1.2 The Composition of Air

Air is a mixture of gases the main components of which are shown in Table 1.1.

Water vapour in varying quantities is found in the atmosphere up to a height of approximately 30 000 ft. The amount in any given air mass is dependent on the air temperature and the passage of the air mass in relationship to large areas of water. The higher the air temperature the greater the amount of water vapour it can hold.

Kelvin Scale. Often for scientific purposes temperatures relative to absolute zero are used in formulae regarding atmospheric density and pressure. Temperatures relative to absolute zero are measured in Kelvin. A body is said to have no heat at absolute zero and this occurs at a temperature of –273.15 ◦C.

1.2.1 The Measurement of Temperature

Centigrade Scale. The Centigrade scale is normally used for measuring the air temperature and for the temperature of aero-engines and their associated equipment. On this temperature, scale water freezes at 0 degrees and boils at 100 degrees at mean sea level.

1.2.2 Air Density

Air density is mass per unit volume. The unit of air density is either kg per m3 or gm−3 and the symbol used is ‘ρ’. The relationship of air density to air temperature and air pressure is given by the formula:

where ρ is the density, p is pressure in hPa and T is the absolute temperature.

The Effect of Air Pressure on Air Density. If air is compressed the amount of air that can occupy a given volume increases. Therefore, both mass and density are increased. For the same volume if the pressure is decreased then the reverse is true. From the formula above if the air temperature remains constant then the air density is directly proportional to the air pressure. If the air pressure is doubled so is the air density.

The Effect of Air Temperature on Air Density. When air is heated it expands so that a smaller mass will occupy a given volume and provided that the air pressure remains constant then the air density will decrease. Thus, the density of the air is inversely proportional to the absolute temperature. The rapid decrease of air pressure with increased altitude has a far greater effect on the air density than does the increase of density caused by the decrease in temperature for the same increased altitude. Thus, the overall effect is for the air density to diminish with increased altitude.

The Effect of Humidity on Air Density. Until now it has been assumed that the air is perfectly dry; such is not the case. In the atmosphere, there is always some water vapour present, albeit under certain conditions a minuscule amount. However, in some conditions, the amount of water vapour present is an important factor when determining the performance of an aeroplane. For a given volume the amount of air occupying that volume decreases as the amount of water vapour contained in the air increases. In other words, air density decreases with increased water-vapour content. It is most dense in perfectly dry air.

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