The atmosphere is the layer of gas that surrounds Earth. It is often called air. Other planets, and some of their larger moons, also have atmospheres.
The atmosphere consists of several different gases. The gases are held close to Earth by a force called gravity. Near Earth’s surface, the atmosphere is about three fourths nitrogen and one fifth oxygen. At higher elevations, the atmosphere is mostly hydrogen and helium.
The weight of the air is known as atmospheric pressure. It is also called barometric pressure. The pressure is greatest at sea level. There, the gas particles are pressed together by the weight of the air above them. Air becomes lighter farther away from Earth’s surface. As height increases, the air molecules become separated by more space, and the weight decreases. As the weight of the air decreases, so does the air pressure.
LAYERS OF ATMOSPHERE
Scientists divide the atmosphere into five regions, or layers. The layer closest to Earth is the troposphere. It extends up to about 6 miles (10 kilometers) above Earth’s surface. Most of Earth’s weather, including the wind and most clouds, exists in the troposphere.
The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere, and below the mesosphere.the stratosphere, extends to about 30 miles (50 kilometers) above Earth’s surface. The stratosphere includes the ozone layer. Ozone blocks much of the sun’s radiation that would harm plants.
The mesosphere is the layer of the Earth’s atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as the altitude increases.
The thermosphere is the layer of the Earth’s atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions in the ionosphere.
The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is too low for them to behave as a gas by colliding with each other.