The gas nitrogen makes up 78.08% of the air. Nitrogen is largely unreactive, but it sometimes reacts with oxygen to form oxides of nitrogen.
Nitrogen is continually recycled by the bacteria that consume plant and animal waste.
Oxygen makes up 20.94% of the air. Animals breathe in oxygen. Plants give it out as they take their energy from sunlight in photosynthesis.
Carbon dioxide makes up 0.03% of the air. Carbon dioxide is continually recycled as it is breathed out by animals and taken in by plants in photosynthesis.
The air contains other, inert (unreactive) gases: 0.93% is argon; 0.0018% is neon; 0.0005% is helium.
There are tiny traces of krypton and xenon which are also inert.
Ozone makes up 0.00006% of the air. It is created when sunlight breaks up oxygen.
Hydrogen makes up 0.00005% of the air. This gas is continually drifting off into space.
Up to 10 kin above the ground, the air is always moist because it contains an invisible gas called water vapour.
There is enough water vapour in the air to flood the globe to a depth of 2.5 m.
Water vapour enters the air when it evaporates from oceans, rivers and lakes.
Water vapour leaves the air when it cools and condenses (turns to drops of water) to form clouds. Most clouds eventually turn to rain, and so the water falls back to the ground. This is called precipitation.
Like a sponge, the air soaks up evaporating water until it is saturated (full).
It can only take in more water if it warms up and expands.
If saturated air cools, it contracts and squeezes out the water vapour, forcing it to condense into drops of water.
The point at which this happens is called the dew point.
Humidity is the amount of water in the air.
Absolute humidity is the weight of water in grams in a particular volume of air.
Relative humidity, which is written as a percentage, is the amount of water in the air compared to the amount of water the air could hold when saturated.