Geography Cambridge AS level syllabus 9696
Paper 1 Physical Core
Atmosphere and Weather 2
Global Circulation at Upper Levels:
Rossby Waves are found in the upper-air westerly
circulation of middle and high latitudes
Jet Stream
Jet streams: high-speed air flow in narrow bands
within the upper-air westerlies and along certain
other global latitude zones at high levels
Jet Streams
Jet streams form the
BOUNDARIES of the
Westerlies.
There are 2 types:
polar and subtropical.
They are high speed
winds, and 60 knots
is the minimum to
qualify. See Fig 5-24,
pg 122.
Whenever the paths
of jet streams change
or meander, they’re
called “Rossby
Waves”. Fig 2.23, p.
39.
Rossby Waves, Jet Streams, and the Polar Front
Rossby Waves:
• Arise in the polar front
• Contribute to variable
weather in mid and
high latitudes
Rossby Waves: horizontal
undulations in the flow path
of the upper-westerlies;
upper-air waves
Polar front: zone where cold
polar air meets warm tropical
air
Flow of air along front smooth for days or weeks.
Waves stronger. Tongue of cold air
brought south and warm north.
Undulations begin and
Become stronger. Rossby
Wave forms. Warm air
Pushed poleward,
cold air south
Tongue pinched
off.
Pool of cold air
farther
south than
originally.
Become
cyclones of cold
air
Persist for days
or weeks.
General
Surface
Circulation
•Polar Highs
•Polar Easterlies
•Subpolar Lows/Polar Front
Polar-front jet stream
• Westerly;
associated with
Rossby waves
 Subtropical jet stream
• Westerly; above
subtropical highpressure cells
 Tropical easterly jet
stream
• Runs east to west
• Summer only
• Develops in Asia

Subtropical Highs
(STHs)
Formed by the high pressure
created from descending air of
Hadley Cells.
They are giant anticyclones
(characteristic?)
They are persistent, and are
found about 30 degrees N & S.
Also called the “horse latitudes”
(16th & 17th cent.)
STHs are the source of Trade
Winds & Westerlies.
Horse Latitudes
Subtropical Highs
Responsible for
the world’s
greatest
deserts.
Dry conditions
caused by no
uplift of air due
to high
pressure.
Trade Winds
Major wind
systems of the
tropics, btw
latitudes 25
degrees N & S.
Dominates the
earth more than
any other wind
system.
They are
Easterly (blow
from East).
That’s how
winds are
named
(continued on
next page).
General Surface
Circulation
Trade winds are the most
reliable of ALL winds – their
direction & speed is
extremely consistent
(whether it be summer,
winter, day or night). There
is one exception (discussed
later).
Hence “trade winds”, mean
“winds of commerce”. 16th
century sailors found that it
was the fastest & most
reliable route from Europe to
America.
Intertropical Convergence Zone (ITCZ)
This is where the
Trade Winds
converge.
Known for low
pressure & a lot of
storms.
Also called the
“doldrums” (calm
air) – even though
it’s a zone of
convergence,
horizontal air flow
is weak.
The Westerlies
Why are they called
“Westerlies”?
Found between
30 to 60 degrees
N & S.
Cyclonic Storms

Extratropical Cyclones (nor’easterns)
 Created at the Polar Front.
 Involves two air masses.
 Meandering (wave) creates low pressure center.
 Air from both sides flow inward and begin rotating due to Coriolis
effect (c.cw. in N.hemi.)
 Rising warm moist air condenses, fueling storm energy and creating
strong wind and rains.
 Usually during winter when differences in air masses are extreme.
 Strongest winds hit northeast US and eastern Canada from the
northeast (hence nor'eastern)

Tropical Cyclones (hurricanes, typhoons, cyclones)
 Created by disturbances within a single tropical air mass.
 Genesis involved thunderstorms during periods of low wind shear.
Tropical Cyclones:
Move westward and poleward.
Begins as a tropical depression
over warm water (>25ºC). Most
of the energy associated with a
storm derives from the latent
heat of evaporation which is
given off during condensation of
rising moist air in vertical
thunderstorms.
Creates intense low pressure.
Coriolis effect initiates rotation
of air as it moves inward toward
the low pressure center = "eye".
Upgrades to a hurricane when
the wind velocity increases
above 74 mph. Winds circulate
around the eye in a counterclockwise direction in the
northern hemisphere.
Weakens when they travel
over colder water or land.
Low wind
shear favors
development!
Mitch
(Oct 1998)
Tegucigalpa, Hondurus
Geography Cambridge AS level syllabus 9696
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