By Terry Thomsen, nature guide and writer
Rule number one for visitors to Christchurch: pack for all weather and temperatures.
The weather’s changeability shouldn’t take visitors by surprise – just look at New Zealand on a world map to see why. The islands are smack in the path of an endless procession of anticyclones, depressions and fronts that march across the southern Pacific Ocean, most often from the west.
Don’t let this put you off. New Zealand has an equable, temperate climate. Christchurch itself is the driest of New Zealand’s main cities. Just don’tbe surprised if you get all four seasons in a day.
The nor’wester: Christchurch’s signature wind.
You can tell that a nor’wester is coming by a glance at the weather map. Look for a depression approaching the South Island from the west (as they normally do). The leading edge is a very humid airflow from the north-west, moving from the Tasman Sea on to the West Coast of the South Island.
The rising, cooling air dumps its moisture as rain – an awful lot of rain – on the West Coast and the Southern Alps. With all of its moisture depleted, the wind advances eastwards across the Canterbury plains and roars into Christchurch as the warm, dry nor’wester. This is the New Zealand version of the dry Fohn wind of Norway and the Chinook of North America.
The ‘rain shadow’ effect in Canterbury is one of the most pronounced in the world.
In a single 24-hour period in November 2006, over 300mm (12 inches) of rain fell in the Cropp valley on the western side of the Southern Alps (by comparison, Christchurch and London both get around 650mm of rain per year). In that 24 hours, the same wind brought a dry, mild spring day to Christchurch, with not a drop of rain.
The southerly wind: have your jacket handy
When a depression and cold front moves across the South Island, the weather in Canterbury can change abruptly from a warm nor’wester to a cold southerly, bringing frigid air and rain straight up from the Antarctic Ocean. If you are in Christchurch and see ominous clouds massing over the Port Hills, have your jacket handy!
At times like this, the ‘rain shadow’ works in reverse: while it is cold and wet in Christchurch, the West Coast is enjoying a clear, sunny day.
Christchurch temperatures – monthly
Average Daily Minimum
Average Daily Maximum