The gross convection velocities of various simple point source convective phenomena in an unstratified stationary fluid can be related to each other through empirical constants by using Turner’s model of the starting plume cap. Considered here are ’’weak’’ (Rayleigh number approaching zero), laminar, and turbulent regimes of the phenomena, the thermal, the starting plume cap, and the steady plume. Thus, by determining an empirical constant a, for either the thermal or the starting plume cap, the convection velocity of the other phenomenon can be estimated. Here, a is defined in terms of the thermal, a≡zt2/ktAm, where zt is the thermal height above the buoyancy source at time t, k is a molecular diffusivity, A is the effective Rayleigh number, and m=1/2, 1, or 2 depending on the flow regime. The dependence of a on the Prandtl number Pr is presently unknown. For experimental verification of the model, new measurements of the laminar and of the turbulent starting plume are presented. The constant a, determined independently for the thermal and for the starting plume cap was within 20% of the mean value of 0.038 for the laminar case, while for the turbulent case the constant was within 13% of the mean value of 4.2. The new experimental data further reveals that a change in Prandtl number from 21 to 7 results in a 50% increase in a for the laminar case.