UNIT  6  ATMOSPHERE
The atmosphere is the layer of gases surrounding the Earth.
It is essential for life because:

• it provides air and water
• it keeps the Earth warm
• is protects from harmful radiation

The Earth's atmosphere comprises a layer of gases retained by the Earth's gravity. It contains roughly:

• 78% nitrogen
• 21% oxygen
• 0.9% argon
• 0.04% carbon dioxide
• trace amounts of other gases
• and a variable amount (average around 1%) of water vapour.

This mixture of gases is commonly known as air. The atmosphere protects life on earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.

 

The atmosphere is divided into five layers:

• troposphere: the first layer above the surface and contains half of the Earth´s atmosphere. Weather occurs in this layer.
• stratosphere: the layer above the troposphere. Many jet aircrafts fly in this layer because it is very stable. The ozone layer absorbs harmful rays from the Sun.
• mesosphere: it is the layer in which meteors or rock fragments burn up.
• ionosphere: it is the layer with auroras. Space shuttles orbit in this layer.
• exosphere: it is the upper limit of our atmosphere.