Abstract: It is presented that there is a pumping effect at the cumulus base in the development process of cumulus. As for the air-mass cumulus at the most development stage, the pumping is mainly produced by the buoyency at the base, and may be taked as the first approximation of the updraft at the base. The results of numerical calculation and simulating four observed echoes show that using the first approximation, the air-mass cumulus may be approximately simulated by Haltiner's model without the observed assending speed at the base, and Haltiner's model is characterized by the sensitivity of the development of the cumulus to the virtual temperature excess over environment at the base, and the concept of the pumping is supported.