In the tropics, deep convection requires a threshold SST of 26.5 C. We show that this threshold is associated with the SST at which a significant fraction of air parcels in the boundary layer have positive CAPE, i.e. a pseudoequivalent potential temperature larger than 345 K. For SSTs larger than 28 C, increased SST is offset by decreased wind speed, so that the frequency of deep convection becomes independent of SST.