The troposphere is the lowest and densest atmospheric layer, extending from the Earth's surface to approximately 12 km altitude (varies from ~8 km at poles to ~16 km at equator). It contains ~75% of total atmospheric mass and ~99% of water vapour. All weather phenomena occur here. Temperature decreases with altitude (~6.5°C/km). Above the troposphere lie the stratosphere (ozone layer), mesosphere, thermosphere (ionosphere), and exosphere.