After using an online nuclear war game simulator, I used 5 megaton warheads. (Currently there isn't a simulator to offer multi warhead devices I've been able to find as yet.) After detonating 42 individual devices at an altitude of a half a mile over the intended 42 cities. Before the results I did a check. The US has a population of 316 million. The total loss of life by impact of 42, 5 megaton devices over populated areas only incurred a total of 50 million and change. With a slow kill from radiation sickness and fall out of 60 million and change. world population is 7.4 billion according to wiki. So the total while large from a country stand point is only drop in the ocean. 

The simulator did not include earth after shocks from being struck with 42, 5 megatons over the surface of the planet or the after effects of the heat released into the atmosphere would cause to current weather patterns. I tried to launch a nuke to the antarctic but the simulator had no information since it only tallied fatalities, in given area of effect. The program was too simplistic to offer more detail in scenario like what would a 5 megaton detonation do to an volcanic island. The interesting part was detonation over military complexes to see just how close to where I lived the rings of destruction got. 

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