This is just a lovely analysis by +Randall Munroe
about playing relativistic baseball. The physics seem mostly right, except for the spherical shape of the resulting nuclear explosion; the baseball's motion should instead reshape it into roughly a hyperboloid (same shape as a sonic boom) with additional widening due to secondary fusion of the debris.
The fusion will slow down the baseball more than Munroe claims, too. Although it's going very fast, that also means it's hitting a lot of nuclei per second, and that's a lot of explosions. (Even after the original baseball is wholly consumed, there's still a flying blob of very hot nuclei to fuse) Using figures for a standard MLB baseball and air that's basically pure Nitrogen at STP, and approximating 10MeV energy released per fusion event (assuming that it's dominated by fusion of Hydrogen in the baseball with Nitrogen in the air, or Hydrogen in the air with Carbon in the baseball) you end up with a simple differential equation for the speed of the baseball which you can solve numerically. The baseball is brought to a stop within about 300 microseconds, during which it will have traveled about 84km. Since the cone of fire will be dominated by X-rays during this short time interval, it will also spread at the speed of light, with the result that the fireball will look at first somewhat like a cone lying on its side.
So what you'll really see is a very bright flash streaking across the countryside, leaving in its wake an expanding cone of light brighter than the Sun (but tinged noticeably blue) expanding outwards. It will then go through the whole process of getting briefly dimmer before flashing bright again, and then rising upwards as a ball of flame before curving into a mushroom cloud. This cloud would be more than big enough to bounce up against the upper end of the atmosphere before it properly mushroomed out, so it would more flatten and spread horizontally, probably engulfing a sizeable geography (like, a good-sized Western state) in flame and light. The total energy yield would only be about 20kT, not much bigger than the Hiroshima bomb, but the motion of the ball would spread the explosion out over a much wider area -- less devastation per location, but spread out much more dramatically.
It would be a pretty awesome game to watch.