Chapter 4, The Earth’s Atmosphere 4-1
4 The Earth’s Atmosphere
4.1 Introduction
The Earth ’s atmosphere is a cloud of gas and suspended solids extending from the surface out many
thousands of miles, becoming 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 that people 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 that people live in.
This chapter covers the atmosphere’s composition and vertical structure and the standard atmosphere.
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4.2 Composition
The Earth ’s atmosphere consists of numerous gases (see Table 4-1) with nitrogen, oxygen, argon, and
carbon dioxide 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 4-1. Approximations of the Composition of a Dry Earth’s Atmosphere
Gas Symbol Content (by Volume)
Nitrogen N2 78.081%
Oxygen O2 20.945%
Argon Ar 0.932%
Carbon Dioxide CO2 0.042%
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
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 4 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 daily routine as well
as general life patterns. Virtually all of our activities are affected by weather, but, of all of our endeavors,
perhaps none more so than aviation.
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4.2.1 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, an d air parcels will
be referred to in this handbook.
4.3 Vertical Structure
The Earth’s atmosphere is subdivided into five concentric layers (see Figure 4-1) 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.
4.3.1 Troposphere
The troposphere begins at the Earth’s surface and extends up to about 11 kilometers (km) (36,000 ft) high.
This is where people live. As the gases in this layer decrease with height, the air becomes thinner. Therefore,
the temperature in the troposphere also decreases with height. Climbing higher, the temperature drops from
about 15 degrees Celsius (°C) [59 degrees Fahrenheit (°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 that 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 km [11–12 miles (mi)] high; at 50° N and 50° S latitude, 9 km (5.6 mi)
high; and at the poles, 6 km (3.7 mi) high.
The lowest portion of the troposphere is known as the planetary boundary layer. The height of the boundary
layer varies depending on terrain and time of day and is directly affected by surface heating and cooling. It
has an important role in transporting heat and moisture into the atmosphere.
The transition boundary between the troposphere and the layer above is called the tropopause. Both the
tropopause and the troposphere are known as the lower atmosphere.
4.3.2 Stratosphere
The stratosphere extends from the tropopause up to 50 km (31 mi) 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 tropospher e 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.
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4.3.3 Mesosphere
The mesosphere extends from the stratopause to about 85 km (53 mi) 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 are thick enough to slow down meteorites
hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky.
4.3.4 Thermosphere
The thermosphere extends from the mesopause to 690 km (430 mi) 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.
4.3.5 Exosphere
The exosphere is the outermost layer of the atmosphere and extends from the thermopause to 10,000 km
(6,200 mi) 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.
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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. Please note that in this figure the temperature is not necessarily to scale with the
height but provides an approximation of how the temperature changes with height.
Figure 4-1. Vertical Structure of the Atmosphere
4.4 The Standard Atmosphere
Continuous fluctuations of atmospheric properties create problems for engineers and meteorologists who
need a fixed standard for reference. To solve this problem, they defined a standard atmosphere that
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 perfor mance calculations, aircraft and missile design, ballistic
tables, etc. (see Table 4-2 and Figure 4-2). Weather -related processes are generally referenced to the
standard atmosphere, as are the examples in this handbook.
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Table 4-2. Selected Properties of the Standard Atmosphere
Property Metric Units English Units
Sea level pressure 1013.25 hectopascals (hPa) 29.92 inches of
mercury (inHg)
Sea level temperature 15 °C 59 °F
Lapse rate of temperature in the
troposphere
6.5 °C/1,000 m 3.57 °F/1,000 ft
Pressure altitude of the tropopause 11,000 m 36,089 ft
Temperature at the tropopause -56.5 °C -69.7 °F
Note: 1 hectopascal = 1 millibar.
Figure 4-2. U.S. Standard Atmosphere Within the Troposphere