The year NZ blew strangely hot and cold

Unseasonal weather patterns struck the country last year but Nelson basked in the title of sunniest town. Photo / Supplied

Heatwaves and sudden snow bombarded the country in 2009, a year of extreme heat and cold that left many wondering which season they were in.

Last year may have been fairly average by historical standards - ranking just 0.2C below the long-term temperature average of 12.5C - but the annual average masked a topsy-turvy year of sudden temperature change.

The year got off to a dry start in January, with less than half the normal rainfall over much of the country combined with unusually warm temperatures in many places.

February kicked off with a heatwave from the 7th to the 12th, and many places topped 34C.

But those who thought they were in for a toasty year were due for a shock.

Chilly southerly winds plunged temperatures to their coldest in May in many places, with an extra dose of rain on top.

The National Institute of Water and Atmospheric Research's climate summary for the year reveals May was 1.6C colder than the 1971-2000 average for the month (calculated using records from seven long-standing climate stations).

The coldest May recorded was in 1913, when the national average temperature was a bone-chilling 2.9C below normal.

The early chill ended abruptlyin August, which was thehottest nationally at 10.2C since records began 155 years ago.

All regions broke records for average August heat as northwesterly winds delivered an early spring.

It was not to last. The coldest October since World War II brought heavy snow dumps to Hawkes Bay and the central North Island and trapped unwary motorists in their cars.

"We typically say winter is the months of June, July and August [but in 2009 we said] there was an early onset to winter and spring," said Niwa climate scientist Dr Andrew Tait. "Unfortunately it got cool again in October, so spring didn't last very long."

Dr Tait said it was relatively common over the past 100 years for winds to get stuck in a particular pattern, as they did in May and August, and deliver an unusually hot or cold month.

As two such unusual months occurred close together, it resulted in an off-kilter year.

Dr Tait said the temperature extremes could not be explained by moderate El Nino conditions, which replaced La Nina conditions as the broad climate setting in spring. "Sometimes the weather just happens."

Meanwhile, Whakatane was again pipped at the post for the title of sunniest town after its annual sunshine could not be calculated because of instrument problems.

The Bay of Plenty sun-trap hotly disputed 2008's result, after Niwa questioned the reliability of its sunshine sensor and awarded the gong to Blenheim.

Whakatane's 2009 total from May to December (1614 hours) was one hour less than the sunniest town - Nelson - over the same eight months, but figures for the rest of the year could not be calculated.

Of the main centres, Tauranga was the warmest and sunniest, Wellington the wettest, and Christchurch the driest. Auckland was much drier than usual, having 81 per cent of its usual rainfall. It was 0.7C cooler than normal with a mean temperature of 14.6C, and 8 per cent sunnier than average.

Most of the six main centres were slightly colder and sunnier than usual. Wellington was the exception with higher-than-usual rainfall, cooler-than-average temperatures and about average sunshine.

Avalanches became a risk towards winter's end after a long snow season was topped off with hot weather in August.

WEATHER WINNERS

Warmest place: Whangarei, with an average temperature of 15.8°C.

Highest extreme temperature: Culverden in Canterbury with 38°C, Feb 8.

Coldest extreme temperature: -11.7C recorded at Middlemarch, July 19.
Strongest wind gust: 184km/h at Southwest Cape, Stewart Island, Nov 4.

Most rain in a single day: Mt Cook with the top three one-day rainfalls - 341mm on April 27, 321mm on May 16 and 295mm on April 26.

Driest place: Ranfurly in Central Otago with 263mm of rainfall.
Wettest place: Cropp River in the Hokitika River catchment with 10,956mm.

Wettest main centre: Wellington with 1274mm.

Driest main centre: Christchurch with 589mm.

Sunniest town: Nelson with 2571 hours, followed by Tauranga (the sunniest main centre) with 2540 hours.

Pipped at the post award: Whakatane, whose annual sunshine could not be calculated because of instrument problems. Its eight-month total from May to December (1614 hours) was just one hour less than Nelson's.