H2G2 The Earths Atmosphere

The Earth’s atmosphere is the layer of gases that surrounds the Earth; that does not dissipate due to the action of gravity; and that, seen from space, shows our planet as a sphere of bright blue color.

It is important because it protects us from the dangerous ultraviolet radiation coming from the sun and because it allows the Earth to maintain heat during the night and, on the other hand, does not overheat during the day. Were it not for this, the surface of our planet would have enormous differences in temperature between the shadow and the sunny areas, as occurs on the moon, for example.

It is composed mainly of nitrogen (about 78.09% by volume), followed by oxygen (20.95%) argon (0.93%), carbon dioxide (0.039%), and small amounts of other gases. There is also a variable amount of water vapor.

The more you climb, from the Earth’s surface, the more rarefied you are. For this reason, its mass, which is approximately 5 x 10 15 tons, is concentrated mainly close to the planet’s surface. In the first 11 km of the atmosphere, ¾ of the total mass is found.

The rarefaction of air is progressive and there is no clear separation between the atmosphere and outer space, that is, there is no defined upper limit for the Earth’s gaseous envelope. Usually, it is customary to place this limit at 100 km of altitude, the call Line Kármán . But that is for aviation-related purposes. If we take into account the action of gravity on the gases surrounding the Earth, the outer space begins much later, that is, the thickness of the Earth’s atmosphere is much greater, up to 10,000 km perhaps.

The layers that make up the Earth’s atmosphere

It has been agreed to divide the atmosphere into five layers, three of which are relatively hot and separated by two cooler ones. Contacts between these five layers are called breaks .


It is the lowest atmospheric layer, extending from the Earth’s surface to the base of the stratosphere. It accounts for about 80% of the total weight of the atmosphere and its average thickness is approximately 12 km, reaching 17 km in the tropics and reducing to about 7 km at the poles.

It is the most important layer, because it is the only one in which living beings can breathe normally and because it is there that practically all meteorological phenomena occur.

The boundary between the troposphere and the next layer is called the tropopause . It is located at a variable altitude, which depends on climatic conditions. With climatic turbulence, this limit goes up. Normally, it lies at an average altitude of 17 km over the Equator.


Above the troposphere and up to approximately 50 km of altitude is the stratosphere, which is characterized by horizontal movements of the air and a temperature that increases with altitude.

It has a small concentration of water vapor and is where many jet planes circulate. As the meteorological phenomena occur below, in the troposphere, there is a zone of stability, without the typical turbulence of storms, rains, gusts, etc.

In the stratosphere is the ozone layer (ozonosphere), very important because it is a barrier that protects humanity from ultraviolet radiation from the sun. It is mainly in the range of 15 to 35 km of altitude, although the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.

The upper limit of the stratosphere is the stratopause , where the temperature stops rising. Thereafter you enter the mesosphere.

From 50 km to 80-85 km of altitude is the mesosphere. In it, the temperature, which had been increasing with altitude, starts to decrease sharply, reaching minus 90 degrees Celsius at its top.

It is there that the meteoroids start to burn, becoming incandescent and forming the so-called shooting stars, which can be seen with the naked eye on the Earth’s surface.

Its upper limit is mesopause .


In the thermosphere, the temperature increases again with altitude, and quickly. It reaches, on average, 1,500 ° C.

Its thickness varies between 350 and 800 km, but it can reach only 80 km, depending on the solar activity. When the intensity decreases, the thickness of the thermosphere also decreases.

It is in the thermosphere that space shuttles orbit.

The upper limit of the thermosphere marks the beginning of outer space and is called thermopause. Physically, all incident solar radiation acts below the thermopause.


From the thermopause onwards is the exosphere. In it, the atmosphere is so tenuous that the phenomena that occur there are practically not noticed. It is composed mainly of hydrogen and helium, but the particles that exist there are so sparse that they can travel hundreds of kilometers without colliding with each other. In addition, they follow straight paths and, therefore, do not behave like a fluid. The thermosphere and the exosphere make up the ionosphere, part of the atmosphere that ranges from 50 to 1,000 km in altitude and is ionized by solar radiation.

The ionosphere influences the propagation of radio waves over the Earth and is responsible for the magnificent spectacle of the northern lights.