AP3456 the Central Flying School (CFS) Manual of Flying: Volume 1

AP3456 - 1-1 - The Atmosphere CHAPTER 1 - THE ATMOSPHERE Introduction 1. The atmosphere is the term given to the layer of air which surrounds the Earth and extends upwards from the surface to about 500 miles. The flight of all objects using fixed or moving wings to sustain them, or air-breathing engines to propel them, is confined to the lower layers of the atmosphere. The properties of the atmosphere are therefore of great importance to all forms of flight. 2. The Earth’s atmosphere can be said to consist of four concentric gaseous layers. The layer nearest the surface is known as the troposphere, above which are the stratosphere, the mesosphere and the thermosphere. The boundary of the troposphere, known as the tropopause, is not at a constant height but varies from an average of about 25,000 ft at the poles to 54,000 ft at the equator. Above the tropopause the stratosphere extends to approximately 30 miles. At greater heights various authorities have at some time divided the remaining atmosphere into further regions but for descriptive purposes the terms mesosphere and thermosphere are used here. The ionosphere is a region of the atmosphere, extending from roughly 40 miles to 250 miles altitude, in which there is appreciable ionization. The presence of charged particles in this region, which starts in the mesosphere and runs into the thermosphere, profoundly affects the propogation of electromagnetic radiations of long wavelengths (radio and radar waves). 3. Through these layers the atmosphere undergoes a gradual transition from its characteristics at sea level to those at the fringe of the thermosphere, which merges with space. The weight of the atmosphere is about one millionth of that of the Earth, and an air column one square metre in section extending vertically through the atmosphere weighs 9,800 kg. Since air is compressible, the troposphere contains much the greater part (over three quarters in middle latitudes) of the whole mass of the atmosphere, while the remaining fraction is spread out with ever-increasing rarity over a height range of some hundred times that of the troposphere. 4. Average representative values of atmospheric characteristics are shown in Fig 1. It will be noted that the pressure falls steadily with height, but that temperature falls steadily to the tropopause, where it then remains constant through the stratosphere, but increasing for a while in the warm upper layers. Temperature falls again in the mesosphere and eventually increases rapidly in the thermosphere. The mean free path (M) in Fig 1 is an indication of the distance of one molecule of gas from its neighbours, thus, in the thermosphere, although the individual air molecules have the temperatures shown, their extremely rarefied nature results in a negligible heat transfer to any body present. Revised Mar 12 Page 1 of 8 AP3456 - 1-1 - The Atmosphere 1-1 Fig 1 The Atmosphere Physical Properties of Air 5. Air is a compressible fluid and as such it is able to flow or change its shape when subjected even to minute pressures. At normal temperatures, metals such as iron and copper are highly resistant to deformation by pressure, but in liquid form they flow readily. In solids the molecules adhere so strongly that large forces are needed to change their position with respect to other molecules. In fluids, however, the degree of cohesion of the molecules is so small that very small forces suffice to move them in relation to each other. A fluid in which there is no cohesion between the molecules, and therefore no internal friction, and which is incompressible would be an 'ideal' fluid - if it were obtainable. 6. Fluid Pressure. At any point in a fluid the pressure is the same in all directions, and if a body is immersed in a stationary fluid, the pressure on any point of the body acts at right angles to the surface at that point irrespective of the shape or position of the body. 7. Composition of Air. Since air is a fluid having a very low internal friction it can be considered, within limits, to be an ideal fluid. Air is a mixture of a number of separate gases, the proportions of which are: Revised Mar 12 Page 2 of 8 AP3456 - 1-1 - The Atmosphere 1-1 Table 1 Composition of Air Element By Volume % By Weight % Nitrogen 78.08 75.5 Oxygen 20.94 23.1 Argon 0.93 1.3 Carbon dioxide 0.03 0.05 Hydrogen Neon Helium Krypton Traces only Xenon Ozone Radon For all practical purposes the atmosphere can be regarded as consisting of 21% oxygen and 78% nitrogen by volume. Up to a height of some five to six miles water vapour is found in varying quantities, the amount of water vapour in a given mass of air depending on the temperature and whether the air is, or has recently been, over large areas of water. The higher the temperature the greater the amount of water vapour that the air can hold. Measurement of Temperature 8. Temperature can be measured against various scales: a. The Celsius scale (symbol º C) is normally used for recording atmospheric temperatures and the working temperatures of engines and other equipment. On this scale, water freezes at 0º C and boils at 100º C, at sea level. b. On the Kelvin thermodynamic scale, temperatures are measured in kelvins (symbol K - note there is no degree sign) relative to absolute zero. In the scientific measurement of temperature, 'absolute zero' has a special significance; at this temperature a body is said to have no heat whatsoever. Kelvin zero occurs at –273.15º C. c. On the Fahrenheit scale (symbol º F), water freezes at 32º F and boils at 212º F, at sea level. This scale is still used, particularly in the USA. 9. Conversion Factors. A kelvin unit equates to one degree C, therefore to convert º C to kelvins, add 273.15. To convert º F to º C, subtract 32 and multiply by 5 ; to convert º C to º F, multiply by 9 9 5 and add 32. Standard Atmosphere 10. The values of temperature, pressure and density are never constant in any given layer of the atmosphere, in fact, they are all constantly changing. Experience has shown that there is a requirement for a standard atmosphere for the comparison of aircraft performances, calibration of altimeters and other practical uses. A number of standards are in existence but Britain uses the International Standard Atmosphere (ISA) defined by the International Civil Aviation Organization (ICAO). 11. The ISA assumes a mean sea level temperature of +15º C, a pressure of 1013.25 hPa* (14.7 psi) and a density of 1.225 kg/m3. The temperature lapse rate is assumed to be uniform at the rate of Revised Mar 12 Page 3 of 8 AP3456 - 1-1 - The Atmosphere 1.98º C per 1,000 ft (6.5º C per kilometre) up to a height of 36,090 ft (11 km), above which height it remains constant at – 56.5º C (see Table 2). 1-1 Table 2 ICAO Standard Atmosphere Relative Altitude Temperature Pressure Pressure Density Density (ft) (º C) (hPa / mb) (psi) (kg/m3) (%) 0 +15.0 1013.25 14.7 1.225 100.0 5,000 +5.1 843.1 12.22 1.056 86.2 10,000 −4.8 696.8 10.11 0.905 73.8 15,000 −14.7 571.8 8.29 0.771 62.9 20,000 −24.6 465.6 6.75 0.653 53.3 25,000 −34.5 376.0 5.45 0.549 44.8 30,000 −44.4 300.9 4.36 0.458 37.4 35,000 −54.3 238.4 3.46 0.386 31.0 40,000 −56.5 187.6 2.72 0.302 24.6 45,000 −56.5 147.5 2.15 0.237 19.4 50,000 −56.5 116.0 1.68 0.186 15.2 *Note: The most commonly used unit of pressure is the hectopascal (hPa). Some references may refer to the millibar (mb) which is equivalent to the hPa. Density 12. Density (symbol rho (ρ)) is the ratio of mass to volume, and is expressed in kilograms per cubic metre (kg/m3). The relationship of density to temperature and pressure can be expressed thus: p Tρ = constant where p = Pressure in hectopascals and T = Absolute temperature (i.e. measured on the Kelvin scale) 13. Effects of Pressure on Density. When air is compressed, a greater amount can occupy a given volume; i.e. the mass, and therefore the density, has increased. Conversely, when air is expanded less mass occupies the original volume and the density decreases. From the formula in para 12 it can be seen that, provided the temperature remains constant, density is directly proportional to pressure, ie if the pressure is halved, so is the density, and vice versa. 14. Effect of Temperature on Density. When air is heated it expands so that a smaller mass will occupy a given volume, therefore the density will have decreased, assuming that the pressure remains constant. The converse will also apply. Thus the density of the air will vary inversely as the absolute temperature: this is borne out by the formula in para 12. In the atmosphere, the fairly rapid drop in pressure as altitude is increased has the dominating effect on density, as against the effect of the fall in temperature which tends to increase the density. 15. Effect of Humidity on Density. The preceding paragraphs have assumed that the air is perfectly dry.

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