Purdue University has teamed up with Imperial College London to create an impact simulator to calculate what would happen if an asteroid or a comet were to crash into the earth. Enter your various parameters and see what you would experience at a distance from the impact of your choosing.
Here’s the flashy version
There’s also a text-based version if your computer doesn’t like the other (mine only ran it once, then refused to again).
My only regret is that it speaks very little about the long-term effects of such an impact. But they can’t do everything.