xvi
Figure 16-6. Layer Aloft Ceiling Versus Indefinite Ceiling16-9
Figure 17-1. Convective Turbulence17-2
Figure 17-2. Thermals17-2
Figure 17-3. Mechanical Turbulence17-3
Figure 17-4. Mountain Waves17-4
Figure 17-5. Mountain Wave Clouds17-5
Figure 17-6. Wind Shear Turbulence17-5
Figure 17-7. Wind Shear Turbulence Associated with a Temperature Inversion17-6
Figure 18-1. Icing with Fronts18-5
Figure 18-2. Icing with Mountains18-6
Figure 19-1. Necessary Ingredients for Thunderstorm Cell Formation19-1
Figure 19-2. Thunderstorm Cell Life Cycle19-2
Figure 19-3. Multicell Cluster Thunderstorm19-3
Figure 19-4. Multicell Line Thunderstorm19-4
Figure 19-5. Supercell Thunderstorm19-5
Figure 19-6. Factors that Influence Thunderstorm Motion19-5
Figure 19-7. Downburst Life Cycle19-7
Figure 19-8. Landing in a Microburst19-7
Figure 19-9. Thunderstorm with Shelf Cloud19-8
Figure 19-10. Vivian, South Dakota Record Hailstone19-9
Figure 20-1. Radar Antenna20-1
Figure 20-2. Backscattered Energy20-2
Figure 20-3. Wavelengths20-3
Figure 20-4. Precipitation Attenuation20-3
Figure 20-5. Precipitation Attenuation Versus Wavelength20-4
Figure 20-6. Beam Resolution20-5
Figure 20-7. Beam Resolution Comparison Between WSR-88D and
Aircraft Weather Radar20-6
Figure 20-8. Normal Refraction20-7
Figure 20-9. Subrefraction20-8
Figure 20-10. Superrefraction20-8
Figure 20-11. Ducting20-9
Figure 20-12. Reflectivity Associated with Liquid Targets20-10
xvii Figure 21-1. Mean Worldwide Surface Pressure Distribution and Prevailing Winds
Throughout the World in July21-2
Figure 21-2. Mean Worldwide Surface Pressure Distribution and Prevailing Winds
Throughout the World in January21-2
Figure 21-3. A Shear Line and an Induced Trough Caused by a Polar High
Pushing into the Subtropics21-8
Figure 21-4. A TUTT Moves Eastward Across the Hawaiian Islands. Extensive
Cloudiness Develops East of the Trough21-9
Figure 21-5. A Northern Hemisphere Easterly Wave Progressing from A–B21-10
Figure 21-6. Vertical Cross Section along Line A–B in Figure 21-521-10
Figure 21-7. The Tracks of Nearly 150 Years of Tropical Cyclones and their
Strength Weave Across the Globe21-12
Figure 21-8. Radar Image of Hurricane Katrina Observed at New Orleans, Louisiana,
on August 29, 200521-14
Figure 21-9. Hurricane Andrew Observed by Satellite in 199221-14
Figure 22-1. The Arctic Circle22-1
List of Tables
Table 1-1. Composition of a Dry Earth’s Atmosphere1-1
Table 1-2. Selected Properties of the Standard Atmosphere1-6
Table 2-1. Celsius Temperature Conversion Formulae2-2
Table 2-2. Fahrenheit Temperature Conversion Formulae2-2
Table 2-3. Heat (Thermal) Conductivity of Various Substances2-7
Table 2-4. Specific Heat Capacity of Various Substances2-9
Table 3-1. Latent Heat of Water at 0 °C3-7
Table 5-1. Units of Pressure5-3
Table 5-2. Pressure System Symbols5-6
Table 5-3. Common Constant Pressure Charts5-9
Table 6-1. Common Isopleths6-2
Table 6-2. Common Weather Chart Symbols6-5
Table 11-1. Air Parcel Vertical Motion Characteristics11-4
Table 13-1. Cloud Forms13-1
Table 13-2. Approximate Height of Cloud Bases above the Surface13-2
Table 19-1. Enhanced Fujita Scale (Enhanced F Scale) for Tornado Damage19-11
Table 21-1. Wind Speed and Characteristic House Damage for the
Saffir-Simpson Hurricane Wind Scale21-15
1-1 CHAPTER 1. THE EARTH’S ATMOSPHERE
1.1 Introduction. The Earth’s atmosphere is a cloud of gas and suspended solids extending from the surface out many thousands of miles, becoming increasingly thinner with distance, but always held by the Earth’s gravitational pull. The atmosphere is made up of layers surrounding the Earth that holds the air we breathe, protects us from outer space, and holds moisture (e.g., vapor, clouds, and precipitation), gases, and tiny particles. In short, the atmosphere is the protective bubble we live in.
This chapter covers our atmosphere’s composition, vertical structure and the standard atmosphere.
1.2 Composition. The Earth’s atmosphere consists of numerous gases (see Table 1-1) with the top four making up 99.998 percent of all gases. Nitrogen, by far the most common, dilutes oxygen and prevents rapid burning at the Earth’s surface. Living things need it to make proteins. Oxygen is used by all living things and is essential for respiration.
Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket and prevents the escape of heat to outer space.
Table 1-1. Composition of a Dry Earth’s Atmosphere Gas Symbol Content (by Volume) Nitrogen N2 78.084% Oxygen O2 20.947% Argon Ar 0.934% Carbon Dioxide CO2 0.033% Neon Ne 18.20 parts per million Helium He 5.20 parts per million Methane CH4 1.75 parts per million Krypton Kr 1.10 parts per million Sulfur dioxide SO2 1.00 parts per million Hydrogen H2 0.50 parts per million Nitrous Oxide N2O 0.50 parts per million Xenon Xe 0.09 parts per million
1-2 Gas Symbol Content (by Volume) Ozone O3 0.07 parts per million Nitrogen dioxide NO2 0.02 parts per million Iodine I2 0.01 parts per million Carbon monoxide CO trace Ammonia NH3 trace Note: The atmosphere always contains some water vapor in amounts varying from trace to about four percent by volume. As water vapor content increases, the other gases decrease proportionately.
Weather, the state of the atmosphere at any given time and place, strongly influences our daily routine as well as our general life patterns. Virtually all of our activities are affected by weather, but, of all our endeavors, perhaps none more so than aviation.
1.2.1 Air Parcel. An air parcel is an imaginary volume of air to which any or all of the basic properties of atmospheric air may be assigned. A parcel is large enough to contain a very large number of molecules, but small enough so that the properties assigned to it are approximately uniform. It is not given precise numerical definition, but a cubic centimeter of air might fit well into most contexts where air parcels are discussed.
In meteorology, an air parcel is used as a tool to describe certain atmospheric processes, and we will refer to air parcels throughout this document.
1.3 Vertical Structure. The Earth’s atmosphere is subdivided into five concentric layers (see Figure 1-1) based on the vertical profile of average air temperature changes, chemical composition, movement, and density. Each of the five layers is topped by a pause, where the maximum changes in thermal characteristics, chemical composition, movement, and density occur.
1.3.1 Troposphere. The troposphere begins at the Earth’s surface and extends up to about 11 kilometers (36,000 feet) high. This is where we live. As the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height. As you climb higher, the temperature drops from about 15 °C (59 °F) to -56.5 °C (-70 °F). Almost all weather occurs in this region.
The vertical depth of the troposphere varies due to temperature variations which are closely associated with latitude and season. It decreases from the Equator to the poles, and is higher during summer than in winter. At the Equator, it is around 18-20 kilometers (11-12 miles) high, at 50° N and 50° S latitude, 9 kilometers (5.6 miles), and at the poles, 6 kilometers (3.7 miles) high. The transition boundary between the troposphere and the
1-3 layer above is called the tropopause. Both the tropopause and the troposphere are known as the lower atmosphere.
1.3.2 Stratosphere. The stratosphere extends from the tropopause up to 50 kilometers (31 miles) above the Earth’s surface. This layer holds 19 percent of the atmosphere’s gases, but very little water vapor.
Temperature increases with height as radiation is increasingly absorbed by oxygen molecules, leading to the formation of ozone. The temperature rises from an average -56.6 °C (-70 °F) at the tropopause to a maximum of about -3 °C (27 °F) at the stratopause due to this absorption of ultraviolet radiation. The increasing temperature also makes it a calm layer, with movements of the gases being slow.
Commercial aircraft often cruise in the lower stratosphere to avoid atmospheric turbulence and convection in the troposphere. Severe turbulence during the cruise phase of flight can be caused by the convective overshoot of thunderstorms from the troposphere below. The disadvantages of flying in the stratosphere can include increased fuel consumption due to warmer temperatures, increased levels of radiation, and increased concentration of ozone.
Figure 1-1. Vertical Structure of the Atmosphere Exosphere 10,000 km km mi 140 →87 690 km 130 Temperature change with height 81 120 - 75 Thermosphere 110 68 Thermosphere 100 62 Shuttle 90 56 Mesopause 80 50 70 Mesosphere 43 Aurora 60 37 85 km 50 Stratopause 31 Mesosphere Meteors 40 25 50 km 30 Stratosphere - 19 Weather Balloon 20 12 Tropopause 11 km 10 Troposphere -100 -80 -60 -40 Temperature 0 20 40 60 -20 Mount Everest The regions of the stratosphere and the mesosphere, along with the stratopause and mesopause, are called the middle atmosphere. The transition boundary that separates the stratosphere from the mesosphere is called the stratopause.
1.3.3 Mesosphere. The mesosphere extends from the stratopause to about 85 kilometers (53 miles) above the Earth. The gases, including the number of oxygen molecules, continue to become thinner and thinner with height. As such, the effect of the warming by ultraviolet radiation also becomes less and less pronounced, leading to a decrease in temperature with height. On average, temperature decreases from about -3 °C (27 °F) to as low as -100 °C (-148 °F) at the mesopause. However, the gases in the mesosphere 1-4
1-5 are thick enough to slow down meteorites hurtling into the atmosphere where they burn up, leaving fiery trails in the night sky.
1.3.4 Thermosphere. The thermosphere extends from the mesopause to 690 kilometers (430 miles) above the Earth. This layer is known as the upper atmosphere.
The gases of the thermosphere become increasingly thin compared to the mesosphere.
As such, only the higher energy ultraviolet and x ray radiation from the sun is absorbed.
But because of this absorption, the temperature increases with height and can reach as high as 2,000 °C (3,600 °F) near the top of this layer.
Despite the high temperature, this layer of the atmosphere would still feel very cold to our skin, because of the extremely thin air. The total amount of energy from the very few molecules in this layer is not sufficient enough to heat our skin.
1.3.5 Exosphere. The exosphere is the outermost layer of the atmosphere, and extends from the thermopause to 10,000 kilometers (6,200 miles) above the Earth. In this layer, atoms and molecules escape into space and satellites orbit the Earth. The transition boundary that separates the exosphere from the thermosphere is called the thermopause.
1.4 The Standard Atmosphere. Continuous fluctuations of atmospheric properties create problems for engineers and meteorologists who require a fixed standard for reference.
To solve this problem, they defined a standard atmosphere, which represents an average of conditions throughout the atmosphere for all latitudes, seasons, and altitudes.
Standard atmosphere is a hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc. (see Table 1-2 and Figure 1-2). Weather-related processes are generally referenced to the standard atmosphere, as are examples in this document.
Table 1-2. Selected Properties of the Standard Atmosphere Property Metric Units English Units Sea level pressure 1013.25 hectopascals 29.92 inches of mercury Sea level temperature 15 °C 59 °F Lapse rate of temperature in the 6.5 °C/1,000 meters 3.57 °F/1,000 feet troposphere Pressure altitude of the tropopause 11,000 meters 36,089 feet Temperature at the tropopause -56.5 °C -69.7 °F Note: 1 hectopascal = 1 millibar.
Figure 1-2. U.S. Standard Atmosphere within the Troposphere FEET 200 -39,000 TROPOPAUSE 11
- 35,000 10- 300 9- - 30,000 ALTITUDE (U.S. STANDARD ATMOSPHERE) 8- PRESSURE (MILLIBARS) - 25,000 400 7 -
- 20,000 500 5—
- 15,000 600 700 3 - 10,000 800 2 5,000 900 1'050 .000 0- 0 -60 -50 -40 -30 -20 10 20 30 40°C -60 -40 -20 20 40 60 80 100 OF
TEMPERATURE 1-6
2-1 CHAPTER 2. HEAT AND TEMPERATURE
2.1 Introduction. Temperature is one of the most basic variables used to describe the state of the atmosphere. We know that air temperature varies with time from one season to the next, between day and night, and even from one hour to the next. Air temperature also varies from one location to another, from high altitudes and latitudes to low altitudes and latitudes. Temperature can be critical to some flight operations. As a foundation for the study of temperature effects on aviation and weather, this chapter describes temperature, temperature measurement, and heat transfer and imbalances.
2.2 Matter. Matter is the substance of which all physical objects are composed. Matter is composed of atoms and molecules, both of which occupy space and have mass.
The Earth’s gravity acting on the mass of matter produces weight.
2.3 Energy. Energy is the ability to do work. It can exist in many forms and can be converted from one form to another. For example, if a ball is located at the edge of a slide, it contains some amount of potential energy (energy of position). This potential energy is converted to kinetic energy (energy of motion) when the ball rolls down the slide.
Atoms and molecules produce kinetic energy because they are in constant motion. Higher speeds of motion indicate higher levels of kinetic energy.
2.4 Heat. Heat is the total kinetic energy of the atoms and molecules composing a substance.
The atoms and molecules in a substance do not all move at the same velocity. Thus, there is actually a range of kinetic energy among the atoms and molecules.
2.5 Temperature. Temperature is a numerical value representing the average kinetic energy of the atoms and molecules within matter. Temperature depends directly on the energy of molecular motion. Higher (warmer) temperatures indicate a higher average kinetic energy of molecular motion due to faster molecular speeds. Lower (colder) temperatures indicate a lower average kinetic energy of molecular motion due to slower molecular speeds. Temperature is an indicator of the internal energy of air.
2.5.1 Temperature Measurement. A thermometer is an instrument used to measure temperature.
Higher temperatures correspond to higher molecular energies, while lower temperatures correspond to lower molecular energies.
2.5.2 Temperature Scales. Many scientists use the Kelvin (K) scale, which is a thermodynamic (absolute) temperature scale, where absolute zero, the theoretical absence of all thermal energy, is zero Kelvin (0 K). Thus, the Kelvin scale is a direct measure of the average kinetic molecular activity. Because nothing can be colder than absolute zero, the Kelvin scale contains no negative numbers.
The Celsius (°C) scale is the most commonly used temperature scale worldwide and in meteorology. The scale is approximately based on the freezing point (0 °C) and boiling point of water (100 °C) under a pressure of one standard atmosphere (approximately sea level). Each degree on the Celsius scale is exactly the same size as a degree on the Kelvin scale.
2-2 Table 2-1. Celsius Temperature Conversion Formulae From Celsius To Celsius Fahrenheit [°F] = ([°C] x 9/5) + 32 [°C] = ([°F] – 32) x 5/9 Kelvin [K] = [°C] + 273.15 [°C] = [K] – 273.15 For temperature intervals rather than specific temperatures, 1 °C = 274.15 K and 1 °C = 33.8 °F The United States uses Fahrenheit (°F) scale for everyday temperature measurements.
In this scale, the freezing point of water is 32 degrees Fahrenheit (32 °F) and the boiling point is 212 degrees Fahrenheit (212 °F).
Table 2-2. Fahrenheit Temperature Conversion Formulae From Fahrenheit To Fahrenheit Celsius [°C] = ([°F] – 32) x 5/9 [°F] = ([°C] x 9/5) + 32 Kelvin [K] = ([°F] + 459.67) x 5/9 [°F] = ([K] x 9/5) – 459.67 For temperature intervals rather than specific temperatures, 1 °F = 255.93 K and 1 °F = -17.22 °C
2-3 Figure 2-1. Comparison of Kelvin, Celsius, and Fahrenheit Temperature Scales
A thermometer changes readings due to the addition or subtraction of heat. Heat and temperature are not the same, but they are related.
2.6 Heat Transfer. Heat transfer is energy transfer as a consequence of temperature difference. When a physical body (e.g., an object or fluid) is at a different temperature than its surroundings or another body, transfer of thermal energy, also known as heat transfer (or heat exchange) occurs in such a way that the body and the surroundings reach thermal equilibrium (balance). Heat transfer always occurs from a hot body to a cold body. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed down.
The heat source for the surface of our planet is the sun. Energy from the sun is transferred through space and through the Earth’s atmosphere to the Earth’s surface. Since this energy warms the surface and atmosphere, some of it becomes heat energy. There are three ways heat is transferred into and through the atmosphere: radiation, conduction, convection, or any combination of these. Heat transfer associated with the heat change of water from one phase to another (i.e., liquid water releases heat when changed to a vapor, liquid water absorbs heat when it changes to ice) can be fundamentally treated as a variation of convective heat transfer. The heat transfer associated with water will be discussed later.
2.6.1 Radiation. If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this type of heat transfer. Heat lamps that keep food warm work in the same way.
2-4 Radiation is the transfer of heat energy through space by electromagnetic radiation. These electromagnetic waves travel at the speed of light and are usually described in terms of wavelength or frequency. Frequencies range from gamma rays on the high end to radio waves on the low end. Also contained in the spectrum are x ray, ultraviolet, visible, infrared, and microwave.
Figure 2-2. Radiation Example
All objects emit (radiate) energy as the heat energy within the object is converted to radiation energy. This transmitted radiation passes through entities such as air, water, or space. Along the way, the radiation can be reflected, which occurs when the wave energy changes direction when encountering an object. Eventually, the radiation is absorbed and the electromagnetic wave energy is converted to heat energy by the absorbing object. The emitting object loses heat energy, and the absorbing object gains heat energy during this process.
2.6.1.1 Solar and Terrestrial Radiation. All objects emit radiation energy, including the sun (solar radiation) and the Earth (terrestrial radiation). An object’s wavelength of maximum radiation is inversely related to its temperature; the hotter (colder) the object, the shorter (longer) the wavelength. The sun’s wavelength of maximum radiation is relatively short and is centered in the visible spectrum. The Earth’s wavelength of maximum radiation is relatively long and is centered in the infrared spectrum.