While poring through old temperature records, climate scientists recently discovered New Zealand's coldest known daily minimum reading – a shivering minus 25.6ºC (originally recorded in the old Fahrenheit scale as minus 14ºF).
That coldest of snaps struck in 1903, at Ranfurly in Central Otago.
On 18 July that shivery winter, The Tuapeka Times reported: "It is some years now since frost of equal severity to the present has been experienced in the district". Blocks of ice were seen tumbling down the Mataura River, and The Otago Witness reported that "ice-skating was general among the townspeople".
"I was quite excited [to find the record]," says NIWA climate scientist Dr Georgina Griffiths. "It's like detective work".
"We know the early 1900s were really cold, with extremely cold spells, and that Central Otago is really unique – it's the only region of New Zealand with a continental climate. It's not surprising that the new record is from this area," says Griffiths.
NIWA maintains New Zealand's national climate database, a collection of instrument measurements and written or oral records of temperature from the past 150 years.
Records must be analysed carefully, to identify the influence of any nonclimate factors, such as changes in observation site or method, or encroaching urban development.
To check the accuracy of the new record, the NIWA climate scientists examined other meteorological data from stations in the area. They chose stations at sites with a similar topography, over a range of dates, which also experienced similar freezing cold conditions.
"It was consistent," says Griffiths. "Southland and Otago were extremely cold, with cold air trapped for days on end. The other observations support the extremes. The paper record from Ranfurly was consistent, and the media reporting supports it".
The World Meteorological Organization (WMO) recently verified the new record, which contrasts sharply with our hottest: 42.4°C, recorded at Rangiora, in Canterbury, and Jordan, in Marlborough's Awatere Valley, on 7 February, 1973.
