Troposphere, lowest region of the atmosphere, bounded by the Earth beneath and the stratosphere above, with its upper boundary being the tropopause, about 10-18 km (6-11 miles) above the Earth's surface. The troposphere is characterized by decreasing temperature with height and is distinguished from the overlying stratosphere by a region of nearly constant temperature in the lower stratosphere.
Stratosphere, layer of Earth's atmosphere lying between the troposphere and the mesosphere. The lower portion of the stratosphere is nearly isothermal (a layer of constant temperature), whereas temperatures in its upper levels increase with altitude. The stratosphere extends from the tropopause at about 10 to 17 km (about 6 to 11 miles) altitude to its upper boundary (the stratopause) at about 50 km (30 miles), and it also contains the ozone layer.
Mesosphere, region of the upper atmosphere between about 50 and 80 km (30 and 50 miles) above the surface of the Earth. The base of the mesosphere is defined as the temperature maximum existing at the top of the stratosphere, with the boundary between the two regions usually called the stratopause. The mesosphere extends upward to the next temperature minimum, which defines the base of the thermosphere; the boundary between the two regions is called the mesopause.
Thermosphere, region of increasing temperature in Earth's atmosphere that is located above the mesosphere. The base of the thermosphere (the mesopause) is at an altitude of about 80 km (50 miles), whereas its top (the thermopause) is at about 450 km.
Exosphere, outermost region of a planet's atmosphere, where molecular densities are low and the probability of collisions between molecules is very small. The base of the exosphere is called the critical level of escape because, in the absence of collisions, lighter, faster-moving atoms such as hydrogen and helium may attain velocities that allow them to escape the planet's gravitational field. Most molecules, however, have velocities considerably lower than the escape velocity, so their rate of escape to outer space is quite low. The Earth's exosphere begins about 500 km (300 miles) above the terrestrial surface and extends out through the magnetosphere and beyond to the interplanetary medium.
Space is the boundless three-dimensional extent in which objects and events have relative position and direction.