A confirmed tornado ripped through Hornsby yesterday afternoon, ripping trees out of the ground and leaving a demountable building on its side. The term 'mini-tornado' was once again heard frequently in relation to this event, however no such classification exists. The Bureau of Meteorology has since confirmed that the damage in and around Hornsby station and shopping centre was due to a full fledged tornado. The main reason why many people immediately suspect tornadoes to be 'mini' is that tornadoes in Australia rarely produce the extensive damage produced by the particularly newsworthy tornadoes in America. Tornadoes typically form within severe thunderstorms, like the one that hit Hornsby, and require an unstable atmosphere and plenty of wind shear. In this case, instability was produced by warm, moist air flowing off the sea into a low pressure trough extending through eastern New South Wales, leading to the development of squally showers and thunderstorms. The uplift isn't usually enough to produce tornadoes on its own. Rotation in the atmosphere is also very helpful. Yesterday the surface winds were fresh southerlies, while the mid level winds were gale force easterlies. As the thunderstorm formed, the rising air gained rotation and then as the storm reached Hornsby the rotation narrowed and intensified into a tornado, causing serious damage. Classification of tornadoes is based on the damage inflicted according to the Enhanced Fujita Scale. This scale rates tornadoes from EF0 to EF5, with EF5 being the most intense and damaging. The Bureau of Meteorology is investigating the Hornsby tornado today to determine its strength. From early reports of the damage it is expected that an EF1 tornado hit Hornsby with potential winds between 117-180km/h, but this is yet to be determined.