Air facts for kids

Kids Encyclopedia Facts
Air composition pie chart
This is what the air is made of
Kawasaki-Electric Fan
A fan is used to move air

Air is the Earth's atmosphere. Air around us is a mixture of many gases and dust particles. It is the clear gas in which living things live and breathe. It has an indefinite shape and volume. It has no color or smell. It has mass and weight. It is a matter as it has mass and weight. Air creates atmospheric pressure. There is no air in the vacuum of the cosmos.

Air is a mixture of about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, and very small amounts of other gases. There is an average of about 1% water vapour.

Animals need the oxygen in the air to live. In the human body, the lungs give oxygen to the blood, and give back carbon dioxide to the air. Plants need the carbon dioxide in the air to live. They give off oxygen that humans can breathe again.

Air can be polluted by some gases (such as carbon monoxide), smoke, and ash. This pollution is one of the reasons for global warming, by causing the "greenhouse effect". The average temperature of the atmosphere at the surface of earth is 14 °C.

Aircraft use propellers to move air over a wing, which allows them to fly. Pneumatics use air pressure to move things.

Temperature and the atmospheric layers

Perlemorskyer Polar stratospheric cloud
Polar stratospheric cloud

The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies between the different atmospheric layers:

  • troposphere: the lowest layer of the atmosphere starting at the surface going up to between 7 km at the poles and 17 km at the equator with some variation due to weather factors
  • stratosphere: from that 7–17 km range to about 50 km, temperature increasing with height
  • mesosphere: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height
  • thermosphere: from 80–85 km to 640+ km, temperature increasing with height

Atmospheric regions are also named in other ways:


Top of Atmosphere
Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving Earth a blue halo when seen from space

Minor components of air not listed above include:

The evolution of the Earth's atmosphere

Oxygen content of the atmosphere over the last billion years

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, this remains an active area of research.

The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen. Heat (from the still-molten crust, and the sun) dissipated this atmosphere.

About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere", which was primarily carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had approximately 100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the cyanobacteria.

Earth Eclipses Sun-ap12-s80-37406
Earth's atmosphere back-lit by the Sun in an eclipse observed from deep space onboard Apollo 12 in 1969

Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving organisms. They were responsible for the initial conversion of the earth's atmosphere from a state without oxygen to a state with oxygen. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.

Photosynthesizing plants would later evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.

As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere, resulting in mass extinctions and further evolution.

With the appearance of an ozone layer (ozone is an allotrope of oxygen) life forms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

Ozone cycle
Ozone cycle

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Air Facts for Kids. Kiddle Encyclopedia.