Friday, June 17, 2011

Earth's atmosphere


The atmosphere is the gaseous envelope is often about 800 km around the Earth, held back by the force of gravity into contact with the earth's surface.
The atmosphere has an important mainly including:

    allows breathing through oxygen in the air
    protects the planet from the fall of meteorites that enter the atmosphere, they disintegrate to the friction
    prevents harmful radiation (UV) reaching the earth, this is thanks to the ozone layer
    regulates the heating of the earth, thanks to the greenhouse effect (due to gases such as carbon dioxide and methane which trap the solar radiation)
    with gas in it participates in the water cycle, carbon and nitrogen

The atmosphere are linked to phenomena such as the rainbow, the different color of the sky at sunrise or sunset, the smooth transition from day to night and vice versa, so the aurora.
The lower part of the atmosphere, one in which we live and breathe, has the following composition:

    Nitrogen 78%
    Oxygen 20.99%
    Argon 0.9%
    carbon dioxide 0.03%
    trace, other gases: neon, hydrogen, methane, sulfur dioxide, ozone
    atmospheric dust
    water vapor

The atmosphere has a history that has seen major changes over millions of years.

    Step 1: 4.5 billion years ago, when our planet was born, the primitive atmosphere was probably formed by light gases like hydrogen, helium, methane and ammonia, derived from the nebula from which the solar system was formed.
    Step 2: Most of these gases, also due to their low density and high temperature on the planet, is dispersed in space. The residue is mixed with the gases released from the forming planet as a result of intense volcanic activity. In the new atmosphere, made up of water vapor, carbon dioxide, nitrogen, oxygen was not present.
    Step 3: water vapor and carbon dioxide were reduced by formation of seas, lakes, rivers and oxygen appeared after the splitting of water molecules in the atmosphere by solar and cosmic radiation, and the band began to form ozone.
    Step 4: the atmosphere was enriched with oxygen through photosynthetic activity of the first fotoautotrofi organisms (cyanobacteria) and its composition was similar to the current one. We are approximately 1.5 billion years ago.

The atmosphere is divided into 5 concentric bands different in composition, density and temperature and separated by discontinuities such breaks (this is called thermal breakdown or classification of the atmosphere):


Troposphere, the lower and denser, in contact with the ground. It has an average thickness of 10 km and all weather phenomena occur. The temperature decreases by 6 ° C per kilometer. It ends with the tropopause.
Stratosphere, extends 50 km from the ground. It 'consists of stratified gas. The temperature first decreases, then is increased, due to the presence of an ozone layer (ozone layer), which blocks ultraviolet radiation of sunlight, generating heat. It ends with the stratopause.
Mesosphere, extends 80 km from the ground. It 'consists of rarefied gases. The temperature decreases as you climb up to -70 ° C and above. Most meteorites from outer space in this layer disintegrates, giving rise to the phenomenon of "shooting stars". It ends with the mesopause.
Thermosphere, ranges from 80 to 600 km altitude. The gases are extremely sparse, and the molecules move at high speeds, there is the presence of electrically charged atoms or ions (the ionosphere) that reflect radio waves, allowing communications over vast distances. Above 500 km of electrically charged particles are quite stable and are affected by Earth's magnetic field. Here begins the magnetosphere where the aurora takes place.
Exosphere, extends over 600 km from the Earth's surface, the thin air more than the molecules move at high speeds and are beyond Earth's attraction.

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