Earth's atmosphere is divided into four main layers based on temperature patterns. The troposphere, closest to Earth's surface, extends approximately 8–18 kilometers (5–11 miles) high, with greater thickness at the equator and less at the poles. This layer contains nearly all weather and clouds, and temperature decreases with altitude. Above it lies the stratosphere, extending from about 18 to 50 kilometers (11 to 31 miles) altitude. The stratosphere contains the ozone layer, which absorbs ultraviolet radiation and causes temperature to increase with altitude. The mesosphere extends from roughly 50 to 85 kilometers (31 to 53 miles) high, where temperature again decreases with altitude—making it the coldest atmospheric layer. Finally, the thermosphere begins around 85 kilometers and extends to about 600 kilometers (370 miles) or higher, where temperature increases dramatically due to absorption of solar radiation by oxygen and nitrogen. Above the thermosphere lies the exosphere, sometimes considered a fifth layer. The exact boundaries between layers are not sharply defined but rather represent zones where the rate of temperature change shifts. These layers are crucial for different atmospheric processes: the troposphere drives weather, the stratosphere protects life from UV radiation, the mesosphere burns up meteors, and the thermosphere hosts the aurora and contains satellites.