The lower levels of the atmosphere extending from the surface of the Earth or another celestial body up to the tropopause. It is characterized by convective air movements and a large vertical temperature change.
the uppermost layer of a planet's atmosphere
the lowest atmospheric layer; from 4 to 11 miles high (depending on latitude)
an extremely thin atmosphere, as on Saturn's moon Dione
The troposphere is the first layer of the atmosphere of the Earth, and contains 75% of the mass of the planetary atmosphere and 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. The average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics, 17 km (11 mi; 56,000 ft) in the middle latitudes, and 6 km (3.7 mi; 20,000 ft) in the polar regions in winter; thus, the total average height of the troposphere is 13 km (8.1 mi; 43,000 ft).
the outermost atmospheric layer
The exosphere (Ancient Greek: ἔξω éxō , Ancient Greek: σφαῖρα sphaĩra ) is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collisionless. In the case of bodies with substantial atmospheres, such as Earth's atmosphere, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space.
‘outside, external, beyond’; ‘sphere’;