Lenntech Water treatment & purification

FAQ air chemistry and physics Frequently Asked Questions

The question library on air related issues

What is air?

Air is nothing but a mixture of a variety of gasses. The air in the atmosphere consists of nitrogen, oxygen, which is the life-sustaining substance for animals and humans, carbon dioxide, water vapour and small amounts of other elements (argon, neon, etc.). Higher in the atmosphere air also contains ozone, helium and hydrogen. People can only clearly notice the prescence of air when the wind blows.

What does air consist of?

The air we breathe consists mostly of the following gases:

Nitrogen (N2)


Oxygen (O2)


Noble gasses


Carbon dioxide (CO2)


Water vapor (H2O)


The amount of water in the air greatly varies. When large amounts of water are present in the air other elements are present in lower amounts. The amount of water in the air can rise up to 4%. The lowest percentage of water in the air is 0,5%. Water presses present gases closer together, so that it can take up enough space.
When the amount of water in the air is very low, air is called 'dry'. The weight of 22,4 dm3 of dry air is 28,96 grams. When the air contains sufficient amounts of water it is called humid. Humid air is lighter (less dense) than dry air.

Apart from the elements that have been summed up earlier, other elements are also present in air. However, the percentages of these elements are very low. Aerosols can be found in air. These are dust particles that are blown off the earth's surface by wind, or emitted during volcanic activity. When burning processes take place, ashes and grime particles also end up in the air.

The composition of air varies greatly with height. Over ninety kilometres above the earths' surface oxygen molecules fall apart and only oxygen atoms are left. Over one hundred kilometres above the earths' surface, nitrogen molecules also decompose. At this height the air does not have the composition we know. The atmosphere is completely different there.

What are gases?

All chemical elements can be found in several different states (phases). One of these phases is the gaseous phase. Apart from gaseous an element can also be solid or liquid. Water is called ice when it is solid, it is wet when it is liquid, and when it is gaseous it exists merely as gas or steam.
When temperatures rise, the molecules of a substance part, causing the substance to become gaseous and less visible. This phase change causes air to be invisible.
When temperatures fall air molecules are closer together and a substance eventually becomes solid. The temperatures at which each 'phase-change' takes place differs for each substance.

Where on earth do we find air?

We can state that air can be found everywhere on earth exept where there is water. Air is even located in the surface layer of the earth; in the soil.
Air is not only located on earth, it is also located around the earth in an air layer called the atmosphere. The atmosphere can be divided up in separate layers depending on temperature and height. These lines are not clearly separated by straight borders; they gradually overflow one another.

The first layer of air, which is located closest to the earth is called the troposphere. This layer is 11 kilometres in height. When moving up in the troposphere temperatures fall six or seven degrees per kilometre. Consequentially the weather on earth is mainly determined by circumstances within the troposphere.
The upper layer of the troposphere is called tropopause. Near the Antarctic, the tropopause layer is located eight to ten kilometres above earth. However, at the equator the tropopause layer is located at seventeen to eighteen kilometres above earth.

The second layer of air above the troposphere is called stratosphere. Temperatures stop decreasing in the lower part of this layer. The temperature is around -55 degrees Celsius here.
In the higher stratosphere temperatures are rising to zero degrees Celsius at forty-seven kilometres above earth.
Within the stratosphere solar radiation creates ozone (O3) from oxygen (O2), between twenty and forty kilometres above the earth's surface. This reaction causes this part of the stratosphere to be referred to as 'ozone-sphere'. The upper part of the stratosphere is called stratopauze.

The third layer of air is called the mesosphere. This layer can be found over fifty-two kilometres above the earths' surface. The upper part of the mesosphere is called the mesopause. Within the mesosphere, temperatures are decreasing once more. Mesosphere temperatures are around -90 degrees Celsius.

The fourth layer of air, the thermosphere is located over ninety kilometres above earth. Temperatures rise enormously in this layer, causing the highest temperature to be above one thousand degrees Celsius. The density of air is very low in this layer, therefore the forces between molecules nearly vanish.
The lightest molecules can escape through the lowest layer of the thermosphere, the exosphere. The exosphere does not have a clear border, because it fades into space.

The lowest ninety kilometres of the atmosphere are often referred to as the hemisphere, because the air composition is fairly constant. All air above this layer is referred to as hydrosphere, because the air has a very different composition in this area.

What types of air are there?

When a large quantity of air has one identical dampness and temperature, it is rated as one separate type of air. The air type must cover a horizontal area of one thousand kilometres. The height of an air type can vary between one hundred metres to coverage of the entire troposphere.
An air type forms when an air mass has been circulating three to nine days in an area that is located completely above either land or sea and where the wind does not blow. Within this area the air mass receives its specific properties. Above land these areas can be deserts or savannas.
As soon as the air leaves the area its specific properties will slowly fade and eventually disappear entirely.
Air types that have received their specific properties in an area above sea are much more humid than air types that have received specific properties above land. The air types that are shaped above oceans are called marine air types. Air types that are shaped above land are called continental air types.

We can distinguish four separate basic air types, which can all be divided up between marine and continental types:
1. Equatorial air. The temperatures lie between 25 and 30 degrees Celsius and its moisture content is high.
2. Tropical air. Marine tropical air has a high moisture content and a temperature of around 25 degrees Celsius. Continental tropical air has a low moisture content and a temperature of over 50 degrees Celsius.
3. Polar air. Marine polar air is always humid and is relatively hot in winter and cold in summer. Continental polar air very dry and cold in winter. Temperatures can fall to below -50 degrees Celsius. In summer this type of air is warm, yet still very dry.
4. Arctic air. This type of air is very cold. Marine arctic air is warmer than continental arctic air in winter.

What is wind?

Wind is basically air that is in motion. The air moves as a consequence of various types of air pressure on earth. Wind direction and wind-force can vary greatly. Wind-force is often referred to as cardinal points.
Wind-force is expressed as a number called a Beaufort number, on the Beaufort scale. Wind-force is always determined about ten metres above land.

Description of wind speed according to the Beaufort scale



Wind speed
(Metres per second)





Little wind



Little wind



Moderate wind



Moderate wind



Rather strong wind



Strong wind



Rough wind



Stormy wind






Heavy storm



Very heavy storm





Simply put, wind exists because solar radiation warms the earth. Heat and hot air rise, causing the air in the atmosphere to start moving. This motion is known as wind. During this motion of air, warm air is transported from the equator to the poles and cold air is transported back to the equator. This effect causes the equator to cool down and the poles to slightly warm up to prevent extreme temperatures. Apart from wind, oceans also aid in spreading heat.
Air does not move directly from the equator to the poles and back, as you might expect. The spinning motions of the earth influence wind directions. Consequentially, the wind that blows from the equator to the North Pole is turned slightly eastwards. The wind that blows from the equator to the South Pole is turned slightly westwards. Wind is always called after the direction it comes from. Wind from equator to North Pole is called west wind. Hot air that is blown from the equator to the poles cools down on its way.

On 30o north and south latitude, air falls back to the earth's surface. Not all air will be blown back to the equator from there.

Wind blows in a circle, called a cell. On earth we can distinguish three kinds of cells:
- The Hadley cell is a cell between the equator and 30o north and south latitude. The wind in this cell that is located near the earths' surface is called trade wind.
- The Ferrell cell is located between 30o and 60o north and south latitude. The wind in this cell blows harder in winter. At the 30o latitude border, wind falls and blows towards the poles. At 60o latitude, the air rises and the wind blows back to the 30o latitude border. Not all wind blows back, part of the wind from the Ferrell cell is taken up in the Pole cell.
- The Pole cell is located on the poles, up to 60o north and south latitude. At 60o latitude the air rises, but above the poles the air falls. Winds in the Pole cell are usually cold and dry.

This is only an impression of how the wind blows on earth. The earth does not only consist of water, but also of land. Land influences wind directions. This causes wind to be able to blow in a different direction in separate regions.

What is air pressure?

Air practises a certain force on objects, called air pressure. All air found in the atmosphere presses itself upon earth as a result of magnetic attraction force of the earth. Air pressure is practised upon each object and life form on earth, such as tables, roofs and houses, but also humans, animal and plants. You will not notice this air pressure, because there is a force inside your body that creates a pressure equally strong on the air around you. Under a table the pressure equals the pressure on the table, otherwise it would directly collapse.
Air pressure is defined as the pressure that the total weight of a column of air practises upon a piece of earth of one square metre (1 m2). The unit of pressure is Pascal (Pa).
The highest air pressure is located at the bottom of the atmosphere, just above the ground. Higher in the atmosphere the air pressure decreases. On earth you experience the pressure of a large amount of air particles; higher in the air there are less particles present that practise pressure upon you.
The average air pressure on earth is 1013 hPa. This is not an exact number and as such, it can vary a little bit. People will hardly notice this, because their bodies adapt to new air pressures. However, some people are more sensitive to changes in air pressure. For those people a change in air pressure can cause migraines.

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