On the basis of certain evidence it is concluded that mixing between the cumulus cloud and its environment affects the development of the cloud. It is shown that, for the case of mixing, the lapse rate of temperature within the cloud is greater than the moist-adiabatic rate and also that low relative humidities within the environment disfavor the cloud growth. A synoptic study supports the hypothesis that the forecast of convective showers should take into consideration the relative humidity above the convective condensation level.