THE six chasers had waited in position for most of the day, just outside the giant hail and rain, buffeted by 55km/h winds screaming into the base of the ”supercell” thunderstorm, feeding its explosive appetite.Then just after sunset, a huge cone forced its way downwards, out of the cloud base, unmistakable even in the low light. It was a large tornado, carving out a path on the Colorado high plains.”We had almost given up and then there it was, tornado,” said Jimmy Deguara, one of a growing number of Australian storm chasers making the pilgrimage to America each year to witness the incredible show.”It’s called Tornado Alley for a reason. It’s the ultimate in chasing, the pinnacle,” Mr Deguara, a high-school teacher in western Sydney, said.A storm chaser for 17 years, Mr Deguara has spent almost every season of the past decade studying severe weather in the US and then bringing his expertise home.He was, for example, able to alert the Australian Bureau of Meteorology to the October 2007 tornado in Dunoon, on the NSW north coast, which blew out the walls of a church, tore the roofs off about 20 homes and caused a power substation to explode, leaving 3000 homes without power.Mr Deguara and five other Australian storm spotters went to the US this month to further hone their forecasting skills.Michael Bath, a forecaster with the Australian Early Warning Network, which provides alerts on severe weather and natural disasters, said US storms were ”on a whole new level”.”It just seemed that everything was perfect for tornadic thunderstorms day after day, the size and scale and how quickly it all happened was awesome,” he said.The Bureau of Meteorology relies heavily on storm spotters – there are about 2000 in Australia – and its severe weather warning service is one of the bureau’s highest priorities.While they are not as intense or as frequent as the tornadoes in the US, the bureau believes they occur more frequently in NSW than people are aware.However, because they often develop in sparsely populated areas, it is difficult to obtain accurate data, with only 383 tornadoes recorded in NSW from 1795 to December 2007. Eight people have died due to tornadoes since 1918.The twister that tore through Lennox Head this month, demolishing homes and flipping caravans, showed only the destructive ability of a weak tornado.The tornado is yet to be classified on the Fujita scale, which rates a tornado’s intensity between EF1 and EF5, based on the amount of damage it inflicts on man-made structures.An EF5 may leave only the concrete foundations of solid buildings and scour asphalt off roads.The only EF5 tornado recorded in Australia was in NSW on January 1, 1970, which left a trail of damage 22 kilometres long and 1.6 kilometres wide through Bulahdelah State Forest, destroying 1 million trees.When these supercell thunderstorms become organised they can produce powerful straight-line winds called microbursts, giant or ”gorilla” hail more than 10 centimetres in diameter and a tornado which can produce winds at speeds of more than 400km/h.They are like an engine in the sky, with a complex system of moving updrafts, downdrafts, inflow and outflow.A tornado is thought to form when horizontal rotation in the atmosphere created by wind sheer above the supercell is bent vertically towards the ground.
南京夜网