This study investigates the relationship between the vertical development of tropical cumulus convection and the vertical profiles of environmental temperature and moisture in order to identify the factors responsible for determining the tropical cumulus heights. The analyses of observational data obtained in the tropical western Pacific region reveal a strong correlation between the development of shallow and middle-topped cumulus clouds and the existence of dry layers in the middle to upper troposphere. In contrast, the difference in static stability profiles is insignificant between different cloud regimes. The observed importance of the tropospheric moisture in modulating cumulus modes is supported by cloud-resolving numerical simulations. The model results show a strong sensitivity of cumulus heights to the mid- to upper-level relative humidity, while the mid-level stable layer has less impact.