What are the main causes of convection currents in the atmosphere

Radiation of the sun
The gases released by factories and other manmade industries

There are three types of heat transfer or heat propagation; conduction, convection and radiation. Heat transfer is the process by which heat projects externally however, depending on the temperature and pressure. Also called the movement of heat from a low temperatured state which increases as heat progresses.
Conduction is the heat transfer by contact, immediate contact.
Convection is the transfer of heat through air and water.
Radiation is the transfer of heat regardless of the presence of atoms or particles.