The Science of Flight
Chapter 1-1
The Atmosphere
Overview
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What is the Atmosphere
Describe the Atmosphere
Roles of Water and Particulate Matter
Atmosphere in Motion
What is the Atmosphere
• An ocean of air that surrounds the Earth
• There is not distinct line between the
atmosphere and space
• There is a gradual increase in the distance
between the molecules that make up the
atmosphere
Describing the Atmosphere
• A complex arrangement of molecules and
atoms
• There are three major ways to look at
atmospheric composition:
– Atmospheric elements
– Atmospheric regions
– Atmospheric pressure
Atmospheric Elements
• The Earth’s atmosphere is a mixture of
gases, suspended solids,
and liquids
– 78% Nitrogen
– 21% Oxygen
– 1% Other gases
• The atmosphere absorbs energy, recycles
water, and works with electrical and
magnetic forces to provide a moderate
temperature
Atmospheric Regions
• The atmosphere can be divided into four
layers based on temperature:
– Troposphere and Tropopause: the region in
which most of us live work and play
Atmospheric Regions
• Troposphere/Tropopause: Contains 78%
of the gases of the atmosphere
– Zone were clouds and rain occur
– Temperatures decrease as height increases
– The Tropopause acts as a ceiling to the
weather zone; contains the Jet Stream
Atmospheric Regions
• The Stratosphere and Stratopause:
– Contains a layer of ozone
– The ozone keeps most of the UV radiation
from the Sun from reaching the surface
Atmospheric Regions
• The Mesophere and Mesopause:
– The coldest zone of the atmosphere
– Extends 50 miles high past the Stratopause
– Temperatures decrease to near -90 C
Atmospheric Regions
• The Thermosphere and Thermopause:
– Extends from 80-600K above the Earth
– Includes the ionosphere and exosphere
– Temps increase rapidly
Atmospheric Regions
• The Thermosphere and Thermopause:
– The ionosphere occurs at about 80K and is
electrically charged
– Charged particles from the Sun are
concentrated into radiation layers in the
exosphere; called the Van Allen Belt
Atmospheric Pressure
• At high altitudes, the atoms and molecules
are very far apart
• The force of air, or air pressure, is
measured with a barometer
• Atmospheric pressure is defined
as “the downward pressure
exerted by the weight of the
overlying atmosphere.”
Atmospheric Pressure
• Changes in atmospheric pressure play a
significant role in weather
– Low pressure regions experience more
stormy weather
– High pressure regions are more likely to be
associate with clear skies, lower humidity, and
more stable conditions
– Winds blow because of differences in air
pressure
Roles of Water and Particulate
Matter
• Water
– Mostly restricted to the Troposphere
– Goes through a cycle from vapor to
condensation to precipitation
Roles of Water and Particulate
Matter
• Water
– If there is a lot of water vapor in the air we say
it is humid
– The amount of water vapor that can exist in
the air increases when the temperature rises
Roles of Water and Particulate
Matter
• Evaporation:
– The air is constantly gaining and losing water;
water gets into the air through evaporation
– A simple example is water boiling and
becoming vapor
– Evaporation on a global scale happens in a
similar manner; a process called solar
radiation
Roles of Water and Particulate
Matter
• Humidity:
– The term that indicates the amount of water
vapor in the air
– The actual water vapor in the air at a given
time is referred to as absolute humidity
– Relative humidity is the amount of water
vapor that can still enter the air mass before it
becomes saturated
– The ratio of absolute humidity to the
saturation point equals the relative humidity
Roles of Water and Particulate
Matter
• Condensation and Precipitation:
– When part of the water vapor in the air returns
to a liquid or solid form
– Happens due to lower temperatures
– Lower temperatures cause the saturation
point to change
Roles of Water and Particulate
Matter
• Dew Point Temperature:
– The key factor in condensation and
precipitation
– The temperature at or below which water
vapor will condense
Roles of Water and Particulate
Matter
• Particulate Matter:
– Dust and other very small particles play an
important role in the water cycle
– The serve as condensation nuclei
– Very small; two millionths of an inch
Atmosphere in Motion
Atmosphere in Motion
• Radiation:
– The heat energy of the Sun reaches the Earth
as radiation or solar energy. Radiation
transfers heat by means of heat waves.
– Radiation that reaches the Earth is absorbed
by land and water surfaces.
Surface features
influence the amount of
radiation absorbed by
the Earth.
Atmosphere in Motion
• Conduction
– The passage of energy through something,
particularly heat and electricity.
– Heated molecules move more rapidly than
cold molecules. Heat is transferred from fast
moving molecules to slow moving molecules
until all are moving at the same
speed.
– A good example of conduction
is a stove heating a pan.
Atmosphere in Motion
• Convection:
– The most efficient method of heating the
atmosphere. Air is first heated by radiation
and conduction.
– The air absorbs the heat
energy. Warm air is forced
upward
as cold air flows in
displacing the
warm air.
Atmosphere in Motion
• Convection
– Convection currents cause a constant
exchange of cold air for warm air until heat is
distributed evenly.
– Convection also determines the movement of
large air masses above the Earth, the action
of the winds, rainfall, ocean currents, and the
transfer of heat from the interiors of the Sun to
its surface.
Atmosphere in Motion
• Advection
– The horizontal transfer of a property such as
heat, caused by air movement.
– When the wind blows, it is simply movement
by or within the local air mass. Advection is
an important factor in the global circulation of
air.
Atmosphere in Motion
• Insolation:
– The rate at which the Earth’s surface
is heated by solar radiation.
– The amount received at any point on
the Earth’s surface is dependent on
the angle that the Sun’s rays make
with the horizon, the distance of the
Earth from the Sun, and the amount
of radiation absorbed by the
atmosphere.
– Greater in the equatorial zone than
anywhere else on the Earth’s surface
due to the angle of incidence.
Atmosphere in Motion
• Heat Balance:
– If there was no balance of heat among the
Earth, its atmosphere, and space, the Earth
would become increasingly warmer.
– Of all the solar radiation arriving at the top of
the atmosphere, 42 percent is reflected into
space by clouds and atmospheric dust; 15
percent is absorbed directly into the
atmosphere; and 43 percent reaches the
Earth directly.
Atmosphere in Motion
• Heat Balance:
– Of the 15 percent absorbed directly into the
atmosphere, 4 percent eventually reaches the
Earth as diffused sky radiation. Thus, a total
of about 47 percent of the incident solar
radiation finally reaches the Earth and heats
it.
– The heating process that tends to maintain
the Earth's heat balance is also primarily
responsible for worldwide weather.
Atmosphere in Motion
• Wind:
– When air is heated, it rises. This occurs
because the heat applied to it has decreased
its density to the point where it is lighter in
weight than the surrounding air. The
surrounding cooler air pushes the lighter,
heated air upward.
Atmosphere in Motion
• Wind:
– When the heated air rises, cooler, higher
pressure air flows laterally to fill the lower
pressure area created. This lateral movement
is referred to as wind.
– Other factors that affect the circulation of the
air are:
• Gravity
• Friction
• Centrifugal force
Atmosphere in Motion
• Coriolis Effect:
– The Earth rotates on its axis in such a way
that an observer in space over the North Pole
would see the Earth turning in a
counterclockwise direction. (Clockwise in the
Southern Hemisphere.)
Atmosphere in Motion
• The Pressure Gradient:
– The atmosphere is a constantly changing
landscape of invisible mountains and valleys.
Some of the influences that cause this are:
• Irregular distribution of oceans and continents.
• Heat-transferring qualities of different Earth
surfaces.
• Daily temperature variations.
Atmosphere in Motion
• The Pressure Gradient:
– The high-pressure areas of the atmosphere
are the mountains, and the low-pressure
areas are the valleys. The wind flows from
these high-pressure mountains into the lowpressure valleys.
Atmosphere in Motion
• The Pressure Gradient:
– The slope of the high-pressure mountain is
called the pressure gradient.
– On weather maps lines called isobars show
the degree of steepness.
– Isobars are drawn through points of equal
sea-level atmospheric pressure.
– Isobars identify five different types of pressure
patterns.
Atmosphere in Motion
• Local and Surface Air Movements:
– The general circulation of air is complicated
by the irregular distribution of land and water
areas.
– Different types of surfaces differ in the rate at
which they absorb heat from the Sun and
transfer heat to the atmosphere.
– In some regions local low-pressure areas
form over hot land surfaces and over warmer
water surfaces in the winter.
Atmosphere in Motion
• Local and Surface Air Movements:
– Convection currents are formed along
shorelines. These currents cause the wind to
flow from the water over the land during the
day. During the night, they cause the wind to
blow from the land toward the water.
– Local air circulation of limited scope is caused
by variations in the Earth’s surface.
Atmosphere in Motion
• Local and Surface Air Movements:
– Some surfaces give off or reflect a great amount of
heat.
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Sand
Rocks
Plowed areas
Barren land
– Other surfaces tend to retain heat.
• Meadows
• Planted fields
• Water
Atmosphere in Motion
• Local and Surface Air Movements:
– Rising air currents are encountered by aircraft
flying over sand, rock, and other surfaces that
give off considerable heat.
– Descending air currents are encountered over
surfaces that retain heat.
Atmosphere in Motion
• Local and Surface Air Movements:
– Moving air flowing around obstructions tends
to break into eddies.
• On the leeward side of the mountain there are
descending air currents. Such conditions cause
turbulent air. The stronger the wind, the greater
the descending air currents and turbulence.
• Aviators flying into the wind toward mountainous
terrain should place enough distance between
their aircraft and the mountain tops to avoid
dangerous descending air currents.
Atmosphere in Motion
• The Jet Stream:
– The jet stream is a narrow current of air, which moves
around the Earth in wavelike patterns. It is like a
“river” of wind moving at a high rate of speed.
– The jet stream varies from about 100 to 400 miles
wide and 1 to 3 miles thick.
– Winds usually have a speed of 150 to 300 mph, but
speeds of 450 mph have been recorded. Its general
motion is from west to east.
Atmosphere in Motion
• The Jet Stream:
– The jet stream shifts positions frequently and actually
migrates with the seasons. Sometimes two streams
flow across the United States, one along the northern
border and the other well toward the south.
– The cruising range of aircraft flying downwind within
the jet stream is greatly increased. Pilots anticipating
high-altitude or long-range flights attempt to discover
the location of the jet stream and use it to their
advantage.
Summary
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What is the Atmosphere
Describe the Atmosphere
Roles of Water and Particulate Matter
Atmosphere in Motion
What’s Next