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Learn about the two main layers of the atmosphere: the troposphere and the stratosphere.

The troposphere is full of weather and ‘bad’ ozone, and above that, is the stratosphere, where ‘good’ ozone protects us against dangerous UV light.

Our atmosphere is made of gases, mostly nitrogen and oxygen. They are held to the earth by gravity. Because air is compressible, it gets less and less dense as you get further from the earth.

Visible sunlight passes through the atmosphere and warms the ground, heating up the air at ground level. As the colder air above sinks, it pushes up the warmed air which is less dense due to expansion. This movement, known as a convection current,gives us our weather systems.

The atmosphere gets colder and colder till you reach about 10 km where the temperature is coldest – the tropopause. Below this is the troposphere, containing 80% of our air and all the weather.

Above the tropopause is the stratosphere. This is a layer of thin air which is hot on top because it is heated from above by UV radiation absorbed by the good ozone and cold underneath, making it dynamically stable.

Ozone is an allotrope of oxygen - its molecules are made of three oxygen atoms bonded weakly together. It is an unstable gas which slowly decomposes back to molecular oxygen. The ozone in the stratosphere is continually being made and broken by ultra-violet light in the ozone-oxygen cycle.

UV gets absorbed in the stratosphere by molecular oxygen, which splits into 2 fast moving atoms. These fast moving oxygen atoms collide with the air molecules around them (nitrogen and oxygen), this slows the free oxygen atoms down, enabling them to bond weakly with molecular oxygen to form ozone. If they are moving too fast they simply bounce off! The energy they shared with molecules in the air heats the upper atmosphere.

The oxygen atom joins with an oxygen molecule to form ozone.

Ozone is very effective at absorbing ultra violet light, which causes the ozone to split back into an oxygen atom and an oxygen molecule, keeping the upper atmosphere warm. The ozone reforms as before once the oxygen atom is moving slowly again.

The ozone is spread out through the stratosphere.

If the ozone were squashed together like a solid it would only be the thickness of a sheet of cardboard, but it is actually a gas spread out over 20 or 30 km.

Before our atmosphere was filled with oxygen the UV reached the earth’s surface and prevented life from emerging onto the land.

Ozone is also present in the troposphere formed mainly by the action of sunlight on pollution from motor vehicles - here it helps to form photochemical smog and is a pollutant.

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