USA Stage 6 Tornadoes Comprehension PDF

STAGE 6 Unit focus: USA Text focus: Information Text How Tornadoes Are Formed North America sees more tornadoes than any other part of the world and particularly the most violent examples. There are upwards of 1000 tornadoes a year in the United States and there is even a region in central USA calle...

STAGE 6 Unit focus: USA Text focus: Information Text How Tornadoes Are Formed North America sees more tornadoes than any other part of the world and particularly the most violent examples. There are upwards of 1000 tornadoes a year in the United States and there is even a region in central USA called ‘Tornado Alley’ where tornadoes – also known as twisters– are concentrated. These spinning columns or funnels of air stretching between the ground and the storm clouds vary in size and intensity but the largest - reaching speeds up to 300mph inside and travelling across land at up to 70mph - can be catastrophic, destroying property and leading to loss of life from flying debris. In the USA, the Storm Prediction Service monitors conditions to identify when tornadoes are likely so that people can be prepared. So, what are these indicators and how do they result in a tornado? 1. The starting point for a tornado is the presence of warm air closer to the ground with much cooler air above. This happens when the ground is heated by the sun and warms the pockets of surrounding air. Since heat rises, these bubbles of warm air (thermals) rise upwards creating strong updraughts and forming storm clouds. 2. The second condition needed for a tornado to form is contrasting wind speeds at different heights above the ground (called wind shear). As the warm air ascends, it encounters higher wind speeds above the ground. The horizontally moving wind combined with the vertically rising air, results in a spinning motion. Clouds which have this rotating air are called supercells. 3. Whilst supercells have the ingredients for tornadoes, not all will ultimately result in them. For a twister to form, cooler air also needs to be sinking downwards to the ground. If these downdraughts of air at ground level are spinning fast enough, it can gain momentum like a figure skater pulling their arms tight to their body to speed themselves up. More and more warm air is sucked upwards and eventually, the column of moving air stretches and meets the ground forming a tornado – a funnel or tube of violently rotating air between the cloud and the ground. Rea e um http://www.literacyshedplus.com C u r r ic ul Th all resources ©2023 Literacy Shed und A ding ro 4. The appearance of a tornado is dictated by a number of factors. They vary in colour according to the local environment and the colour of the debris they accumulate. Most tornadoes begin and end their life like a long thin rope and some never develop beyond this before dissipating. Cone tornadoes are narrower at the base than at the top like an ice-cream cone. Wedge tornadoes are wider than they are tall meaning that they have a large area of destruction as they travel. Tornadoes forming over water are called waterspouts. The strength of a tornado is measured according to the ‘Enhanced Fujita scale (or EF scale) with ratings of zero to five depending on the wind speeds. 5. Tornadoes usually last only a few minutes though occasionally they can stretch to a few hours and cover up to 60 miles as they sweep across the land. However, eventually they weaken as the unstable conditions in the atmosphere which created them are lost. The tornado narrows and then disappears. RETRIEVAL FOCUS 1. What is an informal name for a tornado? 2. How fast can a tornado travel? 3. What is a supercell? 4. What are tornadoes over water called? 5. What do tornadoes look like as they reach the end of their life? V V I E S VIPERS QUESTIONS What is ‘debris’? Find a word that means ‘disappearing’. Why might tornadoes be less common in winter? How can we tell that this text is not aimed at experts such as scientists? Draw a diagram of a tornado. Make sure you include the supercell and the ground and show how air is moving. all resources ©2023 Literacy Shed http://www.literacyshedplus.com