Chapter 5, Heat and Temperature 5-1
5 Heat and Temperature
5.1 Introduction
Temperature is one of the most basic variables used to describe the state of the atmosphere. 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. For additional information on how temperature may affect flight
performance, please see Chapter 8.
Chapter 5, Heat and Temperature 5-2
5.2 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.
5.3 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.
5.4 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.
5.5 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.
5.5.1 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.
5.5.2 Temperature Scales
Many scientists use the Kelvin (K) scale, a thermodynamic (absolute) temperature scale, where absolute
zero, the theoretical absence of all thermal energy, is 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 (100 °C) of water 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.
In the early 1990s, the U nited States aligned with ICAO standards by moving to the metric system for
aviation weather reports. While some websites and app lications provide temperature from the METAR in
degrees Fahrenheit, these are done by the conversion software, as the temperature in the METAR is strictly
reported in degrees Celsius. The United States uses the Fahrenheit (°F) scale for everyday temperature
measurements for non-aviation purposes. In this scale, the freezing point of wat er is 32 °F and the boiling
point is 212 °F.
See Table 5-1 and Table 5-2 for conversion information between temperature scales.
Chapter 5, Heat and Temperature 5-3
Table 5-1. Celsius Temperature Conversion Formulae
From Celsius To Celsius
Fahrenheit [°F] = ([°C] × 9/5) + 32 [°C] = ([°F] – 32) × 5/9
Kelvin [K] = [°C] + 273.15 [°C] = [K] – 273.15
For temperature intervals rather than specific temperatures:
±1 °C = ±1 K = ±1.8 °F
Table 5-2. Fahrenheit Temperature Conversion Formulae
From Fahrenheit To Fahrenheit
Celsius [°C] = ([°F] – 32) × 5/9 [°F] = ([°C] × 9/5) + 32
Kelvin [K] = ([°F] + 459.67) × 5/9 [°F] = ([K] × 9/5) – 459.67
For temperature intervals rather than specific temperatures:
±1 °F = ±.56 °C = ±.56 K
A thermometer changes readings due to the addition or subtraction of heat. Heat and temperature are not
the same, but they are related.
Figure 5-1 gives a comparison of Kelvin, Celsius, and Fahrenheit temperature scales.
Figure 5-1. Comparison of Kelvin, Celsius, and Fahrenheit Temperature Scales
Chapter 5, Heat and Temperature 5-4
5.6 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 he at 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 betwe en them
can never be stopped; it can only be slowed down.
The heat source for the surface of Earth 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 associate d with the he at
change of water from one phase to another ( i.e., liquid wate r absorbs heat when changed to a vapor and
liquid water releases 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 in Chapter 6, Water Vapor.
5.6.1 Radiation
If a person has ever stood in front of a fireplace or near a campfire, then they have felt the heat transfer
known as radiation (see Figure 5-2). The side of the body nearest the fire warms, while the other side
remains unaffected by the heat. Although people 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.
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 5-2. 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.
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5.6.1.1 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.
Figure 5-3. 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.
5.6.1.2 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 5-4 illustrates this concept. Insolation is maximized when the solar zenith angle is zero degrees (0°),
i.e., 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.
Chapter 5, Heat and Temperature 5-6
Figure 5-4. Solar Zenith Angle
5.6.2 Conduction
Conduction is the transfer of energy (including heat) by molecular activity from one substance to another
in contact with 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 conductio n, 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 5-3 provides the
heat (thermal) conductivity of various substances. Note that air is a poor thermal conductor.
Chapter 5, Heat and Temperature 5-7
Table 5-3. 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.
5.6.3 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 plays a vital role in the Earth ’s atmospheric heat transfer process. Figure 5-5
illustrates examples of various heat transfer processes.