(above) air image # 1: AIR: diagram showing Nitrogen N2 & Oxygen O2. This image is 500 x 500 pixels; the original image is 4096 x 4096 pixels.
AIR: forms the atmosphere. It consists mainly of nitrogen. There are approximately four molecules of nitrogen (shown in blue) for every one molecule of oxygen (shown in red). Both of these elements exist in air as molecules of two atoms. Nitrogen atoms are held together by a triple bond and oxygen atoms by a double bond. Other compounds in air include carbon dioxide and methane, both important as greenhouse gases. There is also water vapour in air, which can condense out forming clouds. We are so used to being immersed in air that we rarely give it much thought but it is quite substantial, especially at sea level. The weight of air pressing down can be measured for a given area and knowing this barometric pressure helps us to predict the weather. Air is a fluid and so lighter (strictly speaking less dense) objects will float upwards in it. This is the reason why hot air balloons and airships can fly. Hot air is less dense than cold air and so the hot air balloon becomes bouyant. Hot air balloons can often be seen in the clear skies above Canberra. Airships work by having a large balloon-like structure filled with a gas that is lighter than air. This used to be hydrogen but hydrogen is very explosive and helium is more commonly used nowadays. Heavier-than-air craft such as aeroplanes rely on air to provide lift. They also use air to supply the oxygen necessary to burn their fuel. Rockets, which go where the air is extremely thin, have to furnish their own oxygen.
Air is essential for (most) life and we breathe it it to supply our bodies with oxygen which is used in the slow, highly controlled, metabolic burning that releases carbon dioxide. When we leave the atmosphere we need to take our air supply with us. For example, underwater diving requires compressed air carried in tanks. However, the proportion of the gases can vary from that of normal atmospheric air. Aircraft that travel at high altitudes, where the air is much less dense, have pressurised compartments for their passengers. Pilots of some aircraft breathe from special supplies.
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