Weather Watch: Drop in degrees the only danger from ash

It seems bizarre that an erupting volcano two-thirds of the way around the Southern Hemisphere is disrupting so many travellers here in New Zealand - but that's been the case this week as the ash cloud has been caught up in the Roaring 40s belt of wind.

It was one of those news stories that created that, "what the heck, where did this come from?" response. After the first eruption I said that the ash shouldn't pose a serious travel risk for flights around, to and from New Zealand. But, clearly, a couple of airlines didn't agree. However that changed towards the end of the week when a new plume came in at a much lower altitude, just 3000 metres (10,000 feet), forcing Air New Zealand to also cancel flights.

The ash cloud has been caught up in the jet stream, which has fluctuated between 6kms and 8kms above sea level and can climb to more than twice that height, but Thursday's new plume was significantly lower. The ash is not falling to Earth over New Zealand. It will eventually spread out across the globe and micro-particles - which pose no danger - will, over the next few months, mostly fall into the sea in such a diluted form that we won't even notice it. Some ash particles will simply evaporate.

The one thing we may notice could be a drop in temperature. Dr Jim Salinger says our climate (which is measured over months) could drop 0.2C. But, in the short term, the ash cloud may also have a significant localised effect if the cloud is thick enough. This could drop temperatures further for those in Christchurch who are already dealing with cold weather and a fragile electricity network.

Where the ash cloud is particularly thick, it's possible we may see the local temperature drop by 1C or 2C.

I base this prediction on the events of 9/11. After planes in the United States and Canada were grounded the temperature instantly rose by a couple of degrees after all the jet trails disappeared from the sky. The ash cloud could do the opposite over our skies, acting like a thick jet trail and briefly lowering temperatures here.

I've fielded a number of questions from travellers asking if it is safe to be flying, telling someone ... no pilot would risk their life flying into ash, they're extremely cautious too.