Impacts of kilometer-sized objects are expected to deposit many thousand megatons of energy in the air through the explosive expansion of vapor products. The fireball grows to a radius equal an atmospheric scale-height while the internal pressure is high. The fireball is accelerated upward by interaction with atmospheric pressure and density gradients. Gas velocities will reach orbital speeds before interaction with the magnetic field stops the rise, provided that the original energy deposited in the air exceeds roughly 12,000 Mt. Because the Cretaceous-Tertiary impact probably deposited several times this much energy in the atmosphere, gradient acceleration provides a probable mechanism for the worldwide dispersal of micrometer particles within a few hours after the impact.