MEDIA RELEASE
WEDNESDAY, NOVEMBER 8, 2017
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.
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 figures 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.
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.
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 C 84 51
8.8 C 93 62
9.0 C 100 70
9.5 C 113 85
ends