Table of Contents

The Space Environment and Earth's Atmosphere

Composition and structure of the atmosphere

Chemical Composition

The Earth’s atmosphere, that is, the air we breathe, consists of the following gases:

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Gases composing dry air, and volumes (in %)

 

We can therefore say that air is essentially composed of nitrogen (~80%) and oxygen (~20%).

Layers

The atmosphere is made up of consecutive layers from the ground up to space.

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Troposphere

The troposphere is the lowest portion of the Earth’s atmosphere, extending from the ground to an altitude of approximately 8 km at the poles and 15 km at the equator. Most meteorological phenomena occur in this layer, which is the most dense. The temperature drops at a rate of approximately 6 °C per kilometre as we travel from the lowest point to the apex of the troposphere.

Stratosphere

Immediately following the troposphere is the stratosphere, which extends up to 50 km. The temperature in this part of the atmosphere is constant and even increases slightly to reach -3 °C at high altitude. This phenomenon is attributed to the absorption of ultraviolet rays by the ozone located in this layer.

Ninety percent of the air that makes up the Earth’s atmosphere is located in the troposphere and stratosphere.

Mesosphere

The next layer is the mesosphere, which extends up to 85 km. As is the case for the troposphere, temperature decreases with altitude. At the peak of the mesosphere the temperature reaches approximately -93 °C.

Thermosphere

Beyond the mesosphere lays the last layer of the Earth’s atmosphere, the thermosphere. This layer extends up to 500 or 600 km from the ground. At this altitude, the gases that make up the atmosphere are rarefied, and temperatures can reach approximately 1,700 °C because of the intense radiation emitted by the Sun, which is converted into heat. This is the layer in which orbiters such as the space station and orbital stations are found.

Upward Pressure

We now know that there are 600 km of atmosphere above us. The air that makes up this atmosphere has a force that exerts significant pressure on the Earth’s surface.

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At sea level, the atmospheric pressure (the force of the air “above”) measured by a barometer is approximately 101 kPa (kilopascals). At the top of Mount Everest, at 8,850 m in altitude (remember, at this height, we are close to the troposphere), the same barometer would read approximately 31 kPa.

This drastic drop in pressure is explained by the fact that the portion of the atmosphere above us decreases with altitude, and the air loses density proportional to the altitude at which the measurement is taking place. Gravity forces the atmosphere downward towards the Earth’s surface. The force of the compression is greater at ground level than it is at a few thousand metres higher. At the uppermost limit of the atmosphere, pressure is close to zero.

We do not feel the force of the atmospheric pressure. However, our body is sensitive to sudden changes in pressure, such as during airplane takeoffs and landings, or even during fast elevator rides (we feel our ears block). The relatively stable atmospheric pressure on Earth is necessary for our body to function properly. Mountain climbers know this and, to reach the top of Mount Everest, are conscious about bringing the necessary equipment to offset the rarification of the air and the drop in pressure.

• History of Atmospheric Development