Blue light is scattered more than other wavelengths by the gasses in the atmosphere. This gives the Earth a blue halo when seen from space.
The atmosphere of Earth is a layer of gasses surrounding the planet Earth that is retained by Earth gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (the greenhouse effect) and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21% oxygen, less than 1% argon and carbon dioxide and small amounts of other gasses. Air also contains a variable amount of water vapor, on average around 1%.
The atmosphere has a mass of about five quintillion (5,000,000,000,000,000,000) kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, and there is no definite boundary between the atmosphere and outer space. An altitude of 120 km (75miles) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Karman line, at 100 km, is also often regarded as the boundary between atmosphere and outer space.
The atmosphere is mainly composed of nitrogen, oxygen and argon, which together constitute the major gasses of the atmosphere. The remaining gasses are often referred to as trace gasses. Among these are the greenhouse gasses, such as water vapor, carbon dioxide, methane, nitrous oxide and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts and meteoroids. Other compounds may also be present either as industrial pollutants or naturally, such as chlorine, fluorine compounds, mercury, and sulfur compounds such as sulfur dioxide.
Earth’s atmosphere is divided into five main layers. There layers a re mainly determined by how temperatures increase or decrease with altitude. The principal layers of the atmosphere from top (space) to bottom (the Earth) are the Exosphere, The Thermosphere, the mesosphere, the Stratosphere and the Troposphere. They are briefly presented here:
The outermost layer of the Earth’s atmosphere extends from the exobase at around 690 km (the part nearest to Earth) and upward to 10.000 km. The upper part, the particles are so far apart that they can travel hundreds of km without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind. The exosphere is mainly composed of hydrogen and helium and other light gasses.
Temperature increases with height in the thermosphere from the mesopause (around 50-85 km above Earth, the coldest part of the atmosphere, up to the thermopause), and then it is constant with height. The temperature of this layer can rise to 1.500 C, though the gas molecules are so far apart that temperature in the usual sense is not well defined. The international Space Station orbits in this layer, between 320 and 380 km. The top of the thermosphere is the bottom of the exosphere, called the exobase. It’s height various with solar activity and ranges from about 350-800 km. It is in this part of the atmosphere that the space shuttles navigate.
The mesosphere extends from the stratopause to 80-85 km. It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around minus 100 C. In the Mesosphere, you find meteors.
The stratosphere extends from the tropopause to about 51 km. Temperature increases with height, which restricts turbulence and mixing. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 pm. It is here that weather balloons are places.
The troposphere begins at the surface and extends to between 7 km at the poles and 17 km at the equator; with some variation due to weather: The troposphere is mostly heated by transfer of energy from the Earth’s surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing. The troposphere contains roughly 80% of the mass of the atmosphere. The tropopause is the boundary between the troposphere and stratosphere.
Within the five principal layers determined by temperatures are several layers by other properties.
The ozone layer is contained within the stratosphere. In this layer, ozone concentrations are about 2 to 8 parts per million which is much higher than in the lower atmosphere, but still very small compared to the main components of the atmosphere. It is mainly located in the lower portions of the stratosphere from about 15-35 km., though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.
The ionosphere, the part of the atmosphere that is ionized by solar radiation stretches from 50 – 1.000 km and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical important because it influences, for example, radio propagation on the Earth. It is also responsible for auroras.
Atmospheric circulation is the large-scale movement of air, and the means (with ocean circulation) by which heat is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant, because it is determined by the Earth’s rotation rate and the difference in solar radiation between the equator and poles.
The evolution of Earth’s atmosphere – the second atmosphere
Water related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the the then stable “second atmosphere.“ An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago.
The geological record, however, shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. Sometime during the late Achaean era around 2.5 billion years ago, an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae, the early basic carbon isotope proportions are very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.
The coming together of continents about 3.5 billion years ago added plate tectonics and its effects of constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages.
Free oxygen did not exist until about 2.4 billion years ago. This signifies a shift from a reducing atmosphere to an oxidizing atmosphere. The amount of oxygen in the atmosphere showed major ups and downs until it reached a steady state of more than 15%.
This build-up oxygen in the atmosphere has fuelled the planet’s living beings of humans and animals and myriads of other life forms as we know them today, and these living beings on their part maintain the atmosphere through their life processes.
Currently, however greenhouse gasses are increasing in the atmosphere. According to the intergovernmental Panel of Climate Change this increase is the main cause of global warming and will result in dire consequences for the Earth within the next decades. Global Warming is the threat to future life on the globe, a challenge for mankind as such. All forces of our world, from individuals to groups and peoples, to nations, governments and intergovernmental forces should realize this situation and find ways to counteract it and save the world for the future and its beings.