Ben Winters, author of The Last Policeman (which I reviewed here), sent me a link to a website called Impact Earth! It was developed by Gareth Collins, HJ Melosh, and Robert Marcus at Purdue University. To use it, you enter parameters about the size and density of the hypothetical asteroid, the impact angle, the distance from impact, etc.
What happens when a rock the size of a school bus hits the Earth at an angle of 45° and a speed of 40 km/s? According to Impact Earth!
The projectile bursts into a cloud of fragments at an altitude of 96,500 ft.
No crater is formed, although large fragments may strike the surface.
Energy before atmospheric entry = 544.05 KiloTons TNT.
The average interval between impacts of this size somewhere on Earth is 68.8 years.
That's reassuring! What about a much larger astroid, say 1.3 km in diameter?
Energy before atmospheric entry: 6.6 x 10^5 MegaTons TNT
The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 3.3 x 10^6 years
Final Crater Diameter: 16.7 miles
Final Crater Depth: 2610 feet
Mercalli Scale Intensity at a distance of 10 km: Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly. Underground pipelines completely out of service.
Getting bigger, a 50-kilometer asteroid will make a final crater 603 miles wide and 1.46 miles deep. If you are within 1000 kilometers of the point of impact you will be inside the resulting fireball, which will hit you 37.4 seconds after impact. Fortunately, "the average interval between impacts of this size is longer than the Earth's age. Such impacts could only occur during the accumulation of the Earth, between 4.5 and 4 billion years ago."
Asteroid impact simulator