Tropical Cyclone Larry hit Innisfail, Australia, on Monday morning, ripping the roofs off buildings and damaging 55 percent of the homes in the city. "It looks like an atomic bomb hit the place," said the mayor, and city residents repeated his claim. Could Larry's 180 mile-per-hour wind gusts cause as much damage as an atomic bomb?
It depends how you calculate damage. A Category 4 cyclone like Larry might level buildings over an area of 2,500 square miles. (A hurricane is just what we call a tropical cyclone that originates in our part of the world.) It would take a 10-megaton nuclear weapon to inflict damage over such a wide area. By comparison, the atomic bombs dropped on Hiroshima and Nagasaki were only about 15 and 21 kilotons. (There are 1,000 kilotons in a megaton.) It's a little tricky to compare the two, since a nuke would obliterate everything near to the site of the explosion and inflict less damage as you moved farther away. A hurricane, on the other hand, would cause less severe damage, but the damage would be more uniform and over a larger area.
You can get a better sense of how the two compare by looking at the devastation caused by a nuclear weapon at various distances from the explosion. Then, it's possible to figure out what kind of tropical storm would cause a similar amount of damage at each distance. Savannah, Ga., commissioned this kind of analysis in advance of the G-8 meetings of 2004. Part of the study looked at the structural damage caused by a 25-kiloton nuclear explosion. At a radius of three miles, the damage would be equivalent to that caused by a piddling hurricane with wind speeds averaging 80 mph. At half that distance from the blast, the effects would be more like a Category 5 storm with 160 mph winds.
Nuclear explosions can destroy a city with thermal radiation and a wave of kinetic energy. They also generate high-speed winds that cause damage in much the way a hurricane might. The blast wave consists of an expanding line of very high pressure (called "overpressure") that draws gusting winds in its wake. The wave of overpressure sweeps past structures very quickly, followed by winds that can crash into them at hundreds of miles per hour. Thermal radiation can also produce high winds by igniting flammable materials and setting off a mass fire. The rising heat from a mass fire causes cool air to rush in from the periphery at hurricane velocities. A real hurricane works in a similar way.
Nuclear blast-winds don't affect buildings in quite the same way as hurricane winds of the same speed. The nuclear blast-wind looks like a strong pulse, or gust, rather than the sustained winds of a hurricane. In that sense, it causes damage more like a tornado would—by hitting a building with high intensity for a short period. On the other hand, blast winds generally hit a building from one direction like hurricane winds. Tornado winds cause extra damage by twirling around as they hit.
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Explainer thanks Chuck Watson of Kinetic Analysis Corporation.