How does a single-cell shower differ from a multi-cell thunderstorm, or even a supercell?
A. All are formed within an unstable environment (see Q/A 2A.4), and all require the following to be in place: (i) Instability through a reasonable depth of the troposphere; preferably (but NOT necessarily) extending above the freezing level; (ii) sufficient moisture to sustain the cloud-building process – medium level dryness will often kill shower formation unless low-level inflow of moisture is substantial; (iii) a trigger action – i.e. something to kick the whole process into life by lifting the parcel that goes on to grow into a moderate depth cumulus cloud, or a well-developed ‘supercell’ complex. Once these conditions are met, then consideration of things like shear, CAPE, helicity, etc., are needed as follows:- (for definitions, see the Glossary, and in particular for helicity, see Q/A 2A.24 (below)) >> Single-cell showers: the ‘classic’ growth/decay model of a Cumulus cloud , whereby a single moist convective cell develops in an airmass that is moderately unstable (CAPE values
