NZ’s Winter Shorter by a Month over 100 Years
Studies by the National Institute of Water and Atmospheric Research (NIWA) show that winter has contracted equally at its beginning and end. Winter has gotten shorter by a month over the last 100 years, meaning “true winter” weather with very low temperatures, frosts and snow starts significantly later in the year and ends earlier.
Brett Mullan, from NIWA, crunched the temperature records from two 30-year-periods over the last century to see if New Zealand was experiencing the same contraction of winter weather noted in other parts of the world.
New Zealand’s winter starts at the beginning of June and runs to 31 August, a total of 92 days. In the South Island and some parts of the North Island winter weather usually means snow, frosts, “black ice” on the roads and temperatures as low as -7C overnight.
Mullan defined a winter’s day as one in which the daily average temperature was less than 9C. He examined temperatures between 1909 to 1938, and 1987 to 2016 from seven geographically representative regions around New Zealand.
Mullan discovered there was an average of 100 days per year between 1909 and 1938 when the temperature was less than 9C, compared with only 70 days per year between 1987 and 2016.
Winter has contracted about equally from both ends of the season, he said.
“I was quite surprised to get essentially the same results as the US – a decline of a month,” Mullan told the Guardian.
“It will certainly affect things like kiwi fruit production because the north of the country is becoming too warm, they are gradually moving kiwi production south.”
“Wintertime is also very important for farmers when it kills pests. If it stays too warm pests can reproduce right through the year. There are benefits too. If temperatures are higher you increase the growing-degree days and crops can mature more quickly, but the natural system has evolved to live with the winter conditions and it is quite important.”
New Zealand temperatures would continue to increase and winters would continue to get shorter and shorter, he said.
Original article by Eleanor Ainge Roy, The Guardian, November 10, 2017.