Atmospheric focusing

Atmospheric focusing is a phenomenon occurring when a large shockwave is produced in the atmosphere, as in a nuclear explosion or large extraterrestrial object impact. The shockwave is refracted horizontally by density variations in the atmosphere so that its impact can be felt in localized areas of destruction hundreds of kilometers from the blast site.

This effect operates similarly to the patterns made by sunlight on the bottom of a pool, the difference is that the light is bent at the contact point with the water while the shockwave is distorted by density variations (e.g. due to temperature variations) in the atmosphere. Variations of wind can cause a similar effect. This will disperse the shockwave at some places and focus it at others. For powerful shockwaves this can cause damage farther than expected; the shockwave energy density will increase beyond expected values based on uniform geometry ($1/r^2$ falloff).