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Module #10
Air Currents
Module Introduction:
The unequal heating heating of Earth has wide-ranging effects on the climates of our planet not only
because it determines regional temperatures, but also because it drives the circulation of air around
our planet.
These air currents bring warm and cold air to different regions.
They also move moisture around the planet, which causes some regions to receive more
precipitation than others.
In this module, we will explore how air currents have a major impact on the climates of the world.
We will begin by examining how the properties of air affect the amount of moisture it can
We then explore how the unequal heating of Earth causes air to circulate along the ground
and up into the atmosphere.
As we will see, the direction of this air movement is also affected by the rotation of Earth.
Finally, we will discuss how air currents that travel over mountain ranges can cause different
climates to exist on opposite sides of the mountains.
Module #10: Air Currents
Module #10
Review Questions:
B, A, C, A, D, A, D
to Review
Review Essential
4.5, 4.8
Learning Objectives
After this module you should be able to:
● Explain how the properties of air affect the way it
moves in the atmosphere.
● Identify the factors that drive atmospheric convection
● Describe how Earth’s rotation affects the movement of
air currents.
● Explain how the movement of air currents over
mountain ranges affects climates.
Bozeman: The
Nova: The Coriolis
Essential Knowledge
4.5 Global Wind Patterns (Module 10)
Global wind patterns primarily result from the most intense solar radiation
arriving at the equator, resulting in density differences and the Coriolis
4.8 Earth's Geography and Climate (Modules 9, 10, 11)
Weather and climate are affected not only by the sun’s energy but by
geologic and geographic factors, such as mountains and ocean
A rain shadow is a region of land that has become drier because a higher
elevation area blocks precipitation from reaching the land.
Properties of Air
Air has several important properties that determine how it circulates in the
Saturation point: The maximum amount of water vapor in the air at a given
Adiabatic cooling: The cooling effect of reduced pressure on air as it rises
higher in the atmosphere and expands.
Adiabatic heating: The heating effect of increased pressure on air as it sinks
toward the surface of Earth and decreases in volume.
Latent heat release: The release of energy when water vapor in the
atmosphere condenses into liquid water.
Atmospheric convection current: Global patterns of air movement that are
initiated by the unequal heating of Earth.
Atmospheric convection currents move air and moisture around the globe.
The Saturation
Point of Air
When air cools and its
saturation point drops,
water vapor condenses into
liquid water that forms
These clouds are ultimately
the source of precipitation.
Adiabatic Heating and
As air rises, it experiences less air
pressure and expands in volume.
As the air expands it cools, thereby
lowering the saturation point of the
air mass, resulting in precipitation.
This is known as adiabatic cooling.
The same occurs as air descends,
except in reverse: the air
experiences greater air pressure
causing it to compress and heat up.
This is known as adiabatic heating.
Air Pressure
Warm air is less dense than
cooler air. As air warms, it rises,
generating low air pressure.
Low air pressure is often
associated with precipitation
because as the air rises, it
expands and cools lowering
the saturation point.
Cold air is more dense than
warmer air. As air cools, it sinks,
generating high pressure.
High air pressure is associated
with clear conditions.
Atmospheric Currents
Warming at Earth’s surface causes air
to rise up into the atmosphere where
it experiences lower pressures,
adiabatic cooling, and latent heat
The cool air near the top of the
atmosphere is then displaced
horizontally before it sinks back to
As it sinks, the air experiences
adiabatic heating and then moves
horizontally along the surface of
Earth to complete the cycle.
Atmospheric Convection Currents
► Hadley cell: A convection current in the atmosphere that
cycles between the equator and 30° N and 30° S.
► Intertropical convergence zone (ITCZ): The latitude that
receives the most intense sunlight (highest angle of
incidence), which causes the ascending branches of the
two Hadley cells to converge.
► Polar cell: A convection current in the atmosphere,
formed by air that rises at 60° N and 60° S and sinks at the
poles, 90° N and 90° S.
► Ferrell cell: A convection current in the atmosphere that
lies between Hadley cells and polar cells.
The ITCZ is the
latitude where the
angle of incidence
is closest to 90°. It
moves between
the Tropic of
Cancer (23.5° N)
and Tropic of
Capricorn (23.5° S)
over the course of
the year.
Hadley Cells
Solar energy warms humid air in the tropics.
The warm air rises and eventually cools below
its saturation point.
The water vapor it contains condenses into
clouds and precipitation.
The air, which now contains little moisture, sinks
to Earth’s surface at approximately 30° N and
30° S.
As the air descends, it is warmed by adiabatic
This descent of hot, dry air causes desert
environments to develop at those latitudes.
Hadley cells are atmospheric
convection currents that
operate between the equator
and 30° N and 30° S.
The Coriolis Effect
Coriolis effect: The deflection of an object’s
path due to the rotation of Earth.
As Earth rotates (W → E), its surface moves
much faster at the equator than in
mid-latitude and polar regions.
The faster rotation speeds found closer to the
equator cause objects that are moving
directly north or south to deflect.
The prevailing winds of the world are
produced by a combination of atmospheric
convection currents and the Coriolis effect.
National Geographic: Coriolis Effect (Video)
Note how trade winds are deflected
to the east and westerlies to the west
by the Coriolis Effect.
The Coriolis Effect
► (b) The different rotation speeds
(a) A ball thrown from the North
of Earth at different latitudes
Pole toward the equator would be
cause a deflection in the paths
deflected to the west by the Coriolis
of traveling objects.
Rain Shadow Effect
Rain shadows cause mountains to be dry on
one side.
Rain shadow: A region with dry conditions
found on the leeward side of a mountain
range as a result of humid winds from the
ocean causing precipitation on the
windward side.
Windward: The side of the mountain facing
the wind, usually coming off of the ocean.
Leeward: The side of the mountain opposite
the windward side, in the rain shadow.
Rain Shadows
Air moving inland from the ocean contains a large amount of water
vapor. When it meets the windward side of a mountain range (the
side facing the wind), it rises and begins to experience adiabatic
cooling as it expands under lower pressure.
Because water vapor condenses as air cools, clouds form and
precipitation falls.
The presence of the mountain range causes large amounts of
precipitation to fall on its windward side.
The cold, dry air then travels to the other side of the mountain range
(the leeward side), where it descends and experiences higher
pressures, which cause adiabatic heating.
This air is now warm and dry and produces arid conditions on the
leeward side forming the region called a rain shadow.
Rain Shadows
Rain shadows occur where humid winds
blowing inland from the ocean meet a
mountain range.
On the windward (wind-facing) side of
the mountains, air rises and cools, and
large amounts of water vapor condense
to form clouds and precipitation.
On the leeward side of the mountains,
cold, dry air descends, warms via
adiabatic heating, and causes much
drier conditions.
► The rain shadow of the Rocky Mountains
helped to form the Great Plains.
Warm, wet air (blue arrows) blow
in off the coast and contain water
vapor before they hit the
Cascade Mountain Range (black
line). This causes the air to rise,
expand, cool and generate
precipitation. The resulting dry air
masses (red arrows) provide little
rainfall east of the Cascades.
Note that the Colville, Yakama
Indian and Warm Springs
Reservations are situated in the
rainshadow → environmental
externalities and equity.
Atacama Desert
Deserts are described as
places receiving less than
an average of 250 mm of
rain in a year. The
Atacama, however,
receives less than an
average of 1mm a year.
The Atacama desert is the
driest hot desert on Earth,
largely the result of being
located in the rain shadow
of the Andes Mountains.
Module Review:
In this module, we have seen that the properties of air affect how it
moves in the atmosphere and how much water vapor it contains.
These properties help us understand how the intense sunlight at
latitudes near the equator drives the formation of Hadley cells, which
are responsible for also producing the polar cells and Ferrell cells.
These air convection currents, combined with the Coriolis effect,
cause the predominant wind directions that exist around the planet.
Air convection currents and the effects of rain shadows help to
determine the distribution of heat and precipitation around the