AbstractThis study examines two factors impacting initiation of moist deep convection: free-tropospheric environmental relative humidity (ϕE) and horizontal scale of subcloud ascent (Rsub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a “scale selection” hypothesis: that a minimumRsubis required for moist convection to go deep, and that this minimum scale decreases with increasingϕE. Specifically, the ratio ofto saturation deficit (1 −ϕE) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally varying surface fluxes show strong sensitivity of maximum cumulus height to bothϕEandRsubconsistent with the hypothesis. IncreasingRsubby only 300–400 m can lead to a large increase (>5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly withRsubbut depends little onϕE. However, buoyancy dilution increases as eitherRsuborϕEdecreases; buoyancy above the level of free convection is rapidly depleted in dry environments whenRsubis small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase inRsub. Overall, it is concluded that small changes toRsubdriven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.