A climate of extremes
The mountains give Canterbury a climate of greater extremes than most other parts of New Zealand. Of the country’s main cities, Christchurch has the least rainfall and the greatest range of temperatures.
The mountains lie at an angle to the prevailing westerly air flow. These westerlies lose their moisture when they rise to cross the alps. On the eastern ranges and the plains, rainfall is much less. Rainfall on the plains comes from the south and east, when depressions off the east coast push southerly flows over Canterbury.
Christchurch has an average annual rainfall of 648 millimetres (roughly half that of Auckland and Wellington).
The nor’wester blows across the plains as a hot, dry wind which can send Christchurch’s temperature soaring above 30°C. Associated with the nor’wester are distinctive cloud formations, including the nor’west arch. Typically, this wind is followed by a southerly.
Early settler Mark Stoddart wrote a poem about Canterbury’s nor’westers. One verse runs:
I’ve witnessed all the winds that blow, from Land’s End to Barbadoes –
Typhoons, pamperos, hurricanes eke terrible tornadoes.
All these but gentle zephyrs are, which pleasantly go by ye,
To the howling, bellowing, horrid gusts which sweep down the Rakaia.’ 1
In winter, southerlies occasionally bring snowfall to the plains, and cold air that causes hard frosts.
Christchurch is also affected by easterly winds. Easterly cloud keeps the sunshine hours on the coast to 2,100 hours a year.
Canterbury’s strongest winds blow from the north-west. A north-west gale on 1 August 1975 flattened tree plantations and damaged buildings. The 26–27 December 1957 storm that caused severe flooding in the mountains was also a nor’wester.
Canterbury is subject to two sorts of floods:
- North-west rain in the mountains makes the Waiau, Hurunui, Waimakariri, Rakaia and Rangitātā rivers flood.
- Southerly or easterly rain makes the smaller Ashley, Selwyn, Ashburton and Hinds rivers flood.
The Rakaia River was recognised as a threat to farmland in the 1860s. But the most severe flood hazard in Canterbury is the flood plain of the Waimakariri River. The first major protection works were built in the 1860s, but Waimakariri flood waters flowed through Christchurch in 1868. The last serious break-out of the Waimakariri occurred in 1957.
Much of Christchurch was built on drained wetland, so local flooding after storms has been a periodic problem. This has become particularly acute following the February 2011 earthquake in a number of places, especially the Flockton Basin, Mairehau.
Canterbury is prone to droughts. The average rainfall is adequate for farming, but during nor’westers the rate of evaporation is high, especially from shallow, gravelly soils.
Droughts of a few months to two or more years occur on average once every six years.
Frosts and snowfalls
Frosts can be particularly heavy inland. Some lakes become frozen in winter, and Lake Lyndon and Lake Ida have in the past been popular for ice skating. Snowfalls in Canterbury are usually little more than an inconvenience, except in the high country, where heavy snow can cause severe stock losses – as in 1868 and 1895. Record snowfalls in Christchurch occurred in 1918 and 1945.