CHAPTER 14: THE ATMOSPHERE
SUMMARY
The chapter begins by describing the origin of the atmosphere. You are reminded that
primordial Earth was molten and that the atmosphere developed largely through out-gassing. The
role of the biosphere in shaping atmospheric composition is discussed in the context of presentday atmospheric composition. You are then introduced to the common theme of energy as it
relates to the thermal structure used to define layers of the atmosphere. The troposphere is
defined by its decreasing thermal gradient, discussed in the context of its importance to the
biosphere. The protective role of ozone in the stratosphere is mentioned along with the reason for
the positive temperature gradient found there. The mesosphere and thermosphere are briefly
discussed in the context of their temperature profiles and roles in protecting Earth from meteors
and high-energy charged particles from the sun.
The roles of incoming radiation and energy transfer among states of water are discussed
as major drivers of atmospheric dynamics. You are introduced to the electromagnetic spectrum
and interactions of radiation with matter. This is done in the context of how that energy interacts
in the atmosphere. Since water plays another key role in regulating temperatures, students are
reminded about the hydrologic cycle. Energy transfer among the states of water is used as an
introduction to humidity. Absolute and relative humidity are defined and applied, as are the
consequences of changing air temperatures. The closely related topic of elevation-induced
adiabatic temperature change is also described. These concepts are united as the student learns
about condensation, clouds and cloud formation, and air lifting mechanisms. Lastly, you will
focus on how lateral pressure changes influence winds and the consequences of the Coriolis
effect on air movement.
14.2: The Air Evolves
The atmosphere consists of the air around us which protects us against many potential
hazards from space. The lower boundary of the atmosphere touches Earth's surface and its upper
boundary is a gradational transition into space. Two common gases, nitrogen and oxygen, make
up about 99% of dry air. How did our atmosphere evolve? Read on! (pages 381-383).
14.3: Structure and Processes of the Atmosphere
The atmosphere is comprised of 4 layers: troposphere, stratosphere, mesosphere and
thermosphere. The density of air rapidly decreases as you travel upward from Earth's surface.
See Figure 14.5 for a diagram of the structure of the atmosphere. We live in the troposphere, the
lowest atmospheric layer where air is densist at the Earth's surface, and temperature decreases
with altitude. The air temperature decreases with altitude because the air is warmed by the
Earth's surface (the farther from the surface the less warming of the air). Also the air is thinner
at higher altitudes so retains less heat.
The stratosphere is immediately above the troposphere and is characterized by an
increase in temperature with altitude (this is a stable configuration with colder, denser air
overlain by warmer, lighter air)...note that the troposphere is not a stable configuration (warm air
below colder air) which helps drive our weather systems. Temperatures in the stratosphere
increase upwards because ozone molecules absorb ultraviolet radiation heating the stratosphere
and blocking harmful radiation from reaching Earth's surface.
14.4: Solar Radiation and the Atmosphere.
Our Sun emits electromagnetic radiation. Visible light is the light we see and its colors
are shown in a rainbow (see Figure 14.8).
The Greenhouse Effect is an important concept and process that keeps the Earth's surface
at a livable temperature. Air temperature decreases steadily as distance increases from the warm
Earth's surface (mentioned above), partly because of Earth's heating and partly because the air is
thinner (think about when you drive up into the mountains). Air temperature declines at a rate of
6.5 degrees C/km (12 degrees F) through the troposphere (called the normal lapse rate). The
average temperature at the Earth's surface is 59 degrees F, but it would be much colder and
unlivable without the greenhouse effect.
The greenhouse effect works like this: the Earth's surface warms as it absorbs the Sun's
radiation and re-emits this heat in the form of longwave infrared radiation. Albedo is the
measure of reflectivity of a surface (low-albedo dark surfaces absorb and emit more energy as
heat in the form of long-wavelength infrared radiation than high albedo surfaces such as snowcovered land). Think about an asphalt parking lot that is still warm long after the Sun goes
down. This infrared radiation is absorbed by greenhouse gases such as water vapor, carbon
dioxide and other trace gases (such as methane and nitrous oxide) in the troposphere, creating the
greenhouse effect (keeping the troposphere warm!). What does this have to do with global
warming?? As mentioned above our average temperature on Earth's surface is 59 degrees F, this
is influenced by many factors including the amount of greenhouse gases in the atmosphere
today. One major concern is the burning of fossil fuels which puts more carbon dioxide into the
atmosphere (which can potentially trap more heat) and could raise the overall temperature of the
Earth's surface.
See pages 388-389 for a better understanding of the Earth's albedo.
14.5: The Role of Water in the Atmosphere.
The amount of heat absorbed or released as water changes state is termed latent heat.
See Figure 14.13 on page 391 for a diagram showing how energy is exchanged (lost or absorbed)
when water changes state. Most significant is the heat lost or absorbed during evaporation and
condensation; these processes occur over large areas on Earth's surface and contribute to weather
phenomena and redistribution of heat in the atmosphere. Heat is absorbed during evaporation
(water changing to water vapor) and released during condensation (water vapor changing to
water). This latent heat of condensation provides the power for severe weather.
Checkpoint 14.13: Your body feels cooler when you step out of a warm shower because:
a) water evaporates on your skin
b) water condenses on your skin
c) water evaporates from the surrounding air
d) water condenses in the surrounding air
Humidity: read and understand the difference between absolute and relative humidity
(page 392).
Checkpoint 14.14: Measurements reveal that a cubic meter of air at 12 degrees C actually
holds 6 grams of water. What happens if the temperature of the air increases? Be able to explain
your answer.
a) Absolute and relative humidity both increase
b) Absolute humidity increases and relative humidity remains constant
c) Absolute and relative humidity both decrease
d) Absolute humidity remains constant and relative humidity decreases
When answering the question above it is helpful to look at the graph (Figure 14.15). Plot
a point on the graph that represents 6 grams of water (absolute humidity on vertical axis) and 12
degrees C (horizontal axis). The question reads "what happens if the temperature of the air
increases". Beginning at 12 degrees C see what happens to the relative humidity when you
increase the temperature (does it approach the saturation curve or move away from this curve?).
This should be enough of a hint for this question!
14.6: Air Pressure, Condensation and Precipitation
Read this section carefully, it can be a little confusing with difficult concepts to
understand. Some highlights: The very upper boundary of our atmosphere extends up to 312
miles above the Earth's surface, and the pressure exerted by the weight of this overlying air
column is called atmospheric pressure. This air pressure is influenced by air density. Gravity
pulls most gases close to the Earth's surface, so air density is greater near the Earth's surface in
the troposphere and decreases in the outer layers of the atmosphere. Consequently, 50% of all
air lies below 3 miles altitude!! Think about climbers on Mt Everest where they are climbing to
nearly 30,000 feet above sea level (5-6 miles in elevation)...most of them need oxygen during
their climb to the summit!
Air density changes as air is warmed or cooled. Air contracts when cooled, increasing air
density and generating higher air pressures. Air expands when it is warmed, decreasing the air
pressure. The highest air pressures are recorded in cold regions and lowest in tropical warm
environments (where hurricanes and typhoons form).
Air pressure not only varies with elevation but also with weather systems. Air pressure is
measured with a device called a barometer.
Effects of Air Pressure on Temperature: Compressed air becomes warmer (think about
pumping up your bicycle tire, your tire feels warm); expanding air becomes cooler (think about
letting air out of your bicycle tire, it feels cooler than surrounding air). Pressure changes in
nature result in similar temperature changes and these changes are called adiabatic changes
because they occur without any loss or gain of energy to or from the surrounding air. Why are
these changes in temperature associated with changes in pressure important? These adiabatic
changes help fuel the processes that cause air to rise and promote the development of extreme
weather phenomena such as thunderstorms.
Recall that the normal lapse rate (discussed above) is about 6.5 degrees C/km. This is the
temperature of air that is not rising or falling (it is the amount of cooling you would
experience, for example, when driving up a mountain range where temperature would drop about
6.5 degrees for every km you rise in elevation). A rising parcel of air (this is a volume of air that
can behave adiabatically and independently of the surrounding air), however, cools for two
reasons: it is expanding and cooling because of decreasing air pressure (adiabatic cooling) and it
is moving farther away from the warmth of the Earth's surface. Rising air cools at a faster rate
than the normal lapse rate; this rising air cools at the dry adiabatic lapse rate of 10 degrees C/km
(this is the dry rate for unsaturated air). If, while the air is rising it reaches the dew point and
becomes saturated (and condensation occurs), then latent heat is released (recall condensation
releases heat) and reduces the cooling rate to 6 degrees C/km (the wet adiabatic lapse rate). The
parcel of air will continue to rise until its temperature (and related density) is the same as that of
the surrounding air (as long as it is warmer than the surrounding air, it will continue to rise).
Also, the opposite occurs for sinking air which is compressed and warms at the dry
adiabatic lapse rate.
A parcel of warm air with an average temperature of 29 degrees C is present at sea level.
The surrounding air has a temperature of 18.5 degrees C. The parcel of warm air begins to rise
due to density lifting. Assume that the relative humidity of the air mass steadily increases but
never reaches 100 percent. Analyze how the temperature of the air mass would change as it
rises, and determine at what elevation it would become stable.
The above discussion should help you with analyzing the changes to this air mass...when
analyzing the air mass, recall it cools at the dry adiabatic lapse rate until it reaches 100% relative
humidity (then is cools at the wet adiabatic lapse rate). Also, recall what the normal lapse rate is
for surrounding air, and that the parcel will continue to rise through the surrounding air as long
as its temperature is higher than the surrounding air.
Checkpoint 14.22: on July 2, 1982, truck driver Larry Walters decided to attach 45
helium-filled weather balloons to a lawn chair and go for a ride. Lawn Chair Larry rose to an
altitude of nearly 5 km (16,000 feet). The high elevation and lack of oxygen made him dizzy, so
he decided it was time to deploy his principal altitude control device, a pellet gun. He shot out
several balloons and descended back to Earth. Which process was the most significant in Lawn
Chair Larry's balloon ride? Be able to explain your reasoning.
a) Density lifting
b) Orographic lifting
c) Convergence lifting
d) Frontal lifting
A hint for the above question, helium is lighter than the composition of normal air.
In the following landscape, how would the amount of rainfall change at location X if the
mountain eroded down to the dashed line?
a) Rainfall would increase
b) Rainfall would decrease
c) Rainfall would stay the same
14.8: Winds
Wind is the horizontal movement of air that occurs from differences in air pressure
(above were descriptions of vertical air movement). Wind flows from areas of high pressure to
areas of low pressure. Pressure gradient is the magnitude of the change in pressure between two
points, divided by the distance between those points. The greater the contrast in pressure, the
steeper the gradient will be and the faster the wind will blow. Isobars on a weather map join
points of equal pressure; since they denote differences in pressure (from one isobar to another),
the closer the isobars are together, the greater the drop in pressure with distance and the faster the
wind speed. See Figure 14.27 on page 403. Read the caption and observe, for example, that (a.)
and (c.) cover approximately the same distance on the map, but the isobars are closer together
(and accomodate a greater pressure change) in (c.) so the wind is blowing faster from the high
pressure toward the low pressure system (the pressure gradient is steeper in (c.)).
Cyclones and Anticyclones: A cyclone is another name for a hurricane and typhoon. In
the northern hemisphere a cyclone spins counterclockwise as winds converge and are deflected
by the coriolis effect (to the right in the northern hemisphere), creating this counterclockwise
airflow around a low-pressure center. In contrast, winds blowing outward, away from a highpressure center, will be deflected by the coriolis effect to produce a clockwise flow of air known
as an anticyclone. Cyclones are usually associated with severe weather systems such as
hurricanes and tornadoes, while anticyclones (high-pressure systems) are associated with clear
skies and dry weather. Why? Read on it, and keep in mind that rising air in a low-pressure
system cools and can't hold as much moisture (so relative humidity increases and it can reach
saturation); when saturation occurs water vapor condenses to liquid water and releases latent
heat, the power for storms. The opposite occurs in high-pressure systems where sinking
air contracts and heats up and can hold more moisture (the relative humidity decreases) resulting
in clear skies and calmer, drier weather.
LEARNING OBJECTIVES
1. Students will explain concepts related to the atmosphere.
2. Students will describe the makeup of the atmosphere and its layers.
3. Students will explain the connections between solar radiation and the atmosphere.
4. Students will identify how humidity measurements will differ as atmospheric conditions
change.
5. Students will relate air pressure to air temperature and air density.
6. Students will explain what happens to rising air and analyze how adiabatic temperature
changes occur.
7. Students will describe the process of cloud formation and recognize some basic cloud types.
8. Students will describe the causes of winds, cyclones, and anticyclones.
9. Students will interpret a weather map to determine wind directions and relative wind speeds.