|
Sail: Weather
links: Stormwatch: La
Nina: Profiles: Surf: Sky: Polar: Ocean: Fire: Drought: Weatherfacts: Books: Products:
| Tasman balloon crossing When Australian adventurer Dick Smith crossed the Tasman by balloon in late February, he did so by travelling 'against the prevailing winds'. But what does that mean? Well, in fact, he never travelled against anything. What he did was float with easterly winds across the top of an anti-clockwise spinning high pressure cell, which filled the entire Tasman sea. "We need a high pressure system between Australia and New Zealand that will stay there for a few days, but that only happens a few times a year," Mr Smith told the press before take-off.
"I could go from Oz to NZ (in a hot air balloon) any day of the week, but to go the other way is very hard. That's part of the attraction for me. I like doing things that are a bit hard." While 'highs' in the Tasman are common enough, they tend to move quickly out into the Pacific, caught up in the constant west-to-east migration of our planet's weather. If that had happened, Dick Smith and pilot John Wallington would have have travelled in a great circle, drifting west over the top of the high's centre, then south and then east. Perhaps they would have ended up in New Zealand again. But in practice, the ballooning pair would have ditched their craft into the ocean. What made the journey possible was the fact that the 'high' stayed put for days. Forecasters call it a 'blocking high' and these systems tend to occur during late summer, from February to April. They are particularly prevalent during La Nina years, when stronger-than-normal monsoonal cloud bands form over inland Australia and sit there, helping to stall the easterly movement of weather. This is exactly what the ballooning pair were waiting for.
According to Terry Hart, superintendent meteorologist with the Australian National Meteorological Operations Centre, the flow of air at 20,000 feet helped contribute to this phenomenon. During the flight, the 'jet stream' tracked east over Perth and was forced south over Adelaide by monsoonal clouds soaking central Australia. At this point the jet stream split, with one arm turning dramatically north along the coast and east over the Coral Sea before diving south towards New Zealand. The other arm tracked south into the Southern Ocean. "This pattern where the jet stream splits tends to be stable and can last for a few days," said Hart, who helped advise Dick Smith and his team. Does the jet stream make the high pressure cell stall, or does the monsoonal cloud band over inland Australia do the job? Hart says all the weather systems are in a 'dynamic loop' in which systems feed off each other. "It's kind of like the chicken and egg thing," he says. Both Dick Smith and John Wallington praised the Australian Bureau of Meteorology for their computer modelling of conditions over the Tasman. Hart, who overseas the seven day forecasts for the bureau, says the modelling - which incorporates observations from all over the world - is very good at predicting big events, but no so crash hot at forecasting smaller events. Very localised events are too chaotic for the machines to predict, he says. This is what happened on the morning of the second day when a weak front pushed into the high, creating northerly winds ahead of the change. These northerlies weren't expected, but the computer model did note that if the balloonists rose a few thousand feet, they would find a stream of easterly air. They followed the bureau's advice, found the easterly stream and the rest is history. Dick Smith flight: Tuesday, Feb 22 about 6am EADT to Thursday, Feb 24, about 1pm AEDT. Time taken: 55 hours and 17 minutes. More great reading:
© weather-wise.com 2000 |