Science / Weather / Exosphere: This region is considered the very outer limits of the earth's atmosphere. Its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. It is estimated to be some 400 plus miles (640 kilometers) above the surface.
Science / Weather / Ionosphere: A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is includ MORE
Science / Biology / Atmosphere: The envelope of gases that surrounds the Earth; consists largely of nitrogen (78%) and oxygen (21%). MORE