A tornado is a rapidly spinning column of air that reaches from a thunderstorm down to the ground. Tornadoes can be incredibly destructive, with winds that can exceed 300 kilometres per hour. They occur most often in the central United States, a region known as Tornado Alley, but they can happen in many parts of the world, including Australia. These violent storms form when specific weather conditions come together. Understanding how tornadoes form helps meteorologists predict them and give people time to take shelter. For a tornado to form, three main ingredients are needed: warm, moist air near the ground; cooler, dry air aloft; and wind shear.
Warm air rises because it is less dense, and as it rises, it cools and condenses to form a thunderstorm. Wind shear is a change in wind speed or direction with height. This shear causes the rising air to start spinning horizontally. Think of a rolling pin spinning along the ground. This spinning horizontal tube of air is the first step in creating a tornado. The next stage happens when the thunderstorm's updraft tilts that spinning horizontal tube of air into a vertical column. This creates a rotating wall cloud called a mesocyclone.
Mesocyclones are about 2 to 10 kilometres across and are often visible as a lowering of the storm's base. The updraft then stretches the spinning column vertically, making it spin faster, like an ice skater pulling in their arms. This strong rotation leads to the formation of a funnel cloud. A funnel cloud is a rotating cone of cloud that extends from the mesocyclone but does not yet touch the ground. If the funnel cloud continues to spin faster and reaches the base of the thunderstorm, it becomes a tornado once it makes contact with the earth.
The next stage happens when the thunderstorm's updraft tilts that spinning horizontal tube of air into a vertical column.
The pressure inside the funnel is much lower than outside, which causes air to rush in and cool, creating the visible cloud of condensation made of water droplets. The tornado's damage comes from its fierce winds and the flying debris they pick up. Tornadoes are rated on the Enhanced Fujita (EF) scale based on the damage they cause, from EF0 (weak, winds of 105–137 km/h) to EF5 (incredible, winds over 322 km/h). The width of a tornado can vary from a few metres to over a kilometre.
They usually travel at speeds of 50 to 80 kilometres per hour, but some have stayed still or moved much faster. Most tornadoes last only a few minutes, but some can last over an hour. The most destructive part is not the wind alone but the debris that it flings at high speed. In Australia, tornadoes are less common but still occur, especially in the southeast and along the east coast. They often form from strong thunderstorms associated with cold fronts. Because tornadoes can develop rapidly, it is important to have a plan.
Warnings are issued when a tornado is spotted or indicated on radar. The safest place is in a basement or an interior room on the lowest floor away from windows. By understanding the science behind tornadoes, we can better prepare for these powerful natural events.