Welcome to the depths of winter. Heavy frosts around parts of New Zealand and frosts right up the upper North Island have helped cement the season that, in previous years of the past decade, has occasionally felt absent for some.
I can recall writing a number of news headlines and blogs/columns over the past 10 years declaring spring had arrived in the month of July. That seems a bit hard to fathom considering the recent burst of cold sub-Antarctic air across the nation – but all it takes is for high pressure to be parked east of NZ and suddenly our sub-Antarctic southerly becomes a subtropical northerly. Not a lot of countries on earth can have such a simple switch from cold to warm from the same single air pressure system over a couple of days. But that’s NZ’s unique location.
I’ve always maintained NZ has a short summer and a short winter from a weather point of view – around two months for each of those seasons, while autumn and spring seem to be four months each. In fact, NZ can be so “mild” in winter that some farmers are famous for wearing shorts and singlets during it. We can also get pollen in late winter (especially pine pollen in August) and I’m sure many of us have seen daffodils out in July – again, not the normal sign of the depths of winter in other countries.
High pressure controls our weather a LOT more than we realise. We think of low pressure zones as the headline makers, because they can bring a lot of severe weather. But often damaging winds in NZ are caused by high pressure nearby.
The bigger the mountain of high pressure is, the more isobars there are and the windier it gets. The bigger the high pressure, the greater the “reach” is to scoop up air from the tropics or polar regions. The same high pressure system can do this as it slowly tracks over.
And it’s a single mountain of high pressure that can extend dry periods – like we saw in May (and now again this month) – or can park itself east of NZ (like it did in June) and cause low pressure to stall over NZ for weeks, bringing heavy rain and showers.
Next week, NZ has a mountain of high pressure, and I use that term “mountain” because in my mind that’s what I see. A huge dome moving in with steep edges and a large summit.
The isobars on the weather maps are a version of topographic lines in the sky. The closer those lines are, the steeper the “terrain” gets in the sky –and the steeper the slope, the stronger the winds get.
We get some of our most powerful high pressure zones in our coldest months – now to September, roughly.
So get used to more frosts, cold nights and black ice – but it’s quite possible the same high bringing this current cold weather will bring subtropical or northerly warmth soon, too.
