The deadliest tornado in US history was part of a tornado outbreak that struck the southern states in spring 1925. It touched down in Missouri and tracked north-east through Illinois and on to Indiana where it dissipated. In its wake the F5 monster – the highest possible rating on the Fujita scale – destroyed 15,000 homes and killed hundreds of people. It could move at 110 kilometres (70 miles) per hour, travelled 352 kilometres (219 miles) and, because it was so massive, it appeared as an enormous black, ground-hugging cloud, rather than the characteristic funnel shape.
The Tri-State Tornado was born out of a cold low-pressure system that had been following what we now know is the jet stream, down from Canada, along the Texas- Oklahoma border and into Missouri. It’s here that it hit a warm front from the Gulf of Mexico and conditions were made perfect for a tornado outbreak. Judging by the speed the Tri-State Tornado travelled at, it’s likely the winds in the jet stream were particularly strong at the time.