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.
2-5 Figure 2-3. Temperature’s Effect on Radiation Wavelength
Some of the solar radiation that reaches the Earth’s surface is radiated back into the atmosphere to become heat energy. Dark-colored objects such as asphalt absorb more of the radiant energy and warm faster than light-colored objects. Dark objects also radiate their energy faster than light-colored objects.
2.6.1.2 Solar Zenith Angle. The intensity of incoming solar radiation that strikes the Earth’s surface (insolation) varies with solar zenith angle. Solar zenith angle is the angle measured from the Earth’s surface between the sun and the zenith
(i.e., directly overhead). Solar zenith angle varies with latitude, season, and the diurnal cycle (sunrise/sunset).
Figure 2-4 illustrates the concept. Insolation is maximized when the solar zenith angle is zero degrees (0°), which means the sun is directly overhead.
With increasing solar zenith angle, the insolation is spread over an increasingly larger surface area (y is greater than x) so that the insolation becomes less intense. Also, with increasing solar zenith angle, the sun’s rays must pass through more of the Earth’s atmosphere, where they can be scattered and absorbed before reaching the Earth’s surface. Thus, the sun can heat the surface to a much higher temperature when it is high in the sky, rather than low on the horizon.
2-6 Figure 2-4. Solar Zenith Angle
2.6.2 Conduction. Conduction is the transfer of energy (including heat) by molecular activity from one substance to another in contact, or through, a substance. Heat always flows from the warmer substance to the colder substance. The rate of heat transfer is greater with larger temperature differences and depends directly on the ability of the substance(s) to conduct heat. During conduction, the warmer substance cools and loses heat energy, while the cooler substance warms and gains heat energy.
Heat (thermal) conductivity is the property of a substance that indicates its ability to conduct heat as a consequence of molecular motion. Units are Watts per meter-Kelvin (W m-1 K-1). Table 2-3 below provides the heat (thermal) conductivity of various substances. Note that air is a poor thermal conductor.
2-7 Table 2-3. Heat (Thermal) Conductivity of Various Substances Material Phase Heat (Thermal) Conductivity (W m-1 K-1) Silver Solid 429 Copper Solid 401 Aluminum Solid 250 Iron Solid 80 Sand (saturated) Solid 2.7 Water (ice) Solid (0 °C) 2.18 Sandstone Solid 1.7 Limestone Solid 1.26 – 1.33 Glass Solid 1.05 Water (liquid) Liquid 0.58 Sand (dry) Solid 0.35 Soil Solid 0.17 – 1.13 Wood (oak) Solid 0.17 Wood (balsa) Solid 0.055 Snow Solid (<0 °C) 0.05 – 0.25 Air Gas 0.024 Water (steam) Gas (125 °C) 0.016 All measurements are at 25 °C unless otherwise noted.
Note: 1 K equals -272.15 °C.
2.6.3 Convection. Convection is the transport of heat within a fluid, such as air or water, via motions of the fluid itself. This type of heat flow takes place in liquids and gases because they can move freely, and it is possible to set up currents within them. Water boiling in a pot is an example of convection. Because air is a poor thermal conductor, convection
2-8 plays a vital role in the Earth’s atmospheric heat transfer process. Figure 2-5 illustrates examples of various heat transfer processes.
Figure 2-5. Heat Transfer Examples1
2.7 Thermal Response. Whether by radiation, conduction, convection, or a combination of these, the temperature response to the input (or output) of some specified quantity of heat varies from one substance to another. Specific heat capacity, also known simply as specific heat, is defined as the measure of heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval. Specific heat capacity is typically expressed in units of joules per gram-Kelvin (J g-1 K-1). Thus, two different substances with identical temperature measurements do not necessarily possess the same amount of heat energy. When exposed to the same amount of heat energy, a substance with a low specific heat capacity warms up more than a substance with a higher specific heat capacity.
1 The source of this and other material labeled COMET® is the Web site at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2015 University Corporation for Atmospheric Research. All Rights Reserved.