Earth's Atmosphere

Vertical Structure of the Atmosphere

Troposphere

The Troposphere starts at the ground or sea and reaches up to 10 - 12 km height. It is the troposphere where most weather phenomena like cloud formation take place. Physically it is characterized by a continuous decrease of temperature with increasing height. The uppor boundary of the troposphere is called tropopause. The height of the tropopause is very dependent on geographical latitude and season. Troposphere has it's maxmum height in tropical regions. At the polar regions, the tropopause is located at about 8 km height. The troposphere contains about 80 % of the mass of the atmosphere and also contains almost all of water of the atmosphere. In it's lowest layer of about 1 to 2.5 km, the planetray boundary layer (PBL), the influence of Earth's surface causes heavy variances of meteorologic parameters like temprature, wind and humidy.

In the higher regions of the tropopause, temperature is about -60 °C. At this heights, relatively small bands with very high wind speeds (up to 500 km/h), the jet streams, occur. In this regions very important processes take place that cause vertical splitting, decomposition and new formation of the tropopause.

Stratosphere

The stratosphere lies above the troposphere at 12 - 50 km. It is characterized by an increase of ozone with a maximum ozone concentration at ybout 30 km of height. Temperature also increases with height and reaches 0 °c at about 50 km. This increase is mainly caused by the ozone that absorbs the UV radiation of the sun. Doing this, the ozone layer is of essential important for the life on Earth. The stratosphere is almost without clouds because the transport of water vapour from the troposphere into the stratosphere is very limited because of the extremely low temperatures in the tropopause. The stratosphere may be devided into a lower stratosphere with a constant temperature of about -56 °C and a upper stratosphere above 20 km of height. In the upper stratosphere, temperature increases to about 0 °C from radiation absorption. Because of the increasing temperature with increasing height (like inversion weather condition) there is only very limited vertical movement. Almost 99 % of the mass of the atmosphere is located within the lower 30 km.

Mesosphere

The mesosphere (50 - 85 km) is charaterized by a continuous decrease of temperature. It reaches it minimum with almost -100 °C at about 80 km of height. This also is the upper boundary of the mesosphere.

Thermosphere

In the thermosphere (85 - 500 km) no temperatures but only emittances may be measured because of the very low densitiy of particles.

Exosphere

The exosphere (> 500 km) starts depending on definition at 500 - 1000 km of height. Here, pressure is so low that it can already be called vacuum.

Other Possibilites of Structuring the Atmosphere

The structure as explained above is based on temperature gradients in the atmosphere. But there are other ways of structuring the atmosphere.

Looking at the degree of ionization of the atmosphere is can be separated into a neutrosphere that reaches from the Earth's surface to about 80 km of height. In this layer, almost no ionized particles are found. From 80 km to about 1000 km the atmosphere is called ionosphere and above is the protonosphere. Above the stratosphere (> about 60 km), there is no ozone that can absorb the radiation of the sun, why there is a high degree of ionization that can be separated in characteristic layers. Ionosphere may be separated into D (80 - 100 km), E (about 100 km) and F-layers (F1: 150 - 250 km; F2: 250 - 500 km). These layers play important roles for communications engineering since radio waves (short waves) are reflected by these layers. In higher regions like the exosphere only ionized hydrogen atoms (protons) are found, why this layer may be called protonosphere.

The composition of the atmosphere is another possible way of differentiation. The lower atmosphere is mostly well stirred and therefor called homosphere. In the homosphere, most of the mass of the atmosphere is found, being more than 100 000 fold more than in the whole upper part of the atmosphere. Above about 80 to 100 km of height, there is a separation by molecular weight. Molecules with higher weights are found lower, the lighter molecules higher. This region is called heterosphere.

Finally the friction between the atmosphere and the Earth's surface can be used for differentiation. Doing so, the peplosphere is distinguished from the free atmosphere. The peplosphere reaches up to about 2 km of height, depending on the orography (term used to describe undulations on the surface of the Earth). The term planetary boundary layer (PBL) is much better known for this layer of the atmosphere.

Composition of the Atmosphere

The atmospheric air of the Earth is a mixture of gases with the main components being: nitrogen, oxygen, argon and carbon dioxide. It further contains traces of the other inert gases helium, neon, krypton and xenon. Up to a height of about 20 km, almost always water vapour in heavily changing concentrations (up to 4 % of volume) is found.

* carbon monoxide shows periodic changes>

Mean composition of dry air in the troposphere	volume content in %	ppm (parts per million)
nitrogen	78,08	780 800
oxygen	20,95	209 500
argon	0,934	9 340
carbon dioxide	0,035	350
neon	0,0018	18
helium	0,0005	5
methane	0,00017	1,7
krypton	0,0001	1
xenon	0,000009	0,09
hydrogen	0,00005	0,5
dinitrogen monoxid	0,00003	0,3
carbon monoxide*	0,00002	0,2

Additionally, the atmosphere contains traces of sulfur- and nitrogen compounds (sulfur dioxide, ammonia, nitrogen monoxide nitrogen dioxide), ozone, organic halogen compounds and radicals produces in the atmosphere. The atmosphere also contains solid and liquid particles of different nature and origin as airborne particles, dust and aerosoles.

As already mentioned, the atmosphere contains up to 4 % of water in all states of aggregation. Up to 80 % of all water in the atmosphere is found in heights up to 3000 m. In the stratosphere, only 1 - 10 ppb (parts per billion) water is found. Despite this relatively small amounts, water plays an important role in the atmosphere. By phase transformations between gaseous, liquid and solid it is involved in energy transformation and transport and weather formation. Because of its ability to absorb infrared radiation it plays an important role for the warming of the atmosphere.

Functions of the Atmosphere

• Protection of all life from hazardous or deadly radiation from space (filter for UV- and x-rays from sun).
• Letting pass the vitally important sunlight to the surface of the continents and oceans (energy source).
• Protections from rapid colling at night and heating at day.
• Makes possible a mean temperature on Earth's surface of +15 °C instead of -18 °C as would be without atmosphere.
• Transport of energy (warmth of air that can be felt and latent warmth of water vapour) from the equatorial regions to medium and higher latitudes.
• Transport of water vapour through dynamic processes of general air circulation that determines precipitation.
• Storage of huge amounts of nitrogen (important for plants).
• Reservoir for carbon dioxide and oxygen.
• Is part of different vital cycles of matter.
• Dissipation and decomposition (oxidation, reaction with radicals, photolysis) of natural and anthropogenic (man-made) emissions.
• Protection from smaller meteorites that burn up by heating from the friction when entering the Earth's atmosphere and can not reach the surface.

 Thickness of the atmosphere compared to the radius of the Earth.
(Credit: NASA)