From Glossary of Meteorology
A shower that is produced by local convection within an unstable air mass; the most common type of airmass precipitation.
Such showers are not associated with a front or instability line. They are most frequent within a moist air mass that is sufficiently unstable so that daytime heating at the surface can produce well-developed cumulus clouds. The extreme form of airmass shower is the airmass thunderstorm.