The relationship between the onset of tropical deep convection, column water vapor (CWV) and other measures of conditional instability is analyzed in 16 months of radiosonde, radiometer, and surface meteorological data collected from the mobile Atmospheric Radiation Measurement (ARM) site in Manaus, BR as part of the GOAmazon campaign. Important features seen previously in observations of the tropical western Pacific (TWP) are also found consistently for this tropical continental region. These features include precipitation conditionally averaged as a function of CWV exhibiting a sharp "pick-up" at high values of CWV, as well as the overall shape of the probability distribution of CWV for precipitating points. Similarly, there is a strong association between rainfall and moisture variability in the free troposphere at both locations; an association with boundary layer moisture variability is also observed, although the relationship is less pronounced. As in the TWP, local increases in CWV associated with heavy precipitation at Manaus often result from both a larger-scale and mesoscale moistening of the environment. The CWV is a proxy for the effect of free tropospheric moisture on conditional instability as indicated by entraining plume calculations. For several different mixing schemes, higher CWV at Manaus generally results in progressively greater plume buoyancies, which are consistent with the respective increases in precipitation observed. These buoyancy calculations also suggest that deep convective onset over tropical land may be highly sensitive to microphysics, particularly in the upper troposphere. Overall, leading aspects of the relationship between deep convective onset and CWV are largely consistent over both tropical land and ocean, and important differences in the continental versus oceanic case are seen as departures from these.