. The dashed box outlines the standard winter period, June 1 to August 31. (Note that the tick marks on the x-axis denote the 1stday of each month).


Dr Brett Mullan
Ph 04 386 0508