Natural convection in an enclosure with an opening in the light vertical wall and a heat source on the bottom surface was investigated using a holographic interferometric technique. In particular, emphasis was placed on the effects caused by changing the opening length, divider height, and heat source temperature. When the enclosure was partially opened, warm air from inside escaped from the upper part of the opening and was replaced by surrounding air, which flowed into the enclosure from the lower part of the opening. The flow rates of inflow and outflow through this opening increased with larger opening length, smaller divider height, and higher heater temperature. When the opening length was small, the opening did not significantly affect the upward flow of warm air from the heater, and resulted in a symmetrical temperature distribution. The divider prevented the development of upward flow from the heat source such that the temperature in the absence of a divider was generally higher than that for the longest divider. For cases with a large opening length, the upward flow was forced to move into the enclosure's left-hand side by the increased inflow. The effect of the divider height was not significant due to the increased flow rates through the increased opening length. The temperatures achieved with the longest divider were a little higher than those for the other cases due to the lower cold flow rate and the blocking of the cold air inflow by the longest divider.