The troposphere is the lowest layer of the atmosphere of Earth. Pronounced, the name comes from Ancient Greek τρόπος (trópos) 'turning, change' and -sphere. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary surface of the Earth, the average height of the troposphere is 18 km in the tropics; 11 km in the middle latitudes; and 6 km in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km.