1.The Earth is wrapped in a layer of gas called atmosphere. Atmosphere is tied to Earth by gravitation, so that it cannot disperse in the space. It is 500 km (300 mi) thick, being made of a mix of about 10 gases, called air. The air is made by nitrogen (78 %), oxygen (21 %) and other gases (argon, carbon dioxide, helium, neon). These are chemically inert. Others can react with different chemicals: sulfur dioxide, ammonia, methane, carbon monoxide, ozone and water vapors. There are also contaminants, like toxic gases, smoke, salt, dust and volcanic ash.
2.Atmosphere is made of 4 layers: troposphere, stratosphere, mesosphere and thermosphere.
Troposphere is the thinnest layer (12 km or 7.5 mi), but comprises 80 % of the weight of the atmosphere, being its densest layer. Planes do not fly over 9-11 km (5.5-7 mi). It is the warmest layer, because the reflected sun rays heat up the air. As we go further up into the atmosphere, the temperature decreases with 6.5oC each km (0.6 mi), reaching -55o C at the upper limit of the troposphere (called tropopause).
3.Stratosphere goes up to 50 km (30 mi) over the Earth. At its upper edge, there is the maximum ozone concentration, called the ozone belt. Here, the temperature is higher than in the troposphere, because the ozone absorbs a great amount of the ultraviolet rays. Pollution causes the destruction of the ozone belt, and, without it, the Earth would be exposed to the deadly UV light. Stratopause makes the connection between stratosphere and mesosphere.
4.Mesosphere lies between 50 to 70 km (30-42 mi) over the Earth. It is the coldest layer of the atmosphere (-90o C). This is the formation zone for ice clouds that are visible only during the sunset, when they are lighted from downwards. Usually, meteorites falling over the Earth are burned into mesosphere. Even if the air is rarer there, the resulting friction caused by meteorites choking with oxygen produces a burning temperature that destroys the meteorites.
5.The thermosphere is made of the ionosphere and magnetosphere. The air density is the lowest and this layer comprises just 0,001 % of the total volume of the atmosphere gases. The temperature increases at 500 km (300 mi) to 1,200o C.
In the ionosphere, sun radiations cause ionization: the particles are electrically charged. When the charged particles hit each other, the phenomenon of aurora borealis or aurora australis (polar lights) can be observed. The ionosphere reflects the radio waves employed in telecommunications.
Magnetosphere is located above the ionosphere, at the external limit of the Earth's magnetic field. It behaves like a giant magnet, retaining high energy particles and thus protecting the Earth. This layer has the lowest density of all, as air density gradually decreases as we go further up from the Earth.
At the upper edge of the thermosphere, called exosphere, the air has such a low density that we pass into the inter-planetary space without an evident limit.
6.Meteorological phenomena are restricted to troposphere. They are the result of the sun radiation and Earth's rotation on the atmosphere. Air currents (wind) are produced when the hot air rises up and the colder air replaces it. The Equator, where the sun shines over our head, the air is the hottest, while towards the poles the air gets cooler. The hottest place on Earth is Sahara. The highest average annual temperature is registered in Dalul (Ethiopia), in a depression located 116 m (390 ft) under the sea level: 34.4o C. At Verhojansk (Siberia), the temperature reached -70o C.
7. The biosphere is the part of the troposphere where life can exist. This goes upwards, from the Earth's surface to the maximum height where birds can fly. Through photosynthesis, plants take the carbon dioxide from atmosphere, releasing oxygen. Through respiration, plants and animals consume oxygen, releasing carbon dioxide.
As many tropical forests are destroyed, being replaced by pastures (like in South America), there are increasingly less oxygen producing trees, while the amounts of the greenhouse effect gas carbon dioxide are increasing. Thus, buying products made of tropical woods, you contribute to the global warming.
8.Atmosphere's balance is menaced by human activity, which causes greenhouse effect, global warming, air contamination, ozone belt destruction and acid rains. These effects are mainly due to the development of the industry, which took place in the last 2 centuries. The burning of the fossil fuels produces carbon dioxide, the main greenhouse effect gas. Agriculture's development dumps into the atmosphere large amounts of methane (the most powerful greenhouse effect gas) and nitrogen oxides.
9.The existing gases in the atmosphere must retain the heat (infrared rays) delivered by the sun radiation, reflecting them back to Earth. Without it, the Earth would be so cold that life would not be possible. But the gases dumped by burning fossil fuels increase this effect. By the middle of 21st century, the average temperature of the Earth is believed to be 1.5-4.5o C higher than now.
The warmer clime will melt ice stocked in the polar areas, increasing the sea level and flooding cities like New York or Amsterdam.
10.About 20 % of the Earth's population breathes severely contaminated air, especially with carbon monoxide and sulfur dioxide resulting from industrial processes. This increases the number of respiratory conditions, especially amongst children and elders. 13 % of the British children experience asthma caused by air contamination.
11.The frequency of skin cancers is increased by the thinning of the ozone layer. This is due to chlorinated hydrocarbons and fluorocarbons from aerosols, fridges, detergents and polystyrenes. These gases rise into the atmosphere and decompose into chloride ions that destroy the ozone layer. In 1985, a hole into the ozone layer was observed in Antarctica. In 1995, a similar hole was spotted into the ozone layer over the Arctic. Now, chlorinated hydrocarbons and fluorocarbons are replaced with other chemicals in the industrial processes.
12.Acid rains form when sulfur dioxide or nitrogen oxides combine in the atmosphere with water vapors, forming acids. The resulting acid rains destroy plants and animals, killing entire forests. If this acid reaches rivers or lakes, it destroys all the organisms living there.