This study aims to quantify and understand the rate at which air mass ascends within cumulus clouds in the tropics. This rate is known as convective mass flux. Using fine-scale supercomputer simulations of Earth's atmosphere, we identify that convective mass flux is extremely sensitive to humidity in the lowest few kilometers of the atmosphere. Greater humidity leads to greater convective mass flux. We then test whether environmental humidity increases mass flux by making cloudy air more buoyant (i.e., less dense relative to its surroundings). We reject this hypothesis, finding instead that greater humidity is associated with cloudy air which is less buoyant (i.e., more dense relative to its surroundings). To make sense of these results, we use a simple set of equations to simulate cloudy air as it rises and ingests dry environmental air. As the environment becomes more humid, cloudy air may absorb a greater mass of environmental air without drying out and halting its ascent. This causes mass flux to increase with humidity and causes convective clouds' average density to become more like that of their environment.