Module 8.1 Physics of the Atmosphere PDF

Technical Training Course for Avionics and Airframe and Powerplant Maintenance Degree Programs Module 8 Licence Category B1, B2 and B3 Basic Aerodynamics 8.1 Physics of the Atmosphere  Intentionally Blank Module 8.1 Physics of the Atm...

Technical Training Course for Avionics and Airframe and Powerplant Maintenance Degree Programs Module 8 Licence Category B1, B2 and B3 Basic Aerodynamics 8.1 Physics of the Atmosphere  Intentionally Blank Module 8.1 Physics of the Atmosphere 1-2 Anadolu Üniversitesi © Copyright 2012 Copyright Notice © Copyright. All worldwide rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e. photocopy, electronic, mechanical recording or otherwise without the prior written permission of Anadolu Üniversitesi. Knowledge Levels — Category A, B1, B2, B3 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1, B2, B3 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet the appropriate category B basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 A familiarisation with the principal elements of the subject. Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. Module 8.1 Physics of the Atmosphere 1-3 Anadolu Üniversitesi © Copyright 2012  Intentionally Blank Module 8.1 Physics of the Atmosphere 1-4 Anadolu Üniversitesi © Copyright 2012 Table of Contents Module 8.1 Physics of the Atmosphere ___________________________________________________9 The Atmosphere ___________________________________________________________________________________ 9 Gas Composition ___________________________________________________________________________________ 9 Regions of the Atmosphere __________________________________________________________________________ 9 Temperature _____________________________________________________________________________________ 10 Pressure _________________________________________________________________________________________ 12 Performance Ceilings ______________________________________________________________________________ 18 The Gas Laws _____________________________________________________________________________________ 19 The International Standard Atmosphere (ISA) ___________________________________________________________ 20 Humidity ________________________________________________________________________________________ 27 Module 8.1 Physics of the Atmosphere 1-5 Anadolu Üniversitesi © Copyright 2012  Intentionally Blank Module 8.1 Physics of the Atmosphere 1-6 Anadolu Üniversitesi © Copyright 2012 Module 8.1 Enabling Objectives and Certification Statement Certification Statement These Study Notes comply with the syllabus of EASA Regulation (EC) No.2042/2003 Annex III (Part-66) Appendix I, as amended by Regulation (EC) No.1149/2011, and the associated Knowledge Levels as specified below: Part-66 Licence Category Objective Reference A B1 B2 B3 International Standard Atmosphere (ISA), application to 8.1 1 2 2 1 aerodynamics. Module 8.1 Physics of the Atmosphere 1-7 Anadolu Üniversitesi © Copyright 2012  Intentionally Blank Module 8.1 Physics of the Atmosphere 1-8 Anadolu Üniversitesi © Copyright 2012 Module 8.1 Physics of the Atmosphere The Atmosphere The gaseous envelope surrounding the Earth is called the atmosphere. There is no defined upper limit to the atmosphere, but most aviation activity takes place within the first 60,000 ft and therefore we need not study above that. Gas Composition The gases found in the atmosphere are in the following proportions (by volume): Nitrogen 78% Oxygen 21% Other gases 1% (e.g. argon, carbon dioxide, water vapour) These proportions do not change with altitude. Oxygen is essential for the sustenance of life and the combustion of materials. In the context of aviation, oxygen is required for the combustion of fuel, a deficiency of this gas resulting in incomplete burning and reduced engine efficiency. Water vapour is present in the atmosphere in varying proportions, and is responsible for the weather around the earth, which in turn affects aircraft operations and performance. Additionally the presence of water vapour may cause icing of the airframe or engine which may impair an aircraft's performance. Regions of the Atmosphere The atmosphere is divided into a number of layers: (a) The Troposphere - Temperature decreases with an increase in height. In this region nearly all significant weather occurs. (b) The Tropopause - The upper limit of the troposphere where temperature stops decreasing with an increase of height. The tropopause is therefore the upper limit of significant weather, the first point of lowest temperature, and additionally it is the region for maximum wind strengths. The height of the tropopause varies with latitude, season of the year, and prevailing weather conditions with the result that it is usually higher in low latitudes, in summer and in fine weather. Typical heights for the tropopause are: Latitude Tropopause Height Equator 16 - 47 km 53,000—57,000 ft 45° NIS 10 - 12 km 33,000—39,000 ft Poles 7½ - 9 km 25,000—29,000 ft Module 8.1 Physics of the Atmosphere 1-9 Anadolu Üniversitesi © Copyright 2012 (c) The Stratosphere - From the tropopause to approximately 50 km above mean sea level, and is characterized by the temperature being steady or increasing with height (d) The Mesosphere - From 50 km to 80 km. The temperature generally decreases with height. (c) The Thermosphere or Ionosphere - Temperature increases with height. Temperature (a) Units The temperature scales most commonly used are Celsius (also known as Centigrade), Fahrenheit and Kelvin (also known as Absolute). The first two scales are based on the melting point of ice, being 0°C and 32°F respectively, and the boiling point of water, being 100°C or 212°F Heat is a form of transfer of energy, and is related to the random movement of molecules in a substance. If heat is reduced, the molecules become less active. The minimum temperature to which a substance can be reduced is approximately minus 273°C and this is known as Absolute Zero, or 0 K. Correspondingly, the melting point of ice is equivalent to 273 K and the boiling point of water to 373 K. To convert from one temperature scale to another, the following formulae may be used: 9 F= C + 32 5 5 C= (F – 32) 9 (b) Temperature Variation in the Troposphere At ground level, in general, the temperature increases with a decrease of latitude. With increasing altitude, the conductive and convective effects from the earth are reduced so that temperature will usually decrease with height up to the tropopause. Typical values of temperature found at the tropopause are: Latitude Temperature Equator -80°C 45°N/S -56°C Poles -45°C There is, therefore, a reversal of temperatures with latitude in comparison to those found at ground level. This is partly because the tropopause is higher at the equator and the temperature decrease is effective over a greater height. Module 8.1 Physics of the Atmosphere 1-10 Anadolu Üniversitesi © Copyright 2012  Figure 1.1 – Variations of the Tropopause around the World (c) Lapse Rates The temperature decrease with an increase of height is referred to as lapse rate. A representative value of 2°C/1000 ft is a typical value for the troposphere, and this figure is used as the reference for the Jet Standard. The International Standard Atmosphere (ISA) uses the comparable value of 1.98°C/1000 ft. For meteorological purposes, differentiation between dry (that is, not saturated) and saturated adiabatic lapse rates is made, and the values of 3°C/1000 ft and 1.5°C/1000 ft respectively are used. The difference of lapse rate for saturated air is caused by the release of latent heat during condensation, thus reducing the temperature change. (d)Temperature and Aircraft Performance At a given pressure, an increase of temperature results in a reduction of density. Firstly, considering airframe performance, a reduction of density (ρ) reduces lift (L). This may be counteracted by increasing the true airspeed (V) to achieve the required amount of lift (L): L = C L ½ ρV2S where: C L = coefficient of lift and S = surface area The dynamic pressure is gained at the expense of an increased take-off run, cruising TAS or landing run according to the stage of flight. On the credit side, drag (D) reduces with increase of temperature: Module 8.1 Physics of the Atmosphere 1-11 Anadolu Üniversitesi © Copyright 2012  D = C D ½ ρV2S A piston engine’s performance is related to the temperature of the air being drawn into the cylinder head. The higher the temperature, the lower the density and weight of fuel/air mixture that can be burnt in the combustion chamber. The power output of the engine therefore falls with increase of temperature. For a propulsion system, piston or jet: Thrust = Mass of air x Acceleration to which air is subjected Thus an increase of temperature will reduce the mass flow and, therefore the thrust. Pressure Definition Pressure is the force exerted on a unit area, i.e.: Force Mass × Acceleration Pressure = = Area Area In the atmosphere, pressure is caused by the mass of the gaseous molecules acting under the force of gravity on a given area. As all molecules act under gravity then the pressure can also be considered to be the weight of a column of air on a unit area. Figure 1.2 – A column of air Module 8.1 Physics of the Atmosphere 1-12 Anadolu Üniversitesi © Copyright 2012 Units The metric units of pressure are dynes per square centimetre, where the dyne is the force required to accelerate 1 gram by 1 centimetre per second. The Système international (SI) units of pressure are Newtons per square metre, where the Newton is the force required to accelerate 1 kilogram by 1 metre per second. The Newton is equal to 105 dynes. Although largely obsolete, the Imperial system of units is still encountered, and pressure is expressed in pounds per square inch. In meteorology the unit of pressure is the millibar (mb), which is equivalent to 1000 dynes per square centimetre. Before the introduction of the millibar, meteorological pressure was measured in terms of the length of a column of mercury in a barometer that the weight of the atmosphere could support. Figure 1.3 – Principle of the Mercury Barometer Module 8.1 Physics of the Atmosphere 1-13 Anadolu Üniversitesi © Copyright 2012 Variation of Pressure in the Atmosphere At sea level, pressure generally varies between 950 and 1050 mb. In tropical revolving storms and tornadoes, however, pressures may fall much lower. With increasing altitude the mass of overlying air decreases and so the pressure falls. Pressure values of the International Standard Atmosphere are given below: Altitude Pressure Pressure Pressure Pressure (ft) (mb) (psi) (in Hg) (mm Hg) 40,000 187.6 2.72 30,000 300.9 4.36 20,000 465.6 6.75 10,000 696.8 10.11 0 1013.25 14.7 29.92 760 From the table it should be noted that at about 18,000 ft. the pressure is half the sea level value. Also, it should now be apparent that the rate of pressure decrease with height is not constant. In the first 10,000 ft. the pressure falls at a rate of approximately 1 mb per 30 ft but between 30,000 ft and 40,000 ft the pressure decrease is closer to 1 mb per 88 ft. Pressure Altitude The altitude at which a given pressure occurs in the International Standard Atmosphere is called the pressure altitude. If, for example, the pressure at the top of Mount Everest were determined as 300.9 mb, then the pressure altitude would be 30,000 ft. Assuming the same mean sea level conditions, and two columns of air of the same height, but differing temperatures, then the cold air would have a greater mass than the warm air due to the density difference. The pressure of the atmosphere, however, is caused by the mass of overlying molecules on a unit area. The pressure above the column of warm air is therefore higher than that above cold air. Because a higher pressure is found at a lower level, then the pressure altitude above warm air is lower than the pressure altitude above cold air. Alternatively it can be expressed that the true altitude of an aircraft is more than that indicated (assuming the correct mean sea level pressure has been set on the subscale) above warm air, and less than that indicated above cold air. Module 8.1 Physics of the Atmosphere 1-14 Anadolu Üniversitesi © Copyright 2012  Figure 1.4 – Pressure Altitude – The effect of temperature on pressure Module 8.1 Physics of the Atmosphere 1-15 Anadolu Üniversitesi © Copyright 2012 Density Definition Density is the mass per unit volume of a substance, at a specified temperature and pressure. Mass Density = Volume Units Density is expressed in grams, or kilograms per cubic metre for metric or SI units, respectively. The Imperial units are pounds per cubic feet. Factors affecting density when considering a gas are: Pressure Density = Gas constant × Absolute temperatur e For a given temperature, therefore, an increase of pressure increases density, or, at a given pressure, a decrease in temperature increases density. Variation of Density in the Atmosphere At sea level, densities vary between 1.20 and 1.55 kg per cu m, the higher values being usually associated with the colder temperatures of higher latitudes, and the lower values typical of Equatorial latitudes. Air at lower levels in the atmosphere is compressed by the mass of the air above it. With increasing altitude, the overlying mass reduces and air can now expand, resulting in a further reduction of pressure. With increasing altitude the temperature also decreases, but at a rate lower than the pressure. Density, therefore, decreases with height. Density values of the International Standard Atmosphere are summarized below: Altitude Density Density [ft] [kg/cu m] [lb/cu ft] 40,0000.302 0.019 30,0000.458 0.029 20,0000.653 0.041 10,0000.905 0.056 0 1.225 0.077 At about 22,000 ft, the density is half the sea level value. Module 8.1 Physics of the Atmosphere 1-16 Anadolu Üniversitesi © Copyright 2012 We have already seen that density at sea level tends to be higher at the Poles than at the Equator. However, at 26,000 ft, the density value is similar at all latitudes. Variation of Density with Humidity The total pressure of the atmosphere is equal to the sum of the individual pressures of the gases. The pressure of moist air is less than that for dry air, and so humidity decreases the total pressure. From the gas equation, it can be seen that the reduction in pressure results in a lower density. The greater the humidity, the lower the density. Density Altitude This is defined as the altitude in the International Standard Atmosphere at which a given density is found. Aircraft performance is largely dependent on density altitude as opposed to true or pressure altitude. Density and Performance The effects of density on lift, drag, power and thrust have been described in the section about temperature. There are, however, additional effects of density performance. Above about 300 kt TAS, air becomes significantly compressed, and locally increases the density. At much higher speeds this may give a marked increase in drag, and when increasing altitude, this can offset the otherwise reducing drag value. A similar compressibility effect increases drag on a propeller blade, reducing its efficiency, particularly at higher altitudes. A jet engine's performance, however, is enhanced by this compressibility effect as mass flow is improved. Air Density and the Human Body The reduced density of air with increasing altitude means that in a given volume of air breathed in, the oxygen content has decreased. Above 10,000 ft this reduction leads to hypoxia, its effects ranging from lack of judgment to sleepiness or collapse, according to height. At night, the reduced intake of oxygen impairs night vision at altitudes of 4,000 ft and above. To counter these problems, aircraft operating above 10,000 ft must have an enriched oxygen supply, either in conjunction with a pressurized cabin, or through face-masks. At night, ideally, oxygen should be available from ground level upwards. Module 8.1 Physics of the Atmosphere 1-17 Anadolu Üniversitesi © Copyright 2012 Performance Ceilings Service Ceiling This is defined as the altitude at which the rate of climb of an aircraft falls to a specified figure, usually 100 ft. per minute. Absolute Ceiling The absolute ceiling is the altitude at which the rate of climb of an aircraft falls to zero. Piston-Engined Aircraft For such aircraft operating under 26,000 ft. the improved atmospheric density found in winter in high latitudes will give the highest ceiling. Jet-Engined Aircraft As most jet-engined aircraft operate above 26,000 ft, then the best performance ceiling will be found at the highest pressures and lowest temperature, i.e. in summer, and at low latitudes. Module 8.1 Physics of the Atmosphere 1-18 Anadolu Üniversitesi © Copyright 2012 The Gas Laws Introduction Whilst air is not an ideal gas, it does conform within close limits, to the results of Boyle’s and Charles’ laws. Boyle’s Law The volume (V) of a given mass of gas at constant temperature is inversely proportional to pressure (P): PV = constant This can be expressed in the form: P1 V1 = P2 V2 Charles’ Law 1 The volume of a given mass of gas at constant pressure, increases by of its volume at 0°C for every 273 1°C rise in temperature: V = constant T The alternative expression below is also useful: V1 V2 = T1 T2 Combined Boyle’s and Charles’ Law Equation The results of both laws may be combined in one equation, expressing the behaviour of a gas under varying conditions of pressure, volume and temperature: P1V1 P2 V2 = T1 T2 Module 8.1 Physics of the Atmosphere 1-19 Anadolu Üniversitesi © Copyright 2012 The International Standard Atmosphere (ISA) The International Standard Atmosphere is a tabulation with altitude of the standard variation of pressure, temperature, density, viscosity, etc, appropriate to mid latitudes (45oN), released by the International Civil Aviation Organisation (ICAO). In order to provide a datum for aircraft performance comparison, and instrument calibration, this assumed set of conditions is used. Whilst representative, these conditions do not necessarily reflect actual conditions in the atmosphere. The values used are listed below: Sea Level Conditions Property Metric Value Imperial Value Pressure 101.3 kPa 2116.2 lbf/ft2 Density 1.225 Kg/m3 0.002378 slug/ft3 Temperature 15 oC or 288.2 K 59 oF or 518.69 oR Speed of Sound 340 m/s 1116.4 ft/s Viscosity 1.789x10-5 Kg/m/s 3.737x10-7 slug/ft/s Kinematic Viscosity 1.460x10-5 m2/s 1.5723x10-4 ft2/s Thermal Conductivity 0.02596 W/m/K 0.015 BTU/hr/ft/oR Gas Constant 287.1 J/Kg/K 1715.7 ft lbf/slug/oR Specific Heat Cp 1005 J/Kg/K 6005 ft lbf/slug/oR Specific Heat Cv 717.98 J/Kg/K 4289 ft lbf/slug/oR Ratio of Specific Heats 1.40 1.40 Gravitational Acceleration 9.80665 m/s2 32.174 ft/s2 Module 8.1 Physics of the Atmosphere 1-20 Anadolu Üniversitesi © Copyright 2012 International Standard Atmosphere (ISA) Data ---------------------------------------------------------------------- Altitude Temperature Kinematic Speed o m ft C Pressure Density Viscosity Viscosity of Ratio Ratio Ratio Ratio Sound ----------------------------------------------------------------------- 0 0 15.2 1.0000 1.0000 1.0000 1.0000 340.3 152 500 14.2 0.9821 0.9855 0.9973 1.0121 339.7 304 1000 13.2 0.9644 0.9711 0.9947 1.0243 339.1 457 1500 12.2 0.9470 0.9568 0.9920 1.0367 338.5 609 2000 11.2 0.9298 0.9428 0.9893 1.0493 338.0 762 2500 10.2 0.9129 0.9289 0.9866 1.0622 337.4 914 3000 9.3 0.8962 0.9151 0.9839 1.0752 336.8 1066 3500 8.3 0.8798 0.9015 0.9812 1.0884 336.2 1219 4000 7.3 0.8637 0.8881 0.9785 1.1018 335.6 1371 4500 6.3 0.8477 0.8748 0.9758 1.1155 335.0 1524 5000 5.3 0.8320 0.8617 0.9731 1.1293 334.4 ---------------------------------------------------------------------- 1676 5500 4.3 0.8166 0.8487 0.9704 1.1434 333.8 1828 6000 3.3 0.8014 0.8359 0.9677 1.1577 333.2 1981 6500 2.3 0.7864 0.8232 0.9649 1.1722 332.6 2133 7000 1.3 0.7716 0.8106 0.9622 1.1870 332.0 2286 7500 0.3 0.7571 0.7983 0.9595 1.2020 331.4 2438 8000 -0.6 0.7428 0.7860 0.9567 1.2172 330.8 2590 8500 -1.6 0.7287 0.7739 0.9540 1.2327 330.2 2743 9000 -2.6 0.7148 0.7620 0.9512 1.2484 329.6 2895 9500 -3.6 0.7012 0.7501 0.9485 1.2644 329.0 3048 10000 -4.6 0.6877 0.7385 0.9457 1.2807 328.4 ---------------------------------------------------------------------- 3200 10500 -5.6 0.6745 0.7269 0.9430 1.2972 327.8 3352 11000 -6.6 0.6614 0.7155 0.9402 1.3140 327.2 3505 11500 -7.6 0.6486 0.7043 0.9374 1.3310 326.6 3657 12000 -8.6 0.6360 0.6932 0.9347 1.3484 326.0 3810 12500 -9.6 0.6236 0.6822 0.9319 1.3660 325.4 3962 13000 -10.6 0.6113 0.6713 0.9291 1.3840 324.7 4114 13500 -11.5 0.5993 0.6606 0.9263 1.4022 324.1 4267 14000 -12.5 0.5875 0.6500 0.9235 1.4207 323.5 4419 14500 -13.5 0.5758 0.6396 0.9207 1.4396 322.9 4572 15000 -14.5 0.5643 0.6292 0.9179 1.4588 322.3 ---------------------------------------------------------------------- 4724 15500 -15.5 0.5531 0.6190 0.9151 1.4783 321.7 4876 16000 -16.5 0.5420 0.6090 0.9123 1.4981 321.0 5029 16500 -17.5 0.5311 0.5990 0.9094 1.5183 320.4 5181 17000 -18.5 0.5203 0.5892 0.9066 1.5388 319.8 5334 17500 -19.5 0.5098 0.5795 0.9038 1.5596 319.2 5486 18000 -20.5 0.4994 0.5699 0.9009 1.5809 318.5 5638 18500 -21.5 0.4892 0.5604 0.8981 1.6025 317.9 5791 19000 -22.4 0.4791 0.5511 0.8953 1.6244 317.3 5943 19500 -23.4 0.4693 0.5419 0.8924 1.6468 316.7 6096 20000 -24.4 0.4595 0.5328 0.8895 1.6696 316.0 ---------------------------------------------------------------------- 6248 20500 -25.4 0.4500 0.5238 0.8867 1.6927 315.4 6400 21000 -26.4 0.4406 0.5150 0.8838 1.7163 314.8 6553 21500 -27.4 0.4314 0.5062 0.8809 1.7403 314.1 6705 22000 -28.4 0.4223 0.4976 0.8781 1.7647 313.5 6858 22500 -29.4 0.4134 0.4891 0.8752 1.7895 312.9 Module 8.1 Physics of the Atmosphere 1-21 Anadolu Üniversitesi © Copyright 2012 7010 23000 -30.4 0.4046 0.4806 0.8723 1.8148 312.2 7162 23500 -31.4 0.3960 0.4723 0.8694 1.8406 311.6 7315 24000 -32.3 0.3876 0.4642 0.8665 1.8668 311.0 7467 24500 -33.3 0.3793 0.4561 0.8636 1.8935 310.3 Module 8.1 Physics of the Atmosphere 1-22 Anadolu Üniversitesi © Copyright 2012 ---------------------------------------------------------------------- Altitude Temperature Kinematic Speed o m ft C Pressure Density Viscosity Viscosity of Ratio Ratio Ratio Ratio Sound ----------------------------------------------------------------------- 7620 25000 -34.3 0.3711 0.4481 0.8607 1.9207 309.7 7772 25500 -35.3 0.3631 0.4402 0.8578 1.9484 309.0 7924 26000 -36.3 0.3552 0.4325 0.8548 1.9766 308.4 8077 26500 -37.3 0.3474 0.4248 0.8519 2.0053 307.7 8229 27000 -38.3 0.3398 0.4173 0.8490 2.0345 307.1 8382 27500 -39.3 0.3324 0.4098 0.8460 2.0643 306.4 8534 28000 -40.3 0.3250 0.4025 0.8431 2.0947 305.8 8686 28500 -41.3 0.3178 0.3953 0.8402 2.1256 305.1 8839 29000 -42.3 0.3107 0.3881 0.8372 2.1571 304.5 8991 29500 -43.2 0.3038 0.3811 0.8342 2.1892 303.8 9144 30000 -44.2 0.2970 0.3741 0.8313 2.2219 303.2 ---------------------------------------------------------------------- 9296 30500 -45.2 0.2903 0.3673 0.8283 2.2553 302.5 9448 31000 -46.2 0.2837 0.3605 0.8253 2.2892 301.9 9601 31500 -47.2 0.2772 0.3539 0.8223 2.3239 301.2 9753 32000 -48.2 0.2709 0.3473 0.8194 2.3592 300.5 9906 32500 -49.2 0.2647 0.3408 0.8164 2.3952 299.9 10058 33000 -50.2 0.2586 0.3345 0.8134 2.4318 299.2 10210 33500 -51.2 0.2526 0.3282 0.8104 2.4692 298.6 10363 34000 -52.2 0.2467 0.3220 0.8073 2.5074 297.9 10515 34500 -53.2 0.2410 0.3159 0.8043 2.5463 297.2 10668 35000 -54.1 0.2353 0.3099 0.8013 2.5859 296.5 ---------------------------------------------------------------------- 10820 35500 -55.1 0.2298 0.3039 0.7983 2.6264 295.9 10972 36000 -56.1 0.2243 0.2981 0.7952 2.6677 295.2 10999 36089 -56.3 0.2234 0.2971 0.7947 2.6751 295.1 11277 37000 -56.3 0.2138 0.2843 0.7947 2.7948 295.1 11582 38000 -56.3 0.2038 0.2710 0.7947 2.9324 295.1 11887 39000 -56.3 0.1942 0.2583 0.7947 3.0768 295.1 12192 40000 -56.3 0.1851 0.2462 0.7947 3.2283 295.1 ---------------------------------------------------------------------- 12496 41000 -56.3 0.1764 0.2346 0.7947 3.3872 295.1 12801 42000 -56.3 0.1681 0.2236 0.7947 3.5540 295.1 13106 43000 -56.3 0.1602 0.2131 0.7947 3.7290 295.1 13411 44000 -56.3 0.1527 0.2031 0.7947 3.9126 295.1 13716 45000 -56.3 0.1456 0.1936 0.7947 4.1052 295.1 ---------------------------------------------------------------------- 14020 46000 -56.3 0.1387 0.1845 0.7947 4.3073 295.1 14325 47000 -56.3 0.1322 0.1758 0.7947 4.5194 295.1 14630 48000 -56.3 0.1260 0.1676 0.7947 4.7419 295.1 14935 49000 -56.3 0.1201 0.1597 0.7947 4.9754 295.1 15240 50000 -56.3 0.1145 0.1522 0.7947 5.2203 295.1 ---------------------------------------------------------------------- 15544 51000 -56.3 0.1091 0.1451 0.7947 5.4773 295.1 15849 52000 -56.3 0.1040 0.1383 0.7947 5.7470 295.1 16154 53000 -56.3 0.09909 0.1318 0.7947 6.0300 295.1 16459 54000 -56.3 0.09444 0.1256 0.7947 6.3268 295.1 16764 55000 -56.3 0.09001 0.1197 0.7947 6.6383 295.1 ---------------------------------------------------------------------- 17068 56000 -56.3 0.08579 0.1141 0.7947 6.9652 295.1 17373 57000 -56.3 0.08176 0.1087 0.7947 7.3081 295.1 17678 58000 -56.3 0.07793 0.1036 0.7947 7.6679 295.1 Module 8.1 Physics of the Atmosphere 1-23 Anadolu Üniversitesi © Copyright 2012 17983 59000 -56.3 0.07427 0.09878 0.7947 8.0454 295.1 18288 60000 -56.3 0.07079 0.09414 0.7947 8.4416 295.1 ---------------------------------------------------------------------- Module 8.1 Physics of the Atmosphere 1-24 Anadolu Üniversitesi © Copyright 2012 ---------------------------------------------------------------------- Altitude Temperature Kinematic Speed o m ft C Pressure Density Viscosity Viscosity of Ratio Ratio Ratio Ratio Sound ----------------------------------------------------------------------- 18592 61000 -56.3 0.06746 0.08972 0.7947 8.8572 295.1 18897 62000 -56.3 0.06430 0.08551 0.7947 9.2932 295.1 19202 63000 -56.3 0.06128 0.08150 0.7947 9.7508 295.1 19507 64000 -56.3 0.05841 0.07768 0.7947 10.231 295.1 19812 65000 -56.3 0.05566 0.07403 0.7947 10.735 295.1 ---------------------------------------------------------------------- 20116 66000 -56.3 0.05305 0.07056 0.7947 11.263 295.1 20421 67000 -56.3 0.05056 0.06725 0.7947 11.818 295.1 20726 68000 -56.3 0.04819 0.06409 0.7947 12.399 295.1 21031 69000 -56.3 0.04593 0.06108 0.7947 13.010 295.1 21336 70000 -56.3 0.04377 0.05822 0.7947 13.650 295.1 ----------------------------------------------------------------------- Module 8.1 Physics of the Atmosphere 1-25 Anadolu Üniversitesi © Copyright 2012  Intentionally Blank Module 8.1 Physics of the Atmosphere 1-26 Anadolu Üniversitesi © Copyright 2012 Humidity Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew, frost and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour. The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. This is the absolute humidity of air. Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m3 (grams per cubic metre) of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%. When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Relative humidity is 100%. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer. A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer. The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity. Definitions Absolute humidity: The mass of water vapour in a given volume of air (i.e., density of water vapour in a given parcel), usually expressed in grams per cubic meter Actual vapour pressure: The partial pressure exerted by the water vapour present in a parcel. Water in a gaseous state (i.e. water vapour) exerts a pressure just like the atmospheric air. Vapour pressure is also measured in Millibars. Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapour to liquid water. Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air. Module 8.1 Physics of the Atmosphere 1-27 Anadolu Üniversitesi © Copyright 2012 Dry bulb temperature: The actual air temperature. See wet bulb temperature below. Freezing: The phase change of liquid water into ice. Evaporation: The phase change of liquid water into water vapour. Melting: The phase change of ice into liquid water. Mixing ratio: The mass of water vapour in a parcel divided by the mass of the dry air in the parcel (not including water vapour). Relative humidity: The amount of water vapour actually in the air divided by the amount of water vapour the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapour pressure by the saturation vapour pressure and then multiply by 100 to convert to a percent. Saturation of air: The condition under which the amount of water vapour in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapour is added to the air. Saturation vapour pressure: The maximum partial pressure that water vapour molecules would exert if the air were saturated with vapour at a given temperature. Saturation vapour pressure is directly proportional to the temperature. Specific humidity: The mass of water vapour in a parcel divided by the total mass of the air in the parcel (including water vapour). Sublimation: In meteorology, the phase change of water vapour in the air directly into ice or the change of ice directly into water vapour. Chemists, and sometimes meteorologists, refer to the vapour to solid phase change as "deposition." Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity. Module 8.1 Physics of the Atmosphere 1-28 Anadolu Üniversitesi © Copyright 2012