Air-mass thunderstorm is generally a
thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Rather air-mass thunderstorms are associated with warm, moist and thus unstable
air-masses in the summer months.Therefore the name! They develop locally during the afternoon or late in the day in response to insolation through
convective heating from the surface. Thus they are also known as single-cell thunderstorms. Typically they do not persist very long - usually an hour or so - and dissipate rather quickly after sunset.
Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.