The impact of a large body in the oceans would inject large quantities of water through the tropopause cold trap into the stratosphere and lower mesosophere. We consider the consequences of enhanced water vapor concentrations on the middle atmosphere (50–100 km) chemistry and heat budget. The increased mixing ratio of hydrogen dramatically decreases the ozone concentration above 60 km. Catalytic reactions with odd hydrogen are the main sink of ozone in this region. The ozone reduction causes a lowering of the average height of the mesopause, as well as a lowering of the average temperature. The lower colder mesopause and the creation of saturation conditions over much of the upper mesosphere would have resulted in a permanent layer of mesospheric ice clouds of nearly world-wide extent. (At present, these exist only at high latitudes and are observed in summer as “noctilucent clouds.”) The globally-averaged albedo resulting from these clouds is dependent on the particulate size and shape, and can be as high as several percent, preferentially covering the summer hemisphere. This could have important implications for the short-term climate following a large-body impact. Similar effects would also result from an encounter with a more extended object such as a swarm of cosmic debris or a dense interstellar cloud.