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PREVAILING WINDS
• Prevailing Winds are winds that affect large areas, therefore
affecting weather around the whole world.
• Due to the Earth rotating, the direction of prevailing winds form a
unique pattern.
The Coriolis Effect
• Imagine tossing a ball straight across to a friend on the opposite
side of a spinning surface (i.e., a merry-go-round).
• The friend misses the ball. Why?
• The ball is deflected past the friend. Because of the spinning
surface, although the ball is thrown straight, the path of the ball
appears to curve. This is due to the Coriolis effect.
The Coriolis Effect on Prevailing Winds
• As the Earth rotates, the path of prevailing winds is deflected.
• Since the Earth rotates to the East . . .
 In the Northern hemisphere the
Earth is rotating counterclockwise, so the prevailing wind
is deflected to the right.
 In the Southern hemisphere the
Earth is rotating clockwise, so
the prevailing wind is deflected
to the left.
Direction and Names of Prevailing Winds
• Winds are named for the direction they are coming from.
Prevailing Wind
Name:
Latitude
Wind Direction
Polar Easterlies
Between 60° latitude
and the poles
From East to West
Mid-latitude
Westerlies
Northeast
Trade Winds
Southeast
Trade Winds
Between 30° and 60°
latitude
Near 0°, North of the
Equator to 30°N
Near 0°, South of the
Equator to 30°
From West to East
From Northeast to
West
From Southeast to
West
• Jet Streams are a special kind of prevailing wind. They are
high-speed westerly winds (move from west to east) in the
troposhere, usually around the mid-latitude regions. They are
responsible for moving around major weather systems, such as
low-pressure and high-pressure systems.
Causes of Prevailing Winds
• Prevailing winds are caused by a combination of the Earth’s
rotation and convection currents.
o When the sun’s rays hit the equator, it heats the air. The
warm air rises and moves outward before cooling and sinking.
o Above and below the equator are 3 types of convection
currents.
1. Complete the table, for the Northern hemisphere:
Convection Current
Location
Describe the Movement
- moist, warm air rises and
moves northward
- becomes cooler (more dense)
Equatorial
as it approaches 30° N latitude
0° - 30°N
Convection Current
and sinks
- joins with Northeast Trade
winds to return to the equator
Mid-Latitude
Convection Current
Polar
Convection Current
30°N 60°N
60°N 90°N
(North
Pole)
- some cooler air from
equatorial convection current
joins mid-latitude westerlies
and flows northward
- meets cold air, so since it’s
warmer, it rises and moves
away from the cold north
- air is cold and dense
- meets warm air from midlatitude convection current
which warms it up a bit and
makes it rise
- completes convection cycle by
moving northward again
Convection Currents, Air Pressure and Jet Streams
2. a) Explain how Jet Streams are formed, where they are located,
and how they move.
- air moves from high pressure to low pressure
- troposphere air at equator is more dense than at mid-latitude
- air moves north from equator and twists to the right (due to the
Coriolis effect), forming the Jet Stream at high altitude at about
30°N latitude
- another may form past the next convection current at about 60°N
latitude
b) How is it different in the Southern hemisphere?
- In the southern hemisphere air currents move to the left, so the
Jet Stream will be formed by air moving south from the equator
and twisting to the left.
Effects of Prevailing Winds
• Distributes solar energy (warmth) from the equator to colder parts
of the world.
• Carries moisture, creating precipitation (rain and snow).
• Because warm, moist air rises, weather tends to be cloudy and
rainy near the equator where northern and southern equatorial
convection currents meet and rise.
• At 30° [N] latitude, the cool, falling air is dry, creating desert-like
conditions. (e.g., Gobi Desert in Asia, Sahara desert in Africa,
Mojave and Sonoran Deserts in North America)
• At 60° [N] latitude, unsettling weather conditions of cloud and
precipitation are formed when the mid-latitude convection currents
and polar convection currents meet and rise.
• In the winter, the Arctic region receives less sunlight, causing
greater temperature differences between polar circle and equator,
creating greater pressure differences and stronger winds.
Therefore, winds and storms are more severe in the winter up
north.
Polar
convection
current
3. On the
diagram to the
right, label the
convection
currents and
indicate
where the
jet streams
form.
60°N Jet Stream
Mid-latitude
convection
current
30°N Jet Stream
North Equatorial
convection current
South Equatorial
convection current
30°S Jet Stream
Mid-latitude
convection
current
60°S Jet Stream
Polar
convection
current