Weather Watch: Hurricane season

I've had a fascination with tropical storms since I was a kid. I love so much about them. Their unpredictability. Their power. Their names. (Hurricane Igor last year has got to be one of the best).

I'm especially interested in the Atlantic Hurricane season mainly because of the incredible coverage they have online and on TV. Surprisingly a lot of it isn't hype - the detailed service provided by weather news websites and TV channels comes from public demand - the public don't want to hear about a storm hitting and then wait days for the next update. They want to watch the storm form, grow, threaten and dissipate. Some even feel sadness when a tropical storm falls apart, especially if it's been around for a week or two and the name becomes a household name. You ask most Americans who live near the Atlantic or Gulf of Mexico and they'll tell you - when a hurricane forms they are glued to its development, even several days out.

This, despite the numerous times hurricanes are predicted to hit and end up dying out or making landfall in a different state altogether - sometimes even making landfall in a different country, the public watch every one develop like it's a new TV show with a new director, writers and actors.

Which is a great way of looking at a tropical storm - they all have their own personalities.

The hurricane coverage is taken very seriously because Americans know they are unpredictable and dangerous.

The long range models these days are very accurate at predicting long term storms - but still haven't worked out how to accurately pinpoint where they will make landfall.

Here in New Zealand it's important for us to understand the huge difference between a tropical storm and a storm that might form in the Tasman Sea. There are a number of technical aspects which I won't get in to this time, but there is a general difference which highlights just why cyclones are so dangerous and why they are tricky to accurately pinpoint too far in advance.

Almost all of our wind warnings in New Zealand are sparked by the pressure gradient between an area of low air pressure and an area of high pressure. This is what we call the squash zone or squeeze zone. The isobars on the weather maps show the "terrain" of the air pressure just like the lines on a topographical map show where the hills and valleys are. In this case the highs are the hills and the lows are the valleys. The more lines you have the greater the difference between the top of the high and the bottom of the low.

Spread those two systems out a long way and you have a gentle slope. Push them up together and you get a steep cliff face of air pressure, and like water down a waterslide so too slides the air - and there we get our wind. Certain geographical features such as the Southern Alps, Cook Strait or Kaimai Ranges can further bump up wind speeds.

That's what we normally face here in New Zealand - and the distance of that air pressure gradient can sometimes extend for over the length of the country. So it's very easy to see it coming and which regions will be affected.

With a tropical cyclone it is far more localised and often far more severe. Instead of that energy being spread out over say a 2000km distance it might be spread out over just a couple of hundred kilometres. And within that 200km wall of wind the depth of that low may be far lower than most storm centres that normally cross us.

So the path of a usual storm we have in New Zealand can often be clearly seen and the warnings are given and the system moves in.

Some tropical storms, such as Zelia, can be severe - but only cover a small space. The entire width of Zelia's destructive winds yesterday was just 100kms. That's the distance between Auckland and Hamilton, or an area that's only a little bit larger than Lake Taupo. Now on a satellite map it will shoot clouds out for hundreds of kms (sometimes over 2000kms) and look fairly impressive - but the area of intense wind and rain is much smaller - especially in our part of the world where tropical storms are rarely monsters.

This is why forecasting tropical (or ex-tropical) storms can be so hard. We're talking about an area of severe weather that's only a bit bigger than Lake Taupo coming down towards - but this small area is more damaging than most storms we get. And if one of those tropical storms jogs a little further in one direction it could make a direct hit on somewhere like Auckland - population 1.4 million. It's been a very long time since Auckland had a direct hit by a tropical storm - Drena was the last one in 1997. Zelia is not one that will be remembered.

But there's something about tropical storms that captures our attention - they can be small and very dangerous - and many of us are fascinated by them. Because of La Nina this year's cyclone season will be a busy one - and NIWA says New Zealand remains at an increased risk of being hit by one.