In the tropics, convection outflow is commonly observed around the height of 14 km and 5 km. This fact implies that the mid (5 km)-level and high (14 km)-level outflows are both important to the atmospheric circulation. On the other hand, the outflow around the 5 km level has been recently recognized, and the process responsible for the mid-level outflow is still unclear. The present paper introduces two candidate processes (inhibition of the convection development by stable layer and melting cooling), and concludes that the melting cooling plays a critical role in the mid-level outflow. A few of the essential thermodynamical aspects of individual cumulus clouds are also reviewed briefly.