Thunderstorms — Page 249, Lesson 202

I want to walk you through the rest of the tornado discussion and then introduce the Fujita Scale, which is the standard way we classify tornado intensity based on the damage they cause. First, let's put this in context for European operations. The book notes that fortunately, in Europe we do not have such volatile conditions as the most extreme tornadoes seen elsewhere. Typically, the maximum diameter of a tornado in Europe will be 100 to 150 metres, but most are considerably smaller than that. They are most likely to be associated with air mass thunderstorms in the summer months, and they usually occur in the afternoon. When they occur over the sea, they appear as waterspouts — that's the same phenomenon, just over water. Now, let's look at how we actually classify tornado strength. Dr. Fujita of the University of Chicago devised a scale based on the damage caused by tornadoes. This is the Fujita Scale, often called the F Scale. It's important to note that the scale is based on observed damage, so different construction standards in different regions may result in an erroneous assessment of wind speed. In other words, a building built to a higher standard might survive a stronger wind, making the tornado appear weaker than it actually was, and vice versa. Let me walk you through each category from F0 to F5, and I want you to pay close attention to the wind speeds and the type of damage that defines each level. F0 is called a Gale tornado. The estimated wind speed is 34 to 63 knots, which is 64 to 116 kilometres per hour. These account for about 38.9% of all tornadoes — the most common category. The average damage path width is 10 to 50 metres. The potential damage is light: some damage to chimneys, branches broken off trees, shallow-rooted trees pushed over, and sign boards damaged. F1 is a Moderate tornado. Wind speeds are 64 to 97 knots, or 117 to 180 kilometres per hour. These make up about 35.6% of tornadoes. The average damage path width is 30 to 150 metres. The book points out that the lower limit of F1 is the beginning of hurricane wind speed. Damage includes peeling the surface off roofs, mobile homes pushed off foundations or overturned, moving vehicles pushed off the roads, and attached garages may be destroyed. F2 is a Significant tornado. Wind speeds are 98 to 136 knots, or 181 to 253 kilometres per hour. These account for about 19.4% of tornadoes. The average damage path width is 110 to 250 metres. Damage is significant: roofs torn off frame houses, mobile homes demolished, boxcars overturned, large trees snapped or uprooted, high-rise windows broken and blown in, and light-object missiles generated — meaning debris becomes dangerous projectiles. F3 is a Severe tornado. Wind speeds are 137 to 179 knots, or 254 to 332 kilometres per hour. These make up about 4.9% of tornadoes. The average damage path width is 200 to 500 metres. Damage is severe: roofs and some walls torn off well-constructed houses, trains overturned, most trees in a forest uprooted, and heavy cars lifted off the ground and thrown. F4 is a Devastating tornado. Wind speeds are 180 to 225 knots, or 333 to 418 kilometres per hour. These account for only about 1.1% of tornadoes. The average damage path width is 400 to 900 metres. Damage is devastating: well-constructed houses levelled, structures with weak foundations blown away some distance, cars thrown, and large missiles generated. F5 is an Incredible tornado. Wind speeds are 226 to 276 knots, or 419 to 512 kilometres per hour. These are extremely rare, accounting for less than 0.1% of tornadoes. The average damage path width is approximately 1100 metres. Damage is incredible: strong frame houses lifted off foundations and carried considerable distances before disintegrating, automobile-sized missiles fly through the air in excess of 100 metres, trees debarked — meaning the bark is stripped off — and steel-reinforced concrete structures are badly damaged. Now, I also want to cover a related but distinct phenomenon: dust devils. These are small whirlwinds that usually occur on hot and sunny afternoons when strong convective currents interact. In dry conditions, they will draw up dust off the ground — hence the name. While wind speeds may exceed gale force, they have a small diameter and will only extend up to a maximum height of around 2000 feet. As with tornadoes, dust devils will be hazardous to aircraft and should be avoided. So as a pilot, if you see a dust devil on the ground or near an airfield, you treat it with the same respect as any other convective hazard.