What causes air to rise and sink?
The ascent of air is known as convection.This convection takes place either due to buoyancy or by an external aid.Hence the process may be either forced or free.In the case of an air stream ascending while crossing a mountain barrier or a warm airmass being undercut by cold air and forced aloft,the convection is a forced one;on the other hand,the ascent of air in an unstable atmosphere, is a free convection entirely due to buoyancy.So the necessary condition for the free convection is that,the temperature of the rising air should be higher than that of the surroundings.So, if the air over a place is heated more than that of the surroundings,it rises due to buoyancy. Air rises in a low pressure system also. If there is a low pressure system over a place,winds converge there from all sides and is forced to rise in a whirling motion.Similarly,if there is a high pressure system,there is divergence of winds at the surface level,and to replace this outflowing air,cold air from above,aided by
