# How Tornadoes Form How a supercell builds a tornado, from wind shear rolling air over the prairie to a funnel touching down and then fading into a rope\. By brave\-panda\-ee86eb83 Topic: Earth & Geography [Open the interactive Jig](https://ask.jig.com/j/9WAOKmjZw3B0) ## Transcript ### 1. How tornadoes form A supercell thunderstorm towers over the prairie, and a tornado hangs from its base\. Every point of light is a tracer carried by the modelled wind: amber for warm, moist air near the ground, blue for cool air, grey\-white for cloud\. Step through to build this storm from calm air, one ingredient at a time\. Drag anywhere to look around\. Heights are stretched and the tornado is drawn far wider than life\. A typical tornado is about 80 metres across, under a cloud deck tens of kilometres wide\. ### 2. Warm air, cool air Storms feed on warm, moist air near the ground lying under cooler, drier air aloft\. Warm air that rises cools until its water vapour condenses into cloud\. The moister the low air, the lower the cloud base and the stronger the storm can grow\. Raise the moisture and watch the base come down\. Amber tracers turn grey\-white where they cross the cloud base: that is condensation\. ### 3. Wind shear makes a roll Near the ground a gentle wind blows from the southeast\. A few kilometres up, a strong westerly races the other way\. Air caught between the two speeds starts to turn over, like a pencil rolled between your palms\. The result is an invisible horizontal tube of spinning air across the prairie\. Meteorologists call this horizontal vorticity\. More shear spins the roll faster\. ### 4. The updraft tilts it Warm air rising in the storm's updraft lifts the middle of the roll\. The tube bends upward into an arch, and its two legs now spin about vertical axes, one each way\. A stronger updraft lifts the roll higher and tilts it more steeply\. Tilting turns horizontal spin into vertical spin\. The leg drawn into the updraft becomes the rotating heart of the storm, a few kilometres up\. ### 5. A rotating storm The storm favours one leg, and its whole updraft begins to turn\. This rotating updraft, a few kilometres up and several kilometres wide, is the mesocyclone\. It is what makes a supercell\. Below it, the cloud base lowers into a turning wall cloud\. Most supercells turn counterclockwise because of how the wind veers with height\. Earth's spin is far too weak to set a tornado's direction; a few turn clockwise\. ### 6. The rear\-flank downdraft Rain and hail drag air down behind the updraft\. This rear\-flank downdraft, shown in blue, reaches the ground and wraps around the rotation, squeezing it into a tighter column\. A downdraft only slightly cooler than the inflow helps\. Air that is too cold sinks and spreads out instead, and starves the vortex\. Storms whose downdraft is just a few degrees cooler than the warm inflow are the likeliest to make tornadoes\. ### 7. Touchdown As the rotation tightens, it stretches downward and spins faster, like a skater pulling in their arms\. The pressure in its core falls far enough for water vapour to condense, so a funnel appears to grow down from the wall cloud\. When the circulation reaches the ground it lifts a whirl of dust and debris\. The wind reaches the ground before the funnel does: the debris whirl, not the visible cloud, marks the touchdown\. ### 8. A mature tornado Warm air spirals in low across the ground, races up the core and joins the storm's updraft\. Slide from a narrow rope to a wide wedge\. Width is not strength: the wind speed comes from the storm, so a thin tornado can be as violent as a broad one\. The Enhanced Fujita scale rates tornadoes by their damage, from EF0 \(65 to 85 mph gusts\) to EF5 \(over 200 mph\)\. The widest on record, El Reno in 2013, was 4\.2 km across and rated EF3\. ### 9. Rope\-out Eventually the cold downdraft wraps all the way around the tornado and cuts off its supply of warm, moist air\. Without fuel the vortex weakens\. Its base is dragged behind, the column narrows into a twisting rope and it fades away, often within a few minutes\. Some supercells cycle: a new mesocyclone forms and the storm makes another tornado\. Press again to watch the tornado re\-form and rope out again\.