What the air is made of

What is air made of?

What air is made of

Every dot is a molecule, moving like the real thing. Tap a gas to find it.

• Nitrogen
• Nearly 4 of every 5 molecules. By volume N₂ : O₂ is about 4 : 1; by weight about 3 : 1.
• Oxygen
• About 1 in 5 molecules. The share stays the same as you climb, but the air thins, so each breath holds less. Above 10 000 ft pilots need supplementary oxygen.
• Argon
• An inert gas, just under 1%. It plays no part in weather.
• Carbon dioxide
• Only about 0.035% of the air, yet with water vapour it traps the Earth's heat: a greenhouse gas.
• Water vapour
• Not part of the fixed mix: from almost none over the poles and above 30 000 ft to about 4% in hot, humid tropical air. Warmer air can hold more.

Memory hook: Four nitrogen for every oxygen.

Takeaway: Nitrogen 78%, oxygen 21%, argon about 1%, and a trace of CO2. By volume nitrogen outnumbers oxygen 4 to 1.

What else is floating in the air besides gas?

Dust, salt and smoke

Real pictures. Swipe or tap Next.

1. Air is never just gas Besides gases, the air carries tiny solid particles: salt from evaporating sea spray, dust from dry lands, smoke and particles from industry.
2. They dim the sun and the view The particles block and scatter sunlight, so they change temperatures and reduce visibility. Haze over northern India in winter is mostly this.
3. Seeds for cloud and fog Water vapour needs a surface to condense on. These particles act as condensation nuclei and make condensation start early, even before the air is quite saturated. In large cities and industrial areas this is why fog and smog (fog mixed with smoke) are so frequent. Particles = condensation nuclei → early fog and smog over cities.

Takeaway: Salt, dust, smoke and industrial particles dim the sunlight, cut visibility and give water vapour something to condense on, which is why big cities get frequent fog and smog.

Which gases change from place to place?

Variable gases

A tiny share of the air, but they decide the weather.

1. Most of the air never changes Nitrogen, oxygen and argon are mixed almost evenly everywhere. A few gases are different: their amount changes from place to place.
2. Cities and industry Burning fuel and wood gives carbon dioxide, carbon monoxide, sulphur dioxide and nitrogen dioxide. They are concentrated over towns and industrial areas.
3. Water vapour from water Seas, lakes and rivers keep adding water vapour, the most important variable gas for weather.
4. Plants and oceans take CO₂ back Plants absorb CO₂, keep the carbon as food and release oxygen. A large amount of CO₂ also dissolves in the oceans. Over the poles and at height there is very little of it.
5. How much water can air hold? Warm air can hold more water vapour. In the tropics air can carry up to about 4% by volume; at the poles and above 30 000 ft it is almost nil, because it is so cold. Air holding all the vapour it can is saturated (relative humidity 100%). Anything less is unsaturated, often loosely called dry air. Water in the air comes in three states: gas (water vapour), liquid (drizzle, rain, showers), solid (snow, ice, hail).

Takeaway: Water vapour, CO, SO₂, NO₂ and methane vary from place to place: more over cities, industry and water. Tiny amounts, big effect on weather and life.

Why do a few trace gases matter so much?

Greenhouse gases

Tap Next to build the picture.

1. Sunlight comes straight through The sun's energy arrives as short waves. The air is almost transparent to them, so most of it passes straight through and warms the ground.
2. The ground sends heat back up A warm surface gives off its own energy, but as long waves (heat radiation). This is terrestrial radiation.
3. A few gases catch it Water vapour, carbon dioxide, ozone and methane are only a tiny part of the air, but they absorb long waves well. They are called greenhouse gases.
4. ...and send part of it back down Having absorbed the heat, these gases radiate it again in every direction, some of it back to the ground.
5. The greenhouse effect So the Earth stays warmer than it would be with no air at all, by roughly 33 °C. Life as we know it depends on it.
6. Too much of a good thing Burning fuel adds CO₂ faster than plants and the oceans can take it up, so more heat is held in. This is global warming, and it needs to be limited. Greenhouse gases: transparent to short-wave (solar), absorb long-wave (terrestrial).

Memory hook: Short waves in, long waves trapped.

Takeaway: Water vapour, CO₂, ozone and methane let sunlight in but trap part of the heat the Earth sends back. That keeps the planet warm; too much of them causes global warming.

What does the ozone layer actually do?

Ozone

Tap Next. Heights are to scale.

1. UV arrives from the sun Part of sunlight is ultraviolet (UV): energetic enough to burn skin and damage eyes.
2. Ozone stops it High up, UV splits oxygen molecules; the loose atoms join other oxygen to make ozone (O₃). Ozone then absorbs UV. There is real ozone between about 10 and 50 km, densest at 20-25 km. (Ozone is only about 0.000007% of the air.)
3. This heats the stratosphere The energy ozone absorbs warms the air. That is why the stratosphere gets warmer with height instead of colder.
4. A shield for life Because ozone takes out most of the UV, little reaches the ground. It protects us from sunburn, skin cancer and cataracts.
5. The ozone hole Chlorofluorocarbons (CFCs, once used in fridges and sprays), halocarbon solvents and similar pollutants drift up to the stratosphere. Sunlight breaks them and frees chlorine and other halogen atoms, which break ozone back into ordinary oxygen. The layer has thinned, and big holes open, mostly over the poles. Max ozone 20-25 km. Appreciable from 10 to 50 km.

Memory hook: Ozone: 20-25 km, UV shield, stratosphere heater.

Takeaway: Ozone forms high up, absorbs the sun's ultraviolet light and warms the stratosphere. Most of it is at 20-25 km. CFCs and similar pollutants have thinned it into holes.