Weather Watch: City's island effect puts forecasters in a hard spot

It's common but rain in Auckland is still hard to predict. Photo / Martin Sykes

There are many weather sayings and terms that I loathe. I don't like "fine", which means dry and settled - but not necessarily sunny, as many people think. I don't like "fresh" being used to describe a wind that isn't cold - even though, technically, "fresh" is about speed and not temperature.

The saying I like least of all is "four seasons in one day".

Many other places on Earth claim to have this. It's certainly not unique to New Zealand.

But I concede that our weather is highly changeable.

Our two small mountainous islands in the South Pacific are smack bang in the Roaring Forties, making our weather one of the world's hardest to forecast. Auckland has one of the more fickle climates, making it even more tricky.

That's because Auckland is almost an island on its own. The city, and the North Island for that matter, narrows to just 2km at Otahuhu: the Tasman Sea to the west, Pacific Ocean to the east.

These coastlines nearly meet in several places. In West Auckland, New Lynn sits on the edge of the Waitemata Harbour (Pacific), while 3km south we have Manukau Harbour (Tasman).

With the harbours on both sides bending and twisting around, it makes most of Auckland's suburbs more like islands. That's why it can be sunny at 7am, overcast at 9am, showery at 11am, sunnier in the afternoon and clear at night. To only a handful of days a year would WeatherWatch.co.nz give the city a sun-only category.

Another reason for Auckland's changeability is the lack of big hills. The Waitakere Ranges in the west are only a little over 400m high, similar to the Bombay/Hunuas to the south. These aren't big enough to really impact on rainclouds as the big Kaimai and Coromandel ranges do to the southeast.

The predominant wind in Auckland is a sou'wester, which zips in through the Manukau Harbour, over the city then out into the Hauraki Gulf where it has nothing to stop it. The rain clouds rush though too, so the city also rarely receives solid rain for more than 18 hours, unlike the Bay of Plenty, which is surrounded by large ranges that can trap the rain for days (or trap the sun).

Auckland is like a much cooler tropical island. The weather is always changing but only slightly - from cloudy to clear, showers to dry. With weather like that it's no wonder Kiwis think that the Crowded House song Weather With You was written about Auckland.