What causes the warm air to rise up, cool down and then sink?

• Warm air being less dense rises up and the cool air being dense sinks and takes place of the warm air. 
• The warm air releases heat and becomes cool as it goes up and sinks when more warm air rises up. 
• This is a method of heat transfer called convection.

Therefore, we can say that convection current causes the warm air to rise up, cool down and then sink.