The term Atmosphere refers to a layer of gases that surround a material body of considerable mass. The gravity of the body attracts the gases. They remain for a longer period of time if the gravity is strong and the temperature of the atmosphere is low. Some planets are made of various gases only, which is the reason why they have very deep atmospheres.

A layer of gases veiling the planet Earth and held by the Earth’s gravity is known as the Earth’s Atmosphere. The Earth’s Atmosphere roughly consists of – 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, trace amount of other gases, and roughly 1% of water vapor. Mixture of these gases is generally known as ‘Air’.

Ecosystems on the Earth are protected by the Atmosphere, which absorbs ultraviolet solar radiation and reduces extreme conditions of temperature between day and night.

No well-defined boundary is known to exist between the Atmosphere and the outer space. The Atmosphere dissolves into space by gradually becoming thinner and thinner. Three fourth portion of the mass of the atmosphere is within 11 kilometers of the surface of the Earth.

People traveling above the altitude of 80.5 Km above the Earth’s surface are designated as astronauts. At the altitude of 120 Km the effects of the atmosphere become noticeable. An altitude of 100 Km is marked as the presumed boundary between the atmosphere and outer space. This altitude is also known as the Karman line.

The different layers of the atmosphere are known as the – Troposphere, Stratosphere, Mesosphere, Thermosphere, Ionosphere, and Exosphere. Boundaries between these layers are known as - tropopause, stratopause, mesopause, thermopause and exobase.