The cold atmospheric gases that lie on top of the relatively warm surface of the planet are heated by convection, so that the hot regions of the lower atmosphere rise up bodily through a region of higher density and the transfer their thermal energy to the cooler overlying regions. The troposphere contains about 70 per cent of the total mass of the Earth's atmosphere and much of it is dominated by stratus, cumulus, and altocumulus clouds. All weather occurs within it, so the air is in continual motion, with horizontal airflows and vertical currents.
