Blow-by-blow explanation of why it’s windy here

It’s no secret Wellington is among the windiest cities in the world – the city’s sign was designed to look like it’s being blown away.

But why? What makes New Zealand’s capital a target for average daily gusts of 29kmh?

‘‘Basically, because Wellington is positioned in the gap of a long mountain chain,’’ said National Institute of Water and Atmospheric Research (Niwa) principal climatologist Mike Revell.

The mountainous areas of the South Island and at the bottom of the North Island created a tunnel, with the wind becoming stronger and faster as it funnelled through the small gap in between.

‘‘A good analogy is water flowing down a river and there’s a little gap between two big rocks and the water speeds up as it goes through that gap,’’ Revell said.

‘‘There’s a conservation of momentum – all this water moving slowly and it all has to get through that small gap so it has to speed up to conserve momentum. Wind works in the same way.

‘‘The reason it doesn’t flow over the top is because the atmosphere is reasonably stable, so all that air that pushes against New Zealand has to go through the gaps.’’

If the country wasn’t blessed with its mountainous topography, the wind would just sweep across it. It’s also to do with the capital’s location along Cook Strait. ‘‘Wellington is located on the southern side of the Tararuas and the wind does tend to be stronger than the other side.’’

Picton is on the tip of the South Island but its central position shelters the town from harsh winds.

And, if Cook Strait was land and not water, would Wellington still be as windy? ‘‘Wind strength is less over land than sea because land creates drag. So it would still be windy but slightly less so.’’